A11103 0653SQ

NBS SPECIAL PUBLICATION 260-54
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NATIONAL BUREAU

OF STANDARDS

LIBRARY

SEP 1 1977

Standard Reference Materials:

Certification and Use of Acidic Potassium Dichromate

Solutions as an Ultraviolet Absorbance Standard—SRM 935 loO

^opec^l PSkcdi*" Z^O-M >Vf7

R. W. Burke and R. Mavrodineanu

C ,

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

i NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued August 1977
Library of Congress Catalog Card Number: 77-600027

National Bureau of Standards Special Publication 260-54

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-54,157 pages (Aug. 1977)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON. 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $3—Stock No. 003-003-01828-7
PREFACE

Standard Reference Materials (SRM's) as defined by the

National Bureau of Standards are "wel 1 - character! zed materials, produced in quantity, that calibrate a measurement

system to assure compatibility of measurement in the nation."

SRM's are widely used as primary standards in many diverse

fields in science, industry, and technology, both within the

United States and throughout the world. In many industries

traceability of their quality control process to the national

measurement system is carried out through the mechanism and

use of SRM's. For many of the nation's scientists and technologists it is therefore of more than passing interest to

know the details of the measurements made at NBS in arriving

at the certified values of the SRM's produced. An NBS series

of papers, of which this publication is a member, called the

NBS Special Publication - 260 Series is reserved for this

purpose

.

Tnis 260 Series is dedicated to the dissemination of

information on all phases of the preparation, measurement,

and certification of NBS-SRM's. In general, much more detail will be found in these papers than is generally allowed,

or desirable, in scientific journal articles. This enables

the user to assess the validity and accuracy of the measurement processes employed, to judge the statistical analysis,

and to learn details of techniques and methods utilized for

work entailing the greatest care and accuracy. It is also

noped that these papers will provide sufficient additional

information not found on the certificate so that new applications in diverse fields not foreseen at the time the SRM

was originally issued will be sought and found.

Inquiries concerning the technical content of this

paper should be directed to the author(s). Other questions

concerned with the availability, delivery, price, and so

forth will receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D.C. 20234

J. Paul Cali, Chief

Office of Standard Reference Materials

iii
FOREWORD

Spectrophotometry is a technique widely used in analytical

chemistry. Due to its inherent characteristics of high

sensitivity and good specificity it has been applied to the

establishment of fundamental parameters, as well as to the

determination of the composition of inorganic or organic

compounds in solids, liquids, or gases in science and industry.

For example, in one field of application, clinical analytical

chemistry, spectrophotometry is used in over 50 percent of the

analytical methods commonly employed. The number of spectrophotometry analyses performed in an active hospital laborator)!

exceeds 1,000 measurements per day, with more than 1,000,000

measurements performed in the clinical laboratories in the

U.S. daily. A conservative projection for the future indicates;

that a growth rate of 15 percent per year in spectrophotometry

measurements in clinical analysis should be expected.

The widespread use of spectrophotometric techniques on

samples vital to the health, safety, and industry of the natioiji

makes it imperative that suitable means be available to establi

the accuracy of these analyses. Because of the importance of

these measurements, the Analytical Chemistry Division of the

Institute for Materials Research is conducting an extensive

program to study and develop means which can be used by the

analytical chemist to verify the proper functioning of spectro-;

photometers, and to insure that the data produced are reliable.

A principal ingredient in the Analytical Chemistry Divisicjj

program has been the design and construction of a high-accuracy

spectrophotometer which represents the primary optical transmittance standard. This instrument was used to produce a numbe

of transfer Standard Reference Materials in the form of neutral;!

glass filters and liquid filters certified for transmittance

at various wavelengths and transmittance levels. Quartz

cuvettes with certified paralellism and pathlength were also

produced at NBS.

This Special Publication describes another transmittance/

absorbance transfer standard which has been developed within

the program. This material, crystalline potassium dichromate,

allows certification to be extended into the ultraviolet

spectral region and, additionally, provides a convenient means

to verify the linearity of the absorbance scale of conventiona

spectrophotometers. This material is now available as

Standard Reference Material 935. The details of the selection

preparation, certification, and use of this SRM are described

in detail in this publication.
Improved methods and materials which can be used to verify

the proper functioning of conventional spectrophotometers are

now being actively investigated. Examples are evaporated

metal -on-fused silica filters, which should extend the transmittance/absorbance calibration range from 200 nm to 3000 nm,

and liquid materials, such as holmium perchlorate solutions,

which can be used for verifying the accuracy of the wavelength

scale of spectrometers in transmittance and luminescence

measurements

.

The availability of these Standard Reference Materials,

and their use by the analyst, should contribute significantly

to accurate measurements in spectrophotometry and should

provide a solid base for future progress in this field.

P. D. LaFleur, Chief

Analytical Chemistry Division

v
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference

Materials ( 1975-76 edition), R. W

Seward, ed., NBS Spec. Publ. 260 (June

1975). $1.50*

Michaelis, R. E., and Wyman, L. L.,

Standard Reference Materials: Preparation of White Cast Iron Spectrochemical Standards, NBS Misc. Publ.

260-1 (June 1964). COM74-1 1061**

Michaelis, R. E., Wyman, L. L., and

Flitsch, R., Standard Reference Materials:

Preparation of NBS Copper-Base Spectrochemical Standards, NBS Misc. Publ. 260-2

(October 1964). COM 74-11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A.,

Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric

Low-Alloy Steel Standard, NBS Misc. Publ.

260-3 (October 1964). COM74-1 1060**

Hague, J. L., Mears, T. W., and Michaelis, R. E.,

Standard Reference Materials: Sources of

Information, NBS Misc. Publ. 260-4 (February

1965). COM74-11059

Alvarez, R., and Flitsch, R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys, NBS

Misc. Publ. 260-5 (March 1965). PB168068**

Shultz, J. I., Standard Reference Materials:

Methods for the Chemical Analysis of White

Cast Iron Standards, NBS Misc. Publ. 260-6

(July 1975). COM74-11068**

Bell, R. K., Standard Reference Materials: Methods for the Chemical Analysis of

NBS Copper-Base Spectrochemical Standards,

NBS Misc. Publ. 260-7 (October 1965).

COM74-11067**

Richmond, M. S., Standard Reference

Materials: Analysis of Uranium Concentrates at the National Bureau of

Standards, NBS Misc. Publ. 260-8 (December

1965). COM74-11066**

Anspach, S. C, Cavallo, L. M., Garfinkel,

S. B., Hutchinson, J. M. R., and Smith,

C. N., Standard Reference Materials:

Half Lives of Materials Used in the

Preparation of Standard Reference Materials of

Nineteen Radioactive Nuclides Issued by the

National Bureau of Standards, NBS Misc.

Publ. 260-9 (November 1965). COM74-1 1065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F.

J., and Michaelis, R. E., Standard

Reference Materials: Homogeneity Characterization on NBS Spectrometric Standards II: Cartridge Brass and Low-Alloy Steel, NBS

Misc. Publ. 260-10 (December 1965).

COM74-11064**

Napolitano, A., and Hawkins, E. G.,

Standard Reference Materials: Viscosity of Standard Lead-Silica Glass,

NBS Misc. Publ. 260-1 1 (November

1966). NBS Misc. Publ. 260-11**

Yakowitz, H., Vieth, D. L., and Michaelis,

R. E., Standard Reference Materials:

Homogeneity Characterization of NBS

Spectrometric Standards III: White

Cast Iron and Stainless Steel Powder

Compact, NBS Misc. Publ. 260-12(September

1966). NBS Misc. Publ. 260-12**

Spijkerman, J. I., Snediker, D. K., Ruegg,

F. C, and DeVoe, J. R , Standard

Reference Materials: Mossbauer Spectroscopy

Standard for the Chemical Shift of Iron Compounds, NBS Misc. Publ. 260-13 (July 1967).

NBS Misc. Publ. 260-13**

Menis, O., and Sterling, J. T., Standard

Reference Materials: Determination of Oxygen

in Ferrous Materials - SRM 1090, 1091, and

1092, NBS Misc. Publ. 260-14 (September 1966).

NBS Misc. Publ. 260-14**

Passaglia, E., and Shouse, P. J., Standard

Reference Materials: Recommended Method of

Use of Standard Light-Sensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for

Colorfastness to Light, NBS Misc. Publ. 260-15

(June 1967). (Replaced by NBS Spec. Publ.

260-41.)

Yakowitz, H., Michaelis, R. E., and Vieth,

D. L., Standard Reference Materials:

Homogeneity Characterization of NBS

Spectrometric Standards IV: Preparation and Microprobe Characterization

of W-20% Mo Alloy Fabricated by

Powder Metallurgical Methods, NBS

Spec. Publ. 260-1 6 (January 1969).

COM74-11062**

Catanzaro, E. J., Champion, C. E., Garner,

E. L., Marinenko, G., Sappenfield, K.

M., and Shields, W. R., Standard Reference

Materials: Boric Acid; Isotopic and Assay

Standard Reference Materials, NBS Spec. Publ.

260-17 (February 1970). 65 cents*

vi
Geller, S. B., Mantek, P. A., and Cleveland, N. G.,

Standard Reference Materials: Calibration of

NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using the Reference

Tape Amplitude Measurement "Process A,"

NBS Spec. Publ. 260-18 (November 1969). (See

NBS Spec. Publ. 260-29.)

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold (Certification of Standard Reference Material 745), NBS

Spec. Publ. 260-19 (January 1970). PB190071**

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and

Silver, NBS Spec. Publ. 260-21 (January 1971).

COM74-11359**

Yakowitz, H., Fiori, C. E., and Michaelis, R. E.,

Standard Reference Materials: Homogeneity

Characterization of Fe-3 Si Alloy, NBS Spec.

Publ. 260-22 (February 1971). COM74-1 1357**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23

(December 1970). 25 cents*

Sappenfield, K. M., Marineko, G., and Hague, J.

L., Standard Reference Materials: Comparison

of Redox Standards, NBS Spec. Publ. 260-24

(January 1972). $1.00*

Hicho, G. E., Yakowitz, H., Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials:

A Standard Reference Material Containing

Nominally Four Percent Austenite, NBS Spec.

Publ. 260-25 (February 1971). COM 74-1 1356**

Martin, J. F., Standard Reference Materials:

National Bureau of Standards-US Steel

Corporation Joint Program for Determining

Oxygen and Nitrogen in Steel, NBS Spec. Publ.

260-26 (February 1971). 50 cents*

Garner, E. L., Machlan, L. A., and Shields, W. R.,

Standard Reference Materials: Uranium

Isotopic Standard Reference Materials, NBS

Spec. Publ. 260-27 (April 1971). COM74-

11358**

Heinrich, K. F. J., Myklebust, R. L., Rasberry, S. D., and Michaelis, R. E., Standard Reference

Materials: Preparation and Evaluation of

SRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM7 1-50365**

Geller, S. B., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic

Tape (Computer Amplitude Reference) Using

vi

the Reference Tape Amplitude Measurement

"Process A-Model 2," NBS Spec. Publ. 260-29

(June 1971). COM71-50282**

Gorozhanina, R. S., Freedman, A. Y., and

Shaievitch, A. B. (translated by M. C. Selby),

Standard Reference Materials: Standard

Samples Issued in the USSR (A Translation

from the Russian), NBS Spec. Publ. 260-30 (June

1971). COM71-50283**

Hust, J. G., and Sparks, L. L., Standard Reference

Materials: Thermal Conductivity of Electrolytic

Iron SRM 734 from 4 to 300 K, NBS Spec. Publ.

260-31 (November 1971). COM7 1-50563**

Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes

for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents*

Wagner, H. L., Standard Reference Materials:

Comparison of Original and Supplemental SRM

705, Narrow Molecular Weight Distribution

Polystyrene, NBS Spec. Publ. 260-33 (May

1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference

Materials: Thermal Conductivity of Austenitic

Stainless Steel, SRM 735 from 5 to 280 K, NBS

Spec. Publ. 260-35 (April 1972). 35 cents*

Cali, J. P., Mandel, J., Moore, L. J., and Young,

D. S., Standard Reference Materials: A Referee

Method for the Determination of Calcium in

Serum, NBS SRM 915, NBS Spec. Publ. 260-36

(May 1972). COM72-50527**

Shultz, J. L, Bell, R. K., Rains, T. C, and Menis,

O., Standard Reference Materials: Methods of

Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence Materials: Preparation and Calibration of

Standards of Spectral Specular Reflectance,

NBS Spec. Publ. 260-38 (May 1972). 60 cents*

Clark, A. F., Denson, V. A., Hust, J. G., and

Powell, R. L., Standard Reference Materials:

The Eddy Current Decay Method for Resistivity

Characterization of High-Purity Metals, NBS

Spec. Publ. 260-39 (May 1972). 55 cents*

McAdie, H. G., Garn, P. D., and Menis, O.,

Standard Reference Materials: Selection of

Thermal Analysis Temperature Standards

Through a Cooperative Study (SRM 758, 759,

760), NBS Spec. Publ. 260-40 (August 1972).

65 cents*

i
Wood, L. A., and Shouse, P. J., Standard Refer- ence Materials: Use of Standard Light-Sensitive

Paper for Calibrating Carbon Arcs Used in

Testing Textiles for Colorfastness to Light, NBS

Spec. Publ. 260-41 (August 1972). COM72-

50775**

Wagner, H. L., and Verdier, P. H., eds., Standard

Reference Materials: The Characterization of

Linear Polyethylene, SRM 1475, NBS Spec.

Publ. 260-42 (September 1972). 45 cents*

Yakowitz, H., Ruff, A. W., and Michaelis, R. E.,

Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260-43 (November 1972). 45 cents*

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A.,

Jr., Standard Reference Materials: Preparation

and Use of Superconductive Fixed Point

Devices, SRM 767, NBS Spec. Publ. 260-44

(December 1972). COM73-50037**

Greifer, B., Maienthal, E. J., Rains, T. C, and

Rasberry, S. D., Standard Reference Materials:

Powdered Lead-Based Paint, SRM 1579, NBS

„ Spec. Publ. 260-45 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 K, NBS Spec. Publ. 260-46

(March 1975). $1.00*

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797,

and Austenitic Stainless Steel, SRM 798, from

5 to 280 K, NBS Spec. Publ. 260-47 (February

1974). COM74-50176**

Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory,

SRM 933 and SRM 934, NBS Spec. Publ.

260-48 (May 1974). 60 cents*

Carpenter, B. S., and Reimer, G. M., Standard

Reference Materials: Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ.

260-49 (November 1974). 75 cents*

Hust, J. G., and Giarratano, P. J., Standard

Reference Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Electrolytic Iron, SRM's 734 and 797

from 4 to 1000 K, NBS Spec. Publ. 260-50 (June

1975). $1.00*

Mavrodineanu, R., and Baldwin, J. R., Standard

Reference Materials: Glass Filters As a Standard Reference Material for Spectrophotometry;

Selection; Preparation; Certification; Use -SRM

930, NBS Spec. Publ. 260-51 (November 1975).

$1.90*

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference

Materials 730 and 799, from 4 to 3000 K, NBS

Spec. Publ. 260-52 (September 1975). $1.05*

Durst, R. A., Standard Reference Materials:

Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1975, Revised)*.

Burke, R. W., and Mavrodineanu, R., Standard

Reference Materials: Certification and Use of

Acidic Potassium Dichromate Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (in press).

* Send order with remittance to: Superintendent

of Documents, US Government Printing

Office, Washington, DC 20402. Remittance

from foreign countries should include an

additional one-fourth of the purchase price for

postage.

** May be ordered from: National Technical

Information Services (NTIS), Springfield,

Virginia 22151.

viii
TABLE OF CONTENTS

PAGE

1. GENERAL 2

2. INTRODUCTION 3

3. SELECTION OF POTASSIUM DICHROMATE AS AN

ULTRAVIOLET ABSORBANCE STANDARD 5

4. IONIC COMPOSITION AND SPECTRAL PROPERTIES OF

POTASSIUM DICHROMATE SOLUTIONS 6

5. MEASUREMENT OF THE APPARENT SPECIFIC ABSORBANCES

OF SRM 935 8

5.1 Source and Purity of the Potassium

Dichromate SRM 8

5.2 Preparation of Solutions. 9

5.3 Instrumentation 10

5.4 Selection of Wavelengths 11

5.5 Details of the Transmittance/Absorbance

Measurements 12

5.6 Calculation of the Apparent Specific

Absorbances of SRM 935 Potassium Dichromate

in 0.001N Perchloric Acid 18

5.7 Statistical Treatment of Experimental

Apparent Specific Absorbances 24

6. OTHER EXPERIMENTAL PARAMETERS AFFECTING THE

APPARENT SPECIFIC ABSORBANCES OF SRM 935 ... . 28

6.1 Wavelength Accuracy 28

6.2 Spectral Bandpass 30

6.3 Stray Radiation 32

6.4 Beam Geometry 33

6.5 Temperature 33

6.6 Acidity 35

6.7 Water Purity 35

7. INSTRUCTIONS FOR USING SRM 935 AS AN

ABSORBANCE STANDARD 37

8. USE OF SRM 935 TO VERIFY ABSORBANCE LINEARITY. . . 41

9. SUMMARIZING REMARKS 44

10. ACKNOWLEDGEMENTS . . . , 4 5

11. REFERENCES

.

46

12. APPENDICES 48

ix
LIST OF TABLES

TABLE NO. PAGE

1. Calibration of the microbalance 10

2. Teletype printout of transmittance/absorbance

data for a set of acidic K 2 Cr 2 0 7 solutions. . . 16

3. Apparent specific absorbances, e , calculated

from transmittance/absorbance measurements on

five concentration levels of SRM 935 potassium

dichromate in 0.001N perchloric acid at wavelengths corresponding to absorbance maxima or

minima 20

4. Apparent specific absorbances, e , calculated

from transmittance/absorbance measurements on

five concentration levels of SRM 935 potassium

dichromate in 0.001N perchloric acid at wavelengths near the two isosbestic points 22

5. Composition of chromium (VI) solutions in 0.001N

perchloric acid at 25 °C 25

6. Certified values of the apparent specific

absorbances, e , of SRM 935 potassium

dichromate in 0.001N perchloric acid at

wavelengths 235 , 257 , 313 and 350 nm 26

7. Experimental conditions used in establishing

the apparent specific absorbances, e , of

SRM 935 ? 27

8. Recommended values of the apparent specific

absorbances, e , of SRM 935 potassium

dichromate in 0.001N perchloric acid at four

wavelengths approximating the two isosbestic

points 27

9. Characteristic radiations emitted by a quartz,

low pressure, mercury discharge tube 29

10. Variation of transmittance density with spectral

bandpass of a 0.001N perchloric acid solution

of potassium dichromate at wavelengths corresponding to absorbance maxima and minima. ... 31

11. Densities of 0.001N perchloric acid solutions of

potassium dichromate at 23.5 °C 41

x
LIST OF FIGURES

FIGURE NO. PAGE

1. Absorbance spectra of the HCrQit ion and its

dimerizat ion product, Cr 2 0 7 8

2. Absorbance spectrum of a 40 mg kg 1 solution of

K 2 Cr 2 0 7 in 0.001N perchloric acid 12

3. Apparent absorbances, A , of the blank solvent

measured against air m the five cuvettes

used-Nos. 14, 16, 18, 25, and 26 14

4. Test of the distilled water for reducing

impurities 36

5. Correction curve for the absorbance scale of a

precision spectrometer 40

xi

CERTIFICATION AND USE OF ACIDIC POTASSIUM DICHROMATE

SOLUTIONS AS AN ULTRAVIOLET ABSORBANCE STANDARD

The apparent specific absorbances of 0.001N perchloric

acid solutions of a high purity potassium dichromate salt,

Standard Reference Material (SRM) 935, have been certified at

four wavelengths in the ultraviolet—235 , 257 , 313 and 350 nm.

This publication describes in detail the measurements leading

to the certification, and discusses the use of this SRM as

a transfer standard for verifying the accuracy of the absorbance scale of narrow bandpass absorption spectrometers in

this important wavelength region. Apparent specific absorbance data are also reported near two predicted isosbestic

wavelengths in the acidic HCrOi* /Cr 2 0 7 system. The apparent

specific. absorbances at 345 nm are sufficiently independent

of concentration that absorbance measurements at this wavelength can be used for verifying absorbance linearity to

about one part in a thousand.

Key Words: Absorbance linearity; accuracy of transmittance

or absorbance scale; acidic potassium dichromate

solutions; calibration of ultraviolet

spectrometers; certification of apparent specific

absorbances; isosbestic wavelengths; liquid

filters; Standard Reference Material; transfer

standards

.

R. W. Burke and R. Mavrodineanu

Analytical Chemis

Institute for Mater

National Bureau o

Washington, D.

try Division

ials Research

f Standards

C. 20234

1
1 . GENERAL

The objective of this work was to provide the analyst

with a Standard Reference Material (SRM) which can be used

in laboratory practice to verify the accuracy of the transmittance or absorbance scale of conventional spectrometers in

the ultraviolet region of the spectrum. The material selected

for this purpose was a potassium dichromate salt from which

the user must prepare adequate solutions in 0.001N perchloric

acid. The values given in this work, and in the subsequent

certificate of calibration, are derived from absorbance

measurements which were not corrected for multiple internal

reflections within the cuvette and for buoyancy effects.

The option was believed justifiable in the use of this SRM

as a transfer standard since, in practice, internal reflections are common to all measurements in spectrometers, and

the variation of buoyancy from one location to another is

negligible. Hence, the specific absorbances 1 certified do

not define an absolute physical parameter of this material.

These values are therefore termed apparent specific absorbances, e , and are used throughout this work and in the

certificate to signify that they have not been corrected for

these two effects. These values, however, allow the use of

i this SRM as a transfer standard and do not require additional

data manipulation by the user. This variance from the strict

definition of specific absorbance, e, was made for convenience

but still permits the transfer to the user the accuracy and

traceability derived from direct measurements on the National

Bureau of Standards high accuracy transmission spectrometer.

The reader interested in the specific absorbances of this

material should consult reference 9 in which the corrected

The spectrometry nomenclature used in this publication is

defined in a recent article by K. D. Mielenz. A copy of

this paper is enclosed in the Appendix.

2
values are given. The difference between the corrected and

uncorrected values does not exceed 0.2 percent.

2. INTRODUCTION

During the past forty years molecular absorption spectrometry has been a much used analytical technique in all

areas of science and technology. Despite this widespread

use there has been, until recently, no suitable Standard

Reference Material available for verifying the accuracy of

the transmittance or absorbance scales of the spectrometers

on which the measurements were made. Yet, as early as the

late 1940 T s, the question of photometric accuracy was of

general concern. Soon thereafter the results of various

collaborative testings of absorption spectrometers began

appearing in the literature. Among the most notable of

these were the tests conducted by the British Photoelectric

Spectrometry Group [1,2], by a similar Dutch Group [3] and

by Vandenbelt [4,5] in the United States. All of these

tests showed significant differences between measurements

made on different instruments. However, since no certified

transmittance or absorbance standards were available to

assess accuracy, the only other alternative was to use

agreement with consensus results in order to intercompare

instruments

.

This situation remained essentially unchanged until 1969

when, largely from requests by the American Association of

Clinical Chemists and the College of American Pathologists,

a program was initiated at the National Bureau of Standards

(NBS) in the Analytical Chemistry Division of the Institute

for Materials Research. The express purpose of this program

was to develop and make available to the public Standard

Reference Materials which could be used to establish that an

absorption spectrometer was functioning properly and was

producing meaningful and reliable data. With partial funding

3
from the National Institute of General Medical Sciences,

this program led, in 1970, to the design and construction of

a high-accuracy transmission spectrometer [6,7]. This instrument, in turn, was used to develop and certify two Standard

Reference Materials (SRM's) - SRM 930, Glass Filters for

Spectrophotometry, and SRM 931, Liquid Filters for Spectrophotometry. These SRM's were issued in 1971. A third

Standard Reference Material, SRM 932, Quartz Cuvettes for

Spectrophotometry, was added in 1973 and was produced in

association with the Optical Physics Division of the Institute

for Basic Standards at NBS. SRM 932 was intended to be used

in association with SRM 931, and in general, in all measurements requiring accuracy in determining the transmittance or

absorbance of liquids.

Since the issuance of these Standard Reference Materials,

research interest has focused on developing new SRM's which

can be used in the ultraviolet. The need for such standards

' has been voiced again most strongly by the clinical chemists

and pathologists. They emphasize the importance of these

SRM's by pointing out, for example, that progress in all

areas of enzyme research and analysis is directly related to

the accuracy of absorbance measurements made in the wavelength range from 260 to 350 nm. Neither of the two current

SRM's meet this need. The glass filters do not transmit

below about 350 nm and the liquid filters are certified at

only one wavelength in the ultraviolet, 302 nm.

Progress in this research effort was described in two

papers presented at the Workshop Seminar on Standardization

in Spectrophotometry and Luminescence Measurements held in

the NBS facilities at Gaithersburg , Maryland, on November 19- I

20, 1975. These papers have since been published in the NBS

Journal of Research [8,9]. The first paper described an

extensive evaluation of the solid metal -on-quartz type

filters while the second presented data which showed that

acidic potassium dichromate solutions were the most promising

4
liquid standards that we had studied thus far for use in the

ultraviolet

.

Following that meeting, the decision was made to

certify and issue a high purity potassium dichromate salt in

solid form (SRM 935) , to be used as an SRM for verifying the

accuracy of the absorbance scale of narrow bandpass spectrometers over the range 235-350 nm. In as much as this SRM is

being issued as a solid, it is being certified for its

apparent specific absorbances which have been determined

under a set of selected and well-defined conditions. In

practice, the user will have to prepare solutions containing

known concentrations of the SRM and, using the Beer-Lambert

equation, convert the certified apparent specific absorbance

data into accurate absorbance or transmittance values

.

This publication describes in detail the operations

involved in the certification of SRM 935 and demonstrates,

with examples, its function as a transfer standard for

verifying the accuracy of the transmittance or absorbance

scales of narrow bandpass absorption spectrometers. Reprints

of papers pertinent to the subject matter discussed in this

report have been reproduced and included in the Appendix

Section for the convenience of the reader.

3. SELECTION OF POTASSIUM DICHROMATE AS AN ULTRAVIOLET

ABSORBANCE STANDARD

The decision to certify and recommend acidic potassium

dichromate solutions as transfer standards for verifying the

accuracy of the transmittance or absorbance scale of narrow

bandpass absorption spectrometers in the ultraviolet was

made after extensive studies had shown this system to be the

best of those liquid filters considered for this purpose.

Other systems considered potentially useful included potassium nitrate, potassium acid phthalate, benzoic acid,

p-nitrophenol , alkaline potassium chromate, composite

5
solutions of metal ions, and various water-soluble organic

dyes. These studies have been described previously [10].

Since both the acidic and alkaline chromium systems were

clearly superior to the others in terms of the spectral

range covered, the final choice was reduced to choosing

between these two. Subsequently, the acidic system was

selected because of the following considerations: (a) more

useful distribution and relation of absorbance maxima and

minima, (b) more favorable weighing factor, (c) a smaller

temperature dependence, (d) less reaction with glassware,

and (e) greater ease of transferring and rinsing. These

advantages were considered to more than offset the one

disadvantage that can be cited for the acid potasssium

dichromate system, namely that the absorbance, as a function

of concentration, exhibits a stronger deviation from linearity

than most systems. As will be shown in subsequent sections

^ of this report, this deviation detracts little, if any, from

the practical use of this material as a transfer standard.

4. IONIC COMPOSITION AND SPECTRAL PROPERTIES OF POTASSIUM

DICHROMATE SOLUTIONS

The ionic composition of aqueous chromium (VI) solutions

is dependent on their acidity or alkalinity. Variation of

composition with pH was discussed in detail in a previous

paper [10] . A copy of this publication is included in the

Appendix. In alkaline solution chromium (VI) exists only in

the form of CrOit" ions. In weakly acidic media, the predominant chromium (VI) ion is HCrOi* . In addition, and

depending on the total concentration of chromium present,

there exist varying amounts of a second ion, Cr 2 0 7 ~. This

ion is formed by the dimerization of HCrOi* " . The equilibrium

expression for this dimerization may be written as follows:

6
- KD

ZHCrCU _1 Cr 2 0 7 + H 2 0 (1)

where K

D

, the concentration dimerization constant, is given

by

[Cr 2 0 7 ] K = — (2) U [ECrO h ] 2

Using the value o£ KD

determined previously [10] , it can be

shown that, for the range of potassium dichromate solutions

used in this study, the percentage of the total chromium

(VI) present as the Cr 2 0 7 ~ dimer varies from about one to

four percent.

The variation in the ionic composition with concentration is the principal cause of the observed deviations of

the acid system from Beer's law. These deviations are

easily measured at those wavelengths where the specific

molar absorbances of the Cr 2 0 7 ~ and HCrO^ ions are different.

As shown in figure 1, at most wavelengths in the ultraviolet

the Cr 2 0 7 ~ ion is more strongly absorbing than the HCrOi*

form. Consequently, at these wavelengths, solutions would

be expected to exhibit increasingly positive deviations from

Beer's law with increasing chromium concentration. The

reverse behavior, but to a lesser degree, would be expected

for those wavelengths in the range of approximately 320-345 nm

since, in this region, the Cr 2 0 7 ion is the less strongly

absorbing form. At the two isosbestic points both ions have

the same specific molar absorbances and no deviations from

changes in ionic composition would be expected. The data

presented in the following section will demonstrate better

these various effects.

7
250 300 350 400

Wavelength, nm

Figure 1. Absorbance spectra of the HQrOtt ion and its

dimerization product, Cr 2 0 7 .

5. MEASUREMENT OF THE APPARENT SPECIFIC ABSORBANCES

OF SRM 935

5 . 1 Source and Purity of the Potassium Dichromate SRM

The potassium dichromate (KaCraOy) salt used for this

SRM was an analytical reagent grade material purchased as a

single lot from the J. T. Baker Chemical Company, 2 Phillipsburg, New Jersey 08865. Drying studies performed at 105 °C

showed that the surface moisture of this material was less

than 0.01 percent. No attempts were made to determine possible occluded water. However, a recent paper [11] by

2 In order to describe materials and experimental procedures

adequately, it is occasionally necessary to identify the

sources of commercial products by the manufacturer's name.

In no instance does such identif icaton imply endorsement

by the National Bureau of Standards, nor does it imply

that the particular product is necessarily the best

available for that purpose.

8
Yoshimori and Sakaguchi has shown that K 2 Cr 2 0 7 typically

contains 0.01 to 0.02 percent moisture which can be removed

only by crushing and drying at temperatures in excess of

350 °C. SRM 935 is intended to be used without further

drying. When not in use, it is recommended that it be capped

and stored in a dessicator over magnesium perchlorate to

avoid undue exposure to humidity.

A coulometric assay of the purity of the undried

material was performed in the Analytical Chemistry Division

by G. Marinenko. The purity of SRM 935, expressed as an

oxidant, was found to be 99.972 ± 0.005 percent where the

uncertainty figure represents the 95 percent confidence

interval for the mean based on 11 degrees of freedom. In

addition, this material was examined for trace elemental

impurities by J. A. Norris, also from the Analytical Chemistry

Division, using emission spectrometry. The only significant

impurities detected were sodium and rubidium. Their concentrations were estimated to be in the range 0.02 and 0.03

percent, respectively.

5 . 2 Preparation of Solutions

All concentrations of potassium dichromate solutions

used in this study were prepared, and are expressed, on a

weight basis. Each solution was prepared individually by

weighing milligram amounts of the K 2 Cr 2 0 7 SRM to the nearest

microgram on a sensitive microbalance . The weighed samples

were transferred quantitatively to 1-liter volumetric flasks

which had been pre-weighed to the nearest 0.01 g on a singlepan, top-loading balance. The samples were dissolved in

400-500 ml of distilled water shown to be free of reducing

impurities (see Section 6.7 for recommended test) and 1 ml

of IN perchloric acid, prepared from reagent grade material,

was added to each to adjust acidity. After diluting approximately to volume with distilled water, the flasks were

sealed with tightly- fitting , double-ribbed Teflon stoppers
to prevent evaporation and the solutions were mixed well.-

These stoppers are available from Kontes Glass Company,

Vineland, New Jersey 08360. The flasks and contents were

weighed again on the top-loading balance to the nearest

0.01 g and the net weights of the solutions determined by

difference. The concentrations of each solution were then

calculated and expressed as mg K 2 Cr 2 0 7 /kg of solution. A

solvent blank was prepared by diluting 1 ml of IN perchloric

acid to 1 liter with distilled water.

Prior to preparing the K2Cr2C>7 solutions, the accuracy

of the two balances was checked with weights calibrated in

the Mass Section of the NBS Institute for Basic Standards.

Table 1 shows the corrections that were applied to all

weighings made on the microbalance . No correction was found

necessary for weighings made on the top-loading balance.

Table 1. Calibration of the microbalance.

Nominal Wt

.

mg

True Wt.

mg

Measured Wt

.

mg

Correction

mg

20 19.9993 20. 002 -0.003

40 39. 9932 39. 995 -0. 002

60 59.9941 60.001 -0. 007

80 79.9798 79.976 +0. 004

100 99.9924 99. 992 0. 000

5 . 3 Instrumentation

The Institute for Materials Research high accuracy

transmission spectrometer was used for performing all transmittance measurements which, in turn, were converted to

absorbance by an appropriate computer program. The design

and construction of this instrument have been described in

detail by one of us (RM) in reference 7 and will not be

repeated here. Similarly, the quartz cuvettes used are also

of NBS design and construction and have been described

10
previously [12] . These certified cuvettes are identified as

SRM 932 and are available through the NBS Office of Standard

Reference Materials. A copy of the paper describing the

construction and calibration of these cuvettes is included

in the Appendix.

5 . 4 Selection of Wavelengths

Four wavelengths - 235, 257, 313 and 350 nm - were

chosen for the certification of the apparent specific absorbances of acidic potassium dichromate. These wavelengths

are indicated in the absorbance spectrum shown in figure 2.

They were selected because they are least subject to small

inaccuracies that may be present in the wavelength scales of

the instruments for which this SRM is intended. Although

the spectral bandpass effect has been increased at these

wavelengths in comparison to measurements made along the

slopes of the spectrum, the restrictions are not severe.

The bandwidths of the 257 and 350 nm peaks are approximately

40 and 60 nm respectively and therefore an instrumental

spectral bandwidth of 2 nm or less is sufficent for obtaining

at least 0.999 of the maximum peak intensities (see also

pp. 30 to 32)

.

In addition to the four above-mentioned wavelengths

chosen for certification of apparent specific absorbances,

four additional wavelengths were selected in each of the

regions of the two isosbestic points shown in figure 1.

This approach was necessary because of the uncertainty in

the Cr 2 0 7 spectrum which is a calculated spectrum. The

details of its derivation have been described in reference

[10] . The four wavelengths chosen as the best estimates of

these isosbestic points were 322, 323, 345 and 346 nm.

11
1.0

0.8

0.6

0.4

0.2

-

I i I

\

235

257

350

l

\

313 /

i

i 250 300

Wavelength, nm

350 400

Figure 2. Absorbance spectrum of a 40 mg«kg 1 solution of

K 2 Cr 2 0 7 in 0.001N perchloric acid.

5 . 5 Details of the Transmittance/Absorbance Measurements

All of the transmittance/absorbance measurements were

made relative to air in a temperature-controlled room at

23.5 ± 0.2 °C. Two sets of measurements were necessary

therefore in order to determine the absorbance required for

the calculation of an apparent specific absorbance. These

were: A measurement of the transmittance density,

D = -logioT, of the solvent blank (Dg) and a similar measurement of the sample solution (Dg) . The apparent absorbance

(A ) due to the chromium (VI) ions was then computed from

the relation A = Dc, - D,,

.

a S B

The first transmittance/absorbance measurements performec

on the high-accuracy transmission spectrometer entailed

making a series of runs on a solvent blank. Five of the

certified cuvettes described in reference 12 (reproduced in

the Appendix) were selected for this experiment. This number

was chosen because, in subsequent experiments, five different

concentrations of potassium dichromate were to be measured.

12
The cuvettes were cleaned by immersion in concentrated

(36N) sulfuric acid for several hours. They were then

transferred consecutively to 24, 12 and 6N solutions of

sulfuric acid in order to eliminate excessive generation of

heat upon rinsing directly with distilled water and thereby

avoid the possibility of deforming them. They were then

allowed to air-dry under an inverted Petri dish. Finally,

the windows of the cuvettes were wiped with a lint-free lens

tissue immediately before placing in the spectrometer. Once

placed into their respective holders, they were not removed

again until all the measurements necessary for the certification of SRM 935 had been completed.

Transfer of solvent and sample solutions to and from

the cuvettes were made by means of 23-cm, borosilicate

,

Pasteur- type , disposable pipettes fitted with 2 ml rubber

bulbs. These pipettes and bulbs are readily available from

all major chemical supply houses. Similar pipettes made

from soft glass are also widely available. The soft glass

types are not recommended because, in general, their surfaces

are contaminated with ultraviolet absorbing contaminants and

they also have an alkaline surface which could be an additional source of error [13]. As a precautionary measure,

the borosilicate pipettes were always rinsed thoroughly with

distilled water and dried in an oven at 105 °C before using.

The pattern of results obtained for the five cuvettes

when they were filled with blank solvent and their apparent

absorbances measured relative to air is shown in figure 3.

These initial measurements were made at 235 nm because it

was the shortest wavelength of interest and any problems

associated with cuvette cleanliness or transfer of solution

were expected to be maximal at this wavelength. All of the

cuvettes exhibit the same qualitative absorbance behavior

upon sucessive fillings with solvent. The term "rinse" as

used in figure 3 and throughout the remainder of this report

refers to the exchange of one cuvette volume for a second

13
4.90 1

1 2 3 4 5 6 7

No. OF RINSES

Figure 3. Apparent absorbances, A , of the blank solvent

measured against air in the five cuvettes used—

Nos. 14, 16, 18, 25, and 26; ref. [6].

volume. A minimum and constant absorbance reading was

obtained in each case after 5-6 rinses. Consequently six

rinses were used in the transfer of all blank and sample

solutions. Following these measurements the first set of

acidic potassium dichromate solutions were prepared. Comprising this set were six 0.001N perchloric solutions

having nominal concentrations of 0, 20 40, 60, 80 and 100 mg

K 2 Cr 2 0 7 /kg. The K 2 Cr 2 0 7 solutions were prepared accurately

by weight as described previously and immediately placed in

the controlled- temperature room housing the high-accuracy

transmission spectrometer and allowed to equilibrate. Generally, for convenience, the solutions were prepared one

afternoon and measurements were made the following day.

This series of measurements were begun by repeating the

absorbance measurements on the solvent blank at 235 nm.

14
Since these values were in agreement with those obtained

previously, similar blank measurements were then made at the

other seven wavelengths of interest. Thereupon, the solvent

was removed from the five cuvettes by means of the Pasteurtype, disposable pipettes and immediately replaced with the

acidic potassium dichromate solutions. This operation was

performed in a sequential manner and, as a matter of convention, the lowest K 2 Cr 2 0 7 concentration was always placed in

the first cuvette and the highest concentration in the last.

Six rinses of each cuvette were made with the appropriate

sample solution in order to assure that the concentrations

of the solutions measured were identical to those sampled.

Two different pipettes were used alternately for the

filling and emptying operation associated with the transfer

of each solution. After the third rinse, the pipette used

originally for filling was made the emptying pipette and a

new pipette was taken for the filling step. Such an approach

appeared to be a good technique for avoiding concentration

errors and thereby having a truly representative sample in

the cuvette before the measurements were made. After the

sixth rinse the last transfer was made as quickly as possible

(approximately 10 s) and the cuvette was sealed with a Teflon

stopper. This sequence of operations was repeated for each

solution and cuvette combination until all five K 2'Cr 2 07

solutions were loaded into the high-accuracy spectrometer.

Three cycles of transmittance/absorbance measurements

were then made on each of the solutions at the eight wavelengths selected. Each cycle was separated in time by one

revolution of the rotating sample turn-table [7] . All

results were presented on a teletype printout in the form

illustrated in table 2. The measurements shown were made at

235 nm. A cuvette containing blank solvent was in position

2. The cuvettes containing the five acidic potassium dichromate solutions and having nominal concentrations of 20, 40,

60, 80 and 100 mg K 2 Cr 2 0 7 /kg were in positions 4, 6, 8, 10

15
Table 2. Teletype printout of transmittance/absorbance data

for a set of acidic K.2 L.r £ O7 solutions; X

1 .2001 12E 0 1 . 4423E-03

2 . 1 77946E C 1 . 29 3 2 E- G 3

3 .2G0142E 0 1 . 3373E-03 33.9 16 33 .943 .0 509

4 . 10021 5E 0 1 . 7656E-03

5 .2001 73 E C 1 . 50 61 E- 0 3 50. 067 50. 063 . 3005

6 • 567963E 00 . 6426E-03

7 .200142E 0 1 . 4 7 63 E- 0 3 23 . 375 23 • 370 . 547 1 3 . 3 1 73.6 1 E 00 . 1 1 70E-02

9 .2001 53E 0 1 . 4663E-03 1 5.33 1 1 5.3 79 . 7992

10 . 1 73 730E 00 . 1 354E-02

1 1 .200145E 0 1 . 43 93 E- 0 3 3 .932 3.9 31 1 . 049 1 12 .992660E- 0 1 .2317E-02

13 .200247E 0 1 . 50 60 E- 0 3 4.9 53 4.9 53 1 • 3047

14 . 55253 3E 00 . 109 3E-02

1 5 .200 13 5E 01 . 3479E-03 27. 599 27 • 59 4 . 5 59 2

1 .200142E 0 1 . 43C3E-03

2 . 177936E 0 1 . 40 63 E- 0 3

3 .2001 19E 0 1 . 49 57E-0 3 33.9 10 38 -9 36 . 0509

4 . 1001 63 E 0 1 . 63 41 E- 0 3

5 •200126E 0 1 . 5103E-03 50.053 50 . 0 43 • 300 6

6 . 563272E 00 . 59 7 3 E- 0 3

7 •200176E 01 • 49 1 7 E- 0 3 23 . 39 2 23 • 33 6 • 5469

3 . 31 7599E 00 . 1 235E-02

9 .2001 1 6E 01 . 3510 E- 0 3 1 5.363 15.366 • 79 9 5 10 . 1733 59E CO . 1 1 C2E-02

! 1 . 200073E 0 1 . 323 5 E- 0 3 3 .9 39 3 .9 37 1 . 0 43 3

12 .992523 E- 01 . 1 39 0 E- 0 2

13 . 200 1 43E 01 . 373 3E-03 4.960 4.9 59 1.30 46

14 . 5521 33 E 00 . 6704E-03

15 .200071

E

0 1 . 3046E-03 27. 592 27 . 53 6 • 559 3

1 .200059 E 01 . 37 1 5E-03

2 . 1 77765E 01 . 43 63E-03

3 . 199997E 0 1 . 69 1 1 E- 0 3 33.370 33.397 • 0511

4 . 1001 56E 0 1 . 71 49E-0 3

5 .200041 E 01 . 424 1 E-03 50.073 5 0 . 0 63 . 3004

6 . 567657E 00 . 5464E-03

7 .200050E 01 . 4 1 3 1 E- 0 3 23 . 376 23 . 37 1 • 547 1 8 . 317523E 00 • 3951E-03

9 . 199993E 0 1 . 3614^-03 1 5.3 74 15.3 72 • 799 4

10 . 173623E CO . 1 333E-02

1 1 • 200C49E 0 1 . 43 9 3E-0 3 3.9 30 3 .9 29 1 .0492

12 .99 19 1 3E- 0 1 . 1 74 1 E-02

13 •200025E 01 . 51C0E-03 4.9 59 4.9 53 1 . 3047

14 .552294E 00 . 1 1 26E-02

15 . 200 1 1 5E 01 • 4190E-03 27. 605 27. 599 . 559 1

RUN NO. 170 NO. CYCLES 3 START

POS AVERAGE CORP.. T AVERAGE

2 38 .925 •05097

4 50. 060 . 30051

6 23. 375 . 54705

3 15.372 . 79936

10 8 .932 1 . 04903

12 4.9 53 1 .30463

14 27. 593 . 559 19

LOC. 1 END

ABS3MC

LOC 1 5 NO. PTS. 50

NOTE: The printout contains the data from three sets of

transmittance measurements made on seven samples (I) meas16
ured against air (I ) . The data from the first column identify the positions at which the measurements are performed.

The odd numbers represent the eight I measurements which

bracket the seven I measurements (even numbers) . The second

column gives the average of the 50 individual measurement^

of the photocurrent made in 2.5 s, and corresponding to I and I. Column 3 contains the standard deviation of the 50

individual measurements averaged in column 2. The noncorrected transmittance is given in column 4, which is

deduced from:

l

2

1° + 1°

= T

The corrected transmittance is given in column 5 and the

corresponding calculated transmittance density (-logi 0 T)

is given in column 6. The corresponding averages from the

triplicate measurements are given at the bottom of the

printout. For further details consult reference 7.

and 12, respectively. A filter consisting of inconel-onquartz [8] was placed in position 14 for use as a reference.

In table 2, column 5 is the transmittance (T) relative

to air after correction for detector non-linearity; column 6

contains the corresponding corrected transmittance density

(D) calculated from the relation D = -logi 0 T. At the bottom

of table 2, the average values for corrected T and D are

tabulated for the three cycles of measurements. From the

data the apparent absorbance (A ) was calculated using the

relation A = D , -D, , n. Immediately upon completion

a sample blank / r v

of the first series of measurements for all eight wavelengths, the cuvettes containing the five sample solutions

were rinsed twice with fresh solution, refilled and measured

again. Invariably, since no systematic differences were

observed between the sets of duplicate runs, the values were

simply averaged and used for the subsequent calculation of

apparent specific absorbances

.

For the certification of SRM 935, six samples of each

of the five concentrations of acidic potassium dichromate

17
solutions were prepared and measured according to the procedure

just described. All transmittance/absorbance measurements

were made on a given set of solutions on the same day.

Approximately three weeks were required to complete the

measurements on the six sets.

5 . 6 Calculation of the Apparent Specific Absorbances of

SRM 935 Potassium Dichromate in 0.001N Perchloric Acid

Specific absorbances were calculated at the eight wavelengths of interest for each of the 30 potassium dichromate

solutions measured using the relation

D c

" D-d A

S B _ a

^

a ~T5 • c~~ B • c ^

'

where

e = apparent specific absorbance

a

Dg = transmittance density of sample solution

Dg = transmittance density of the solvent blank

b = internal light path of the cuvette in centimeters

c = weight concentration of potassium dichromate solution in g kg

A

a

= apparent absorbance.

Before calculation of e , the apparent absorbances, A , (A = D - Dg) were corrected for a small systematic error

arising from the fact that the light beam in the IMR highaccuracy spectrometer is not collimated but instead is

slightly convergent at the sample position. This correction

has been mentioned by Mielenz and Mavrodineanu [14] . The

error in absorbance, AA/A, for normal incidence at the

cuvette for this instrument was shown to be 6.3 x 10 h

. All

values of A were subsequently corrected by multiplying by

the factor 0.99937.

The exact values used for the internal light paths of

the five sample cuvettes were the averages of the values

18
given for positions seven through ten in table 2 of reference

12, a copy of which is included in the Appendix. These

averages were the values used for b in Eq. (3).

As discussed previously, no corrections were made to

the measured transmittances or absorbances for internal

multiple reflections since this effect is present to the

same extent, within the uncertainty of the measurements, in

both the certification and the intended use of this SRM.

Internal multiple reflections are not instrument dependent

and therefore corrections for their effect are not necessary

in order to certify and use this SRM as a transfer standard.

On the other hand, if this material were being certified for

specific molar absorbance, appropriate corrections for

internal multiple reflections should be applied. Burnett

[15] has discussed recently the magnitude of the internal

multiple reflection correction as a function of wavelength

and absorbance level. The reader should refer to this paper

before attempting to make and report specific molar absorbance

data. Potential corrections for reflections from other

surfaces such as lenses and slit jaws [14] were disregarded

also because they were believed adequately eliminated by use

of the solvent blank.

Values of the apparent specific absorbances of SRM 935

potassium dichromate were calculated using Eq. (3) for the

eight wavelengths chosen and for the 30 K 2 Cr 2 07 solutions

measured. The only correction applied to the experimental

data was the one for convergent beam geometry. The apparent

specific absorbances calculated are summarized in Tables 3

and 4

.

19
Table 3. Apparent specific absorbances, e , calculated from

transmittance/absorbance measurements on five concentration levels of SRM 935 potassium dichromate

in 0.001N perchloric acid at wavelengths corresponding to absorbance maxima or minima; T = 23.5 °C.

K 2 Cr 2 0 7 Cone . , mg kg 1

20. 346

20.112

20. 269

20. 112

20.136

20. 107

Av.

S.D.

20.180

------- e a

, kg-g ^cm i ------ -

255(1. 2)

a

nm 257(0. 8)nm 313(0. 8)nm 550(0 .8)nm

Min

12. 254

12. 244

12. 264

12 .279

12. 264

12. 275

12. 265

0.015

Max.

14.249

14. 252

14. 268

14. 278

14. 264

14. 267

14. 265

0.011

4

4

4

4

4

4

Min

801

796

80'2

811

806

805

4. 805

0.005

Max

10.668

10.667

10.676

10.681

10.674

10.676

10.674

0.005

Av.

S.D

40.612

40. 292

40. 500

40. 555

40. 288

40.554

40. 565

12. 295

12. 502

12. 505

12. 501

12. 504

12. 507

12.502

0.004

14. 514

14. 520

14.525

14.511

14. 520

14. 515

14.517

0. 005

4.807

4.816

4.815

4. 815

4. 815

4.814

4. 814

0.005

10.674

10.685

10.684

10.682

10.685

10.677

10.681

0.005

Av.

S.D.

60. 561

60.458

60.426

60. 554

60. 558

60. 585

60.417

12. 545

12. 542

12. 548

12. 545

12. 549

12.551

12. 546

0. 004

14. 568

14.570

14. 579

14.572

14.575

14.568

14 . 571

0.004

4.812

4.816

4.817

4.816

4.815

4.816

4.815

0.002

10.685

10.690

10.690

10.688

10.690

10.687

10.688

0.002

20
Table 3 continued.

K 2 Cr 2 0 7 Cone . , mg kg 1

80. 534

81.119

80. 528

80. 999

80.442

80.383

kg'g cm

80. 668

522(1. 2) nm 257(0. 8)nm 313(0. 8)nm 350(0. 8)nm

Min.

12. 390

12. 392

12.397

12.396

12.392

12. 594

12. 394

0.005

Max

.

14.459

14.450

14.450

14.459

14.448

14.442

14.448

0.007

Min.

4.820

4.824

4.823

4.824

4.821

4.821

4 .822

0. 002

Max

.

10.702

10.706

10.705

10. 708

10.704

10.701

10.704

0.003

100.722

100.845

100. 772

101.081

100.458

100.420

100. 715

12.450

12.454

12.458

12. 454

12.457

12.458

12.455

0. 005

14.482

14.488

14.491

14.481

14.486

14.477

14.484

0. 005

4.827

4.825

4. 828

4.826

4.827

4.8 22

4.826

0.002

10.714

10.714

10.711

10.710

10.711

10. 710

10.712

0 .002

Spectral bandpass.

Standard deviation of a single determination.

21
Table 4. Apparent specific absorbances, e , calculated from

transmittance/absorbance measurements on five concentration levels of SRM 935 potassium dichromate

in 0.001N perchloric acid at wavelengths near the

two isosbestic points; T = 23.5 °C.

K 2 Cr 2 0 7 Cone . , mg kg 1 322(0. 8) nm 3 2 3 ( 0 . 8 ) nm 345(0. 8)nm 346(0. 8)n

20. 346 5.855 6.070 10.606 10.621

20.112 5.849 6.068 10.598 10.623

20.269 5.860 6. 078 10.607 10.623

20.112 5 . 849 6.070 10.611 10 . 638

20.136 5.843 6 . 068 10.609 10.630

20.107 5.868 6. 091 10.616 10.629

Av . 20.180 5.854 6.074 10.608 10.627

S.D. b

0. 009 0. 009 0. 006 0. 006

40.612 5.845 6. 071 10. 597 10.621

40.292 5. 858 6.074 10. 602 10.631

40.300 5 .859 6.071 10.601 10.629

40 . 333 5.843 6 . 061 10 . 599 10 . 629 ! 40.288 5.841 6.064 10. 596 10.624

40.354 5.864 6. 081 10.604 10.627

Av. 40.363 5.852 6.070 10. 600 10.627

S.D. 0. 010 0.007 0.003 0.004

1

60. 561 5.844 6.067 10. 597 10.628

60.458 5.846 6.064 10. 597 10.630

60.426 5.851 6. 063 10. 597 10.631

60.334 5.836 6.057 10. 593 10.628

60.338 5.828 6.053 10. 594 10.630

60.385 5.854 6. 074 10. 597 10 . 628 i Av. 60.417 5. 843 6.063 10. 596 10. 629 1 S.D. 0. 010 0.007 0. 002 0. 001

Av

22
Table 4 continued. ------- e a

, kg-g'^cm" 1 -------

K 2 Cr 2 0 7 Cone. , mg kg" 1 5 22(0. 8)

a

nm 323(0. 8)nm 345(0. 8)nm 546(0. 8)nm

80.534 5 .

rt 7 n 859 6

.

062 10

.

600 10.650

81.119 5. 845 6. 062 10. 602 10. 657

80.528 5. 845 6. 062 10. 602 10.655

80 . 999 5 . 833 6

.

055 10 . 602 10 . 656

80.442 5

.

824 6 . 050 10 . 601 10 . 651

80 . 385 5

.

850 6

.

071 10 . 606 10 . 654

A - - Av

.

80 . 668 5 . n 7 a 839 6 . 060 10

.

602 10.654

S . D

.

0

.

009 0 . 007 0

.

002 0 . 005

100 .722 5 . 836 6

.

059 10

.

602 10.655

100.845 5. 855 6. 056 10. 605 10.658

100. 772 5. 858 6. 054 10. 605 10.656

101.081 5. 824 6. 046 10. 598 10.654

100.458 5. 818 6. 045 10. 602 10.655

100.420 5. 842 6. 065 10. 605 10.655

Av. 100. 713 5. 852 6. 054 10. 602 10. 656

S.D. 0. 009 0. 008 0. 002 0.001

a Spectral bandpass.

Standard deviation of a single determination.

25
5 . 7 Statistical Treatment of Experimental Apparent Specific

Absorbances

The increase in the apparent specific absorbances of

acidic potassium dichromate solutions with increasing concentration is clearly evident in the data shown in table 3.

As discussed previously, this effect is due primarily to the

concentration dependence of the HCrOi+ /Cr 2 07 equilibrium.

These data were subsequently analyzed and evaluated by

H. H. Ku of the NBS Statistical Engineering Section. The

average apparent specific absorbances obtained for the five

concentration levels were fitted for each of the four wavelengths as a quadratic function and as a linear function of

the average concentrations. The resulting residual standard

deviations showed that the data fitted a linear function

equally well as a quadratic function. While not expected,

subsequent calculation of HCr0 4 /Cr 2 0 7 ~ speciation using the

dimerization equilibrium expression = [Cr207 j/CHCrCK ] 2

explained the reason for this observation. The relative

amounts of HCrOi+ and Cr 2 0 7 ~ present for the 20-100 mg kg 1

range of K 2 Cr 2 07 concentrations employed are given in table 5

The value used for [32.9 (mol kg l ) *] in this calculation

was taken from an earlier publication [10] . -The speciation

data in table 5 clearly show that the percentage of chromium

present in the dimer form (Cr 2 07 ) is very nearly a linear

function of the K 2 Cr 2 07 concentration over the five-fold

concentration range used and hence explains the equal success

of the linear fit.

24
Table 5. Composition of chromium (VI) solutions

in 0.001N perchloric acid at 25 °C.

K 2 Cr 2 0 7 Percent Cr as Percent Cr as

Cone, mg kg" 1 HCrO*" Cr 2 0 7 =

20 99.10 0.90

40 98.22 1.78

60 97.38 2.62

80 96.56 3.44

100 95.77 4.23

• The results obtained from a linear fit of the apparent

specific absorbances are shown in table 6. The uncertainties

given were derived from estimates of all known systematic

errors and the 95 percent confidence interval for the mean.

A summary of the experimental conditions used in establishing

the certified values of the apparent specific absorbances of

SRM 935 and their uncertainties is given in table 7.

A similar statistical treatment of the average apparent

specific absorbances obtained for the four wavelengths near

the two isosbestic points (table 4) yielded the values given

in table 8. These values are not certified because of their

stronger dependence on wavelength accuracy and temperature

control. They are given here to demonstrate that there are

two isosbestic wavelengths in the weakly acidic potassium

dichromate system which could be useful for verifying

absorbance linearity.

25
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26
Table 7. Experimental conditions used in establishing the

apparent specific absorbances, e , of SRM 935.

Sample and Instrumental Parameters

Weighing of K 2 Cr 2 0 7

Weighing of K2Cr 2 07 solutions

Temperature

Curvette pathlength

Wavelength accuracy

Spectral bandpass

Stray radiation

Beam geometry

Transmittance accuracy

Conditions

Nominal 20 to 100 mg,

±0.004 mg

Nominal 1000 g, ±0.020 g

23.5 ± 0.2 °C

Nominal 10 mm, ±0.0005 mm

±0.05 nm

For X = 23 5 nm, 1.2 nm;

for all other wavelengths,

0.8 nm

Less than 5 in 10 5 at

600 nm

Convergent, f:10, corrected

to normal incidence

1 part in 10 4

Table 8. Recommended values of the apparent specific

absorbances, e , of SRM 935 potassium dichromate in

0.001N perchloric acid at four wavelengths approximating the two isosbestic points; T = 23.5 °C.

K 2 Cr 2 0 7 Cone . , mg kg 1

kg'g 1

• cm 1

522 (0. 8) nm 323 (0. 8)nm 545(0. 8)nm 346(0. 8)nm

20 u 5. 854 6 . 074 10.604 10.626

40 5 . 849 6 . 069 10.6 03 10.628

60 5. 844 6.064 10.602 10 .631

80 5 . 838 6. 059 10.601 10.633

100 5.833 6.054 10.600 10.636

k Wavelength and, ( ) , spectral bandpass

Nominal concentration.

27
6. OTHER EXPERIMENTAL PARAMETERS AFFECTING THE APPARENT

SPECIFIC ABSORBANCES OF SRM 935

6 . 1 Wavelength Accuracy

Except near the maxima and minima, the absorbances of

acidic potassium dichromate solutions- are strongly dependent

on wavelength. Therefore, the accuracy of wavelength scale

must be established before SRM 935 can be used as a standard

to verify the accuracy of the absorbance scale.

In general, the wavelength accuracy of a spectrometer

can be verified by the use of (a) emission sources capable

of producing discrete radiations of suitable intensity and

adequately spaced throughout the spectral range of interest

and (b) solutions or glasses containing rare-earth elements.

Since SRM 935 is being certified for use in the ultraviolet,

the most useful and readily available source for checking

^ wavelength accuracy in this region is the quartz low pressure

mercury discharge tube. To calibrate the wavelength scale,

this tube should be placed immediately in front of the

entrance slit of the monochromator . Adequate eye protection

against ultraviolet radiation should be worn while this

discharge tube is being used.

A detailed description of the characteristic radiations

and the use of low pressure discharge tubes for wavelength

calibration is given in NBS Letter Circular LC-1017; "Standards for checking the Calibration of Spectrophotometers" by

K. S. Gibson, H. J. Keegan, and J. C. Schleter, re-issued in

January 1967. The data on the quartz low pressure mercury

discharge, which are reproduced in table 9, were taken from

this report.

28
Table 9. Characteristic radiations emitted by a quartz, low

pressure, mercury discharge tube.

Wave- Wave- Wave- Wavelength Note length Note length Note length Note

(nm) (nm) (nm) (nm)

205. 29

222.47

230. 21

232.32

235. 25

237.83

239.94)

239.973

244.69

246.41

248 . 20)

248. 27

248.38)

253 . 48 1 X 296. 73 398 . 40

253. 65 3 302. 15

257 . 63 302 . 35 404. 66

260. 32 302. 56 r 6 407. 78

265. 2 0

1 302 . 75 435. 83

265 . 37 4 312. 57 491. 60

265 . 51

J

312. 15 546. 07

269. 95 313. 181

7

576. 96

275 . 28 334. 1 5

J 579. 07

275. 97 623 . 44

280. 35]

5

365. or 671. 64

280 . 45i 365. 48

1

8

'690. 75

284. 78 366. 29

1

1014. 0

289 . 36 366. 33; 1128 . 7

292. 54 390. 64

NOTES:

1. A value of 239.95 is recommended for the unresolved pair.

2. A value of 248.3 is recommended when the three lines are

unresolved

.

3. The intensity of 253.48 is negligible compared to that

of 253.65. The latter value should be used when the

lines are unresolved.

4. The 265.20 line is somewhat stronger than the others and

a value of 265.3 is recommended when the three lines are

unresolved

.

5. These two lines are of approximately the same intensity

and a value of 280.40 is recommended for the unresolved

pair

.

6. The two shorter lines are considerably stronger than the

other two. It is probable that a value of 302.25 should

be used for the unresolved lines.

7. A value of 313.16 is recommended for the unresolved pair

8. With the arc used on the Beckman DU spectrophotometer

the ratio of intensities for 365.01 : 365.48 : 366.29

is 100 : 48 : 36, approximately. The intensity of the

366.29 line appears negligible relative to that of 366.33

9. These two lines are of approximately the same intensity

and a value of 578.0 is recommended for the unresolved

pair

.

29
If additional information is needed, the user is referred

to the NBS Letter Circular LC-1017 mentioned above or to the

Manual on Recommended Practices in Spectrophotometry published

by the American Society for Testing Materials, 1916 Race

Street, Philadelphia, PA 19103. The wavelength scale

should be accurate to ±0.2 nm in order for the SRM to be

used properly.

The accuracy of the wavelength scale of the high-accuracy

spectrometer used in this work was verified with a low pressure mercury discharge lamp using a slit of 0.1 mm which is

equivalent to an effective spectral bandpass of 0.08 nm. The

deviation of the wavelength scale from the true value in the

range from 230 nm to 400 nm was found to be +0.05 nm and

hence this value was applied to correct the scale.

6 . 2 Spectral Bandpass

Failure to select an appropriate spectral bandpass can

lead to inaccurate results when optically non-neutral materials are used as transmittance or absorbance standards. In

general, the degree of inaccuracy will depend on how sharp

the absorbance peaks are. The use of too large a spectral

bandpass will result in underestimating the absorbance at a

maximum and overestimating the absorbance at a minimum.

As pointed out in the introduction of this publication,

the use of acidic potassium dichromate as an absorbance

standard is restricted to the verification of the transmittance/absorbance scale of spectrometers capable of providing

spectral bandpasses of the magnitude indicated in table 10.

In a previous publication [10] a somewhat simplified

mathematical treatment was presented on the effect of spectral

bandpass on the measurement of absorbance at the wavelength

maximum of a Gaussian peak. This study showed that the

ratio of the spectral bandpass of the spectrometer to the

natural bandwidth of the sample at half intensity of the

fully resolved peak should not exceed 0.05 in order that the

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31
measured absorbance be at least 0.999 of the true absorbance.

j A similar treatment of absorbance minima is not as straight

forward because these result from two overlapping peaks.

Based on these considerations and the data presented in

table 10, the following restrictions are placed on spectral

bandpass when SRM 935 is used as an absorbance standard: at

A = 23 5 nm - 1.2 nm; at X = 25 7 nm - 1.4 nm; at A = 313 nm; and

at X=350 nm - 2.0 nm. For certification, a spectral bandpass

of 1.2 nm was used at 235 nm, and 0.8 nm for the remaining

wavelengths

6 . 3 Stray Radiation

Stray radiation is defined as any radiation outside of

the nominal spectral region isolated by the monochromator

that reaches the detector. It is produced by scatter from

the optics and walls of the monochromator and is present in

varying amounts in all spectrometers. Although extensive

literature [16,17,18] exists on the detection and measurement of stray radiation, no Standard Reference Materials are

presently available for this purpose.

The spectrometrist must be aware of the potential

presence of significant stray radiaton errors whenever 1

absorbance measurements are made in a wavelength region

where the spectrometer components are extended to their

performance limit. These regions correspond to wavelengths

where: (a) the source intensity is low, (b) the detector

response in poor, and (c) the transmission of the monochromator is low. A detailed discussion of this instrumental

parameter is given in ASTM Standard Method E 387-72. The

procedures recommended in this work should be used to

determine this parameter in the spectral range of interest.

Measurements performed on the high- accuracy spectrometer

have indicated that the stray radiation in this instrument

is less than five parts in 10 5 at 600 nm.
NOTE : The user is cautioned that SRM 955 will provide

meaningful data only when the three parameters discussed

above—wavelength accuracy, adequate spectral bandpass and

absence of stray radiation—are assured.

6 . 4 Beam Geometry

Since most commercial spectrophotometers use lens

optics, the radiation impinging upon the sample usually is

not collimated. This source of systematic error has been

considered previously [10] . For angles of incidence up to

five degrees, the absorbance correction is less than 0.1

percent. Insofar as the use of SRM 935 is concerned, this

correction can be combined with the transmittance/absorbance

scale correction.

6 . 5 Temperature

A change in temperature normally produces a change in

the absorbance of an absorbing liquid. The sign and magnitude

of the temperature coefficient are determined by the combined

contributions of three effects. These are: (a) a change in

concentration resulting from the thermal expansion or contraction of the solvent, (b) a change in ionization, protonation, or hydration of the absorbing species, and (c) a red

or blue spectral shift resulting from either the addition or

removal of thermal energy to the absorbing species. Generally,

effects (a) and (c) are predominant and the sign of the

temperature coefficient is negative, i.e., the absorbance

decreases with increasing temperature. However, as seen

previously [10] , there are also systems that have positive

temperature coefficients.

The variation of the absorbance of acidic potassium

dichromate solution with temperature is the smallest that we

have observed for any liquid filter that we have studied.

For the four wavelengths selected for verifying the accuracy

of the transmittance/absorbance scale — 235, 257, 313, and

33
350 nm — the specific absorbances decrease with increasing

temperature. Over the range 20-30 °C, the temperature

coefficients, k, are, respectively, -0.05, -0.05, -0.02, and

-0.05 percent per °C. The temperature coefficients near the

two isosbestic wavelengths are slightly larger, -0.06 and

-0.08 percent per °C, respectively.

The apparent specific absorbances given in table 6 are

certified at a temperature of 23.5 °C. However, SRM 935 can

be used as an absorbance standard at any temperature in the

range 20-30 °C provided corrections are made to the apparent

specific absorbance data. The apparent specific absorbance

at any temperature in this range can be calculated from the

certified values and the appropriate temperature coefficient

from the relation

e I

= I 1 + IOT (t " 23 - 5) l W

apparent specific absorbance at temperature t (°C

specific absorbance certified at 23.5 °C

temperature coefficient, percent/°C.

where

t

£

a

23.5

a

For example, the apparent specific absorbance of a 20 mg kg 1

solution of SRM 935 in 0.001N perchloric acid at 30 °C and

a wavelength of 350 nm can be calculated as follows: From

table 6, the certified apparent specific absorbance for this

concentration and wavelength is 10.672 kg»g 1 • cm t. Substituting this value into Eq. (4), the specific absorbance

at 30 °C is:

el° = 10 . 672 [1 - ^| (30 - 23.5)]

= 10.672 [1 - 0.0033]

= 10.637 kg g" 1 cm" 1 a

34
6 . 6 Acidity

Acidic potassium dichromate solutions traditionally

used in collaborative tests have been prepared in 0.01N

sulfuric acid [1,2,3,4,5]. A number of studies, however,

have confirmed the tendency of sulfate to form mixed complexes with chromium (VI) ions in acidic media [10,19,20,21].

Rather than risk making the HCrOi* ~/Cr 2 0 7 = system even more

complex, the decision was made in an earlier study [10] that

acidification with perchloric acid was preferable to acidification with sulfuric acid. For similar reasons it was

decided that an acidity somewhat less than 0.01N was desirable in order to prevent partial formation of H 2 Cr0it from

protonation of HCr0 4 • At the same time, the acidity chosen

could not be too low because HCrCK would partially ionize

to give CrOit , a chromium (VI) species which has quite different absorption characteristics. As a practical compromise

for minimizing the protonation and ionization processes, an

acidity of 0.001N (pH » 3) was finally selected. At this

acidity, less than 0.1 percent of the total chromium in the

absorbing system is present in either the H 2 CrOit or CrO^

form. Close control of acidity at the 0.001N level is not

critical in the use of SRM 935 as an absorbance standard.

The data in tables 6 and 8 are equally valid for perchloric

acid solutions whose pH is in the range 2.5 to 3.5.

6 . 7 Water Purity

Since SRM 935 will be issued in solid form, attention

must be given by the user to the quality of the distilled

water used to prepare the standard solutions. Since acidic

potassium dichromate is a potential oxidant, the water must

be checked and shown to be free of reducing impurities in

order that the SRM provide accurate information. A simple

but definitive test is illustrated in figure 4. In this

test a 500-fold dilution of a 100 mg kg 1 solution of

35
O LU

co E

0.0025

0 0020

gf, 0.0015

<

500—FOLD DILUTION OF 100 mg K

2 Cr2 0

? / kg

2 4 6 8 10 12 14 ' ' 30

DAYS AFTER PREPARATION

60

Figure 4. Test of the distilled water for reducing

impurities

K 2 Cr 2 07 (A 3 1.0) was made with the distilled water to be

tested and its absorbance measured at 350 nm. The measured

absorbance (A = 0.002) agreed with the expected value, thus

indicating that no reduction of chromium (VI) occurred.

Subsequent measurements showed that this solution was stable

for at least two months. Hence, any solution standards

prepared with this water should be stable for the same

period provided they are adequately protected against evaporation losses and exposure to light.

All of the measurements described in this publication

were made using distilled water provided to the Analytical

Chemistry Division from a central supply. The still, condenser system, storage tanks and the copper pipes supplying

this distilled water to the individual laboratories are all

tin-lined. Although this water is not free of organic compounds, the test described above shows that these compounds

do not reduce chromium (VI) under the acidity conditions

used. In this regard, it is interesting to note that the

oxidation potential of potassium dichromate is strongly acid

dependent. By preparing the K 2 Cr 2 0 7 solutions in 0.001N

rather than 0.01N perchloric, the oxidation potential of

36
the chromium (IV) system is lowered by about 13 percent.

Whenever the quality of the distilled water is suspect, it

should be redistilled from an alkaline potassium permanganate

solution in order to preoxidize any reducing impurities.

7. INSTRUCTIONS FOR USING SRM 935 AS AN

ABSORBANCE STANDARD

The procedure recommended for using SRM 935 to verify

the accuracy of the absorbance scales of narrow bandpass

absorption spectrometers is identical to the one used in

the certification of this Standard Reference Material. All

the operations involved in this certification were described

in detail in Section 5 of this publication. Once the solutions of SRM 935 have been prepared accurately, their

absorbances are measured on the spectrometer to be verified

using the instrumental conditions and restrictions described

in Sections 5 and 6. Strict attention must be given to

wavelength accuracy, adequate spectral bandpass, absence of

stray light, and temperature.

The certification of SRM 935 for apparent specific

absorbances provides considerable flexibility in the use of

this material for verifying the accuracy of the absorbance

scale. The user for example, is not required to prepare

specified concentrations of K 2 Cr 2 07 solution. The only

requirement is that the concentration of every solution for

checking the scale be known accurately. Although the

certified values given in table 6 are for nominal K 2 Cr 2 0 7 concentrations of 20, 40, 60, 80 and 100 mg kg

-1

the appropriate specific absorbances for other concentrations can be

obtained by linear interpolation.

The derivation of e

& by linear interpolation and its

subsequent use to calculate an accurate value of A

a

are

37
described below, assuming the following experimental conditions :

A = 350.0 ± 0.2 nm; spectral bandpass < 0.8 nm

b = 0.9982 cm

c = 0.04375 g kg" 1

t = 23.5 °C

From table 6, the e for X = 350 nm and concentrations of

a

0.040 and 0.060 g kg" 1 are 10.682 and 10.692, respectively.

The e o for c = 0.04375 g kg" 1 is:

e a10 - 682 + °:o

4

6o

75

: 0:°!°

(

10 - 692 - 10 - 682 >

e o - 10.682 + 0.0019 (6)

e = 10.684

a

The corresponding apparent absorbance A is given by:

cl

i A

a

= e a

* b ' c

A = 10.684 x 0.9982 x 0.04375

a

A = 0.4666

a

The uncertainty AA in the calculated A is determined from

3. the combined uncertainties in e , b, and c in equation 7,

a

provided no other systematic errors are present. Thus, using

the propagation of error formulas given by Ku [22]

AA = bclAe I + e c I Ab I + e b I Ac I (8) a 'a' a 11 a

1

.

1 v J

To evaluate AA , the appropriate value of Ae is taken from

a a

table 6 and the Ab and Ac values must be provided by the

user. For the example taken, solution of equation 8 for

the Ae term gives

AA = 1 x 0.044(0.020) + e c|Ab| + e b|Ac| (9)

a a , a

38
AA = ±{0.00088 + e clAbl + e b I Ac I

>

a a

1 1 a

1 1 (10)

In the experiments performed to obtain the certified e &

values in table 6, our uncertainties for b and c did not

exceed 1 part in 10 4 and 2 parts in 10**, respectively.

Under these conditions, the combined contribution of Ab and

Ac to AA does not exceed 0.0002 and AA = ±0.0010.

a a

In practice, the user may not know b and c as accurately

as in the above example. Solution of equation (10) for the

case in which b and c are known to 1 part in 10 3 gives

0.00088 + 10.7 (0.044) (0.001) + 10.7 (1)(4.4 x 10" 5 )

0.00088 + 0.00047 + 0.00047

±0. 0018

The calculations of e . A and AA for other concentrations,

a ' a a

' wavelengths, and uncertainties in b and c are performed in

a similar fashion.

The use of SRM 935 to verify the accuracy of the

absorbance scale of a precision transmission spectrometer is

illustrated in figure 5. These measurements were made using

a series of five K 2 Cr 2 07 solutions having nominal concentrations of 20, 40, 60, 80 and 100 mg kg In each instance

the measured absorbance is higher than the certified values

and should be corrected by subtracting the appropriate value

of A

a

(Meas. ) - A (Calc. )

.

AA = a

AA = a

AA = a

39
ft 0.2 0.4 0.6 0.8 1.0

Absorbance

Figure 5. Correction curve for the absorbance scale of a

precision spectrometer. The error bars are the

sum of the errors arising from the uncertainties

in the certified apparent specific absorbances

e , cuvette pathlength b, and concentration c.

Although solutions of this SRM should be prepared

strictly on a weight/weight basis, there are undoubtedly

users who will desire to prepare and express concentrations

on a weight/volume basis. While this approach is not recommended, the certified apparent specific absorbances in

table 6 can be recalculated for the case in which the

K 2 Cr 2 07 concentration is expressed in g«L 1

. This recalculation simply requires dividing the certified values by the

appropriate density given in table 11. The uncertainties

and temperature coefficients remain unchanged. Anyone

wishing to use this approach should calibrate the volumetric

flasks used. NBS recommendations for performing this calibration are given in two papers included in the Appendix.

40
Table 11. Densities of 0.001N perchloric acid solutions of

potassium dichromate at 23.5 °C.

K 2 Cr 2 0 7 Cone, mg'kg 1 Density, kg»L 1

20 0.99746

40 0.99747

60 0.99749

80 0.99750

100 0.99752

8. USE OF SRM 935 TO VERIFY ABSORBANCE LINEARITY

The apparent specific absorbances of 0.001N perchloric

acid solutions of SRM 935 at wavelengths near the two

isosbestic points in the HCrOi* /Cr 2 0 7 spectra (figure 1)

have been presented in table 7. Inspection of these data

show that, of the four wavelengths selected, the calculated

values for 345 nm are, within an uncertainty of 0.1 percent,

independent of concentration over the range of 20-100 mg*kg 1

K 2 Cr 2 0 7 . This relation was not expected since none of the

data in table 7 have been corrected for internal multiple

reflection effects. The constancy of the uncorrected

specific absorbances at 345 nm therefore must result from

the presence of compensating effects.

The most plausible explanation for the observed invariance in the apparent specific absorbance data at 345 nm is

that the internal reflection corrections, which are proportionally larger at lower absorbances [15] , are almost exactly

offset by the effect of the variable HCrOi+ /Cr 2 0 7 ~ speciation

at the higher absorbances. Following this reasoning, the

Cr 2 0 7 ion must have a slightly larger specific molar

absorbance at 345 nm than the HCrOi* ion. Consequently the

true isosbestic point is at a slightly shorter wavelength

41
than those used for measurement. A similar consideration

of the data reported for 322 and 323 nm indicate that this

isosbestic point is also slightly below the wavelengths

chosen.

For the purpose at hand the important fact remains that

the apparent specific absorbances of 0.001N perchloric acid

solutions of K 2 Cr 2 0 7 at 345 nm are independent of concentration at the one part in a thousand level over the range

20-100 mg*kg 2

. Consequently, if the absorbance scale of a

narrow bandpass (<_2 nm) spectrometer is linear, the apparent

absorbances of a series of K 2 Cr 2 07 solutions at this wavelength should be a linear function of concentration. Such a

test may be particularly useful to the clinical chemists who

make a proportionally large number of measurements in the

wavelength range of 340-350 nm.

In summary, an outline of the step-by-step operations

required to use SRM 935 for verification of a spectrometer

is given below. — Weigh 20, 40, 60, 80 and 100 mg of SRM 935 to ±0.02 mg

and transfer quantitatively to five 1-liter borosilicate

volumetric flasks that have been preweighed, with their

individual Teflon stopper, to ±0.01 g. — Dissolve each sample in 200 to 300 mL of distilled

water that is free of reducing impurities, add 1 mL of

IN perchloric acid, and dilute approximately to

volume with distilled water. — Stopper, mix, and weigh each flask and contents to

±0.01 g. — Prepare a blank solution by diluting 1 mL of IN

perchloric acid to 1 liter with the same distilled

water

42
Place a clean, calibrated, fused-silica (non-fluorescent)

cuvette in the temperature-controlled sample compartment

of the spectrometer to be verified. The temperature

should be controlled to ±0.2 °C and in the range of 20

to 30 °C.

Using a borosilicate , Pasteur-type pipette, rinse the

cuvette with six volumes of the blank solution.

Transfer a seventh volume and close the cuvette with a

Teflon stopper.

After 10 minutes measure the apparent absorbance of the

blank solution against air at wavelengths 235, 257, 313

and 350 nm, using the spectral bandpasses given in

table 6. For verification of linearity, measure the

apparent absorbance also at 345 nm. The data obtained

represent the values of Dg in equation (3) on page 18.

In a similar manner perform apparent absorbance measurements on the five solutions of SRM 935 potassium

dichromate. For each concentration, rinse the cuvette

a minimum of six times before beginning measurements.

The data represent the values of Dg in equation (3) on

page 18.

Use the values of Dg and thus obtained to calculate

the measured A of each of the five concentrations.

a

Compare the experimental values of A

&

for these five

concentrations with the accurate A calculated from

a

the certified values of apparent specific absorbances,

e , given in tables 6 and 7 of this publication and

comprising table 1 of the certificate (a copy of the

certificate is included in the Appendix)

.

43
— The differences between the calculated and measured A

a

values are used to correct the absorbance scale of the

spectrometer as illustrated in figure 5. The correction

curves for the four wavelengths at which e is certified ° a

should be similar.

It must be added, however, that the absorbance linearity test described above should not be associated with the

accuracy of the transmittance/absorbance scale of the

spectrometer being tested. Detailed discussions of important

distinctions between ascertaining transmission accuracy and

linearity will be found in references 23 and 24.

9. SUMMARIZING REMARKS

Potassium dichromate is the first SRM to be issued by

the NBS Office of Standard Reference Materials for use as an

"ultraviolet absorbance standard. The fact that this compound has been widely used for many years in the collaborative testing of spectrometers suggests that it should play a

more significant role in the future since, for the first

time, the absorbance measurements have been performed with

an instrument of proven accuracy. The certification of the

apparent specific absorbances of solutions prepared from

this SRM over a range of concentrations should give the

analyst the flexibility necessary to verify the accuracy of

any portion of the absorbance scale that may be desired. In

addition the capability of using the isosbestic data as a

verification of absorbance linearity suggests a number of

other useful applications.

In conclusion, it is believed that SRM 935, when used

in conjunction with a calibrated quartz cuvette (SRM 932)

,

will provide a valid means to verify the accuracy of the

absorbance scale of narrow bandpass spectrophotometers in

the ultraviolet over the wavelength range of 235-350 nm.

44
10. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the assistance of

the following members of the NBS staff:

• K. D. Mielenz, Research Physicist, Analytical Chemistry Division, for many helpful discussions of fundamental

spectrophotometric parameters and for critically reading

the manuscript.

• G. Marinenko, Research Chemist, Analytical Chemistry

Divison, for performing the coulometric assays.

• J. A. Norris, Research Chemist, Analytical Chemistry

Divison, for performing the emission spectrographic analyses

• H. H. Ku, Senior Statistician, Statistical Engineering Section, for the evaluation and statistical treatment

of the experimental data.

• R. W. Seward, Technical Representative, Office of

Standard Reference Materials, for obtaining and providing

the lot of potassium dichromate used for certification.

• I. L. Barnes and J. R. DeVoe, Section Chiefs,

Analytical Chemistry Division, for their kind support and

encouragement throughout this work.

The authors also wish to acknowledge the cooperation

and early financial support of the National Institute of

General Medical Sciences in the research leading to this SRM

The enthusiastic encouragement of George N. Bowers, Jr.,

M.D., Hartford Hospital, Hartford, Connecticut and

Royden N. Rand, Ph.D., Eastman Kodak Co., Research Laboratories, Rochester, New York is likewise gratefully acknowledged .

4 5
11. REFERENCES

[1] Gridgemen, N. T. , Photoelectric Spectrometry Group

Bull. 4, 67-79 (1951) .

[2] Photoelectric Spectrometry Group Bull. 1_6, 441-457

(1965)

[3] Ketelaar, J. A. A., Fahrenfort, J., Hass, C., and

Brinkman, G. A., Photoelectric Spectrometry Group Bull.

8, 176-179 (1955).

[4] Vandenbelt, J. M. , and Spurlock, C. H. , J. Opt. Soc.

Am. 4_5, 967-970 (1955).

[5] Vandenbelt, J. M. , J. Opt. Soc. Am. 5_0 , 24-27 (1960).

[6] Mavrodineanu, R. in NBS Technical Note 544 , 6-16 (July

1969-June 1970; and 584, 2-21 (July 1970-June 1971).

[7] Mavrodineanu, R. , J. Res. Nat. Bur. Stand. (U.S.) 76A

(Phys. and Chem.), No. 5, 405-425 (Sept. -Oct. 1972).

[8] Mavrodineanu, R. , J. Res. Nat. Bur. Stand. (U.S.) 80A

(Phys. and Chem.), No. 4, 637-641 (July-Aug. 1976).

[9] Burke, R. W. , and Mavrodineanu, R. , J. Res. Nat. Bur.

Stand. (U.S.) 80A (Phys. and Chem.), No. 4, 631-636

(July-Aug. 1976), included in the Appendix.

[10] Burke, R. W. , Deardorff, E. R. , and Menis, 0., J. Res.

Nat. Bur. Stand. (U.S.) 76A (Phys. and Chem.), No. 5,

469-482 (Sept. -Oct. 1972), included in the Appendix.

See also NBS Technical Note 544-, 17-34 (July 1967-June

1970; and 584, 21-45 (July 1970-June 1971).

[11] Yoshimori, T. and Sakaguchi, N. , Talanta 22_, 233-238

(1975)

[12] Mavrodineanu, R. and Lazar, J. W. , Nat. Bur. Stand.

(U.S.), Spec. Publ. 260-32, 26 pages (Dec. 1973), and

in Clin. Chem. 1_9 , 1053-1057 (1973), included in the

Appendix

[13] McCormick, P. G. , Burke, R. W. , and Doumas , B. T.

,

Clin. Chem. 18, 854-856 (1972).

46
[14] Mielenz, K. D. and Mavrodineanu , R. , J. Res. Nat. Bur

Stand. (U.S.) 77A, (Phys. and Chem.), No. 6, 699-703

(Nov. -Dec. 1973).

[15] Burnett, R. W. , Anal. Chem. 4_5, 383-385 (1973).

[16] Perry, J. W. , Photoelectric Spectrometry Group Bull.

3, 40 (1950)

.

[17] Slavin, W. , Anal. Chem. 3_5, 561-566 (1963).

[18] Paulson, R. E . , Appl. Opt. 3, 99-104 (1964).

[19] Tong, J. and King, E. L., J. Am. Chem. Soc . 7_5, 6180-

6186 (1953).

[20] Davies, W. G. , and Prue , J. E., Trans. Faraday Soc.

51, 1045-1051 (1955).

[21] Haight, G. P., Richardson, D. C, and Coburn, N. ri. , J. Inorg. Chem. 3, 1777-1780 (1964).

[22] Ku, H. H., J. Res. Nat. Bur. Stand. (U.S.), 70C, (Eng

and Instr.), No. 4, 263-273 (Oct. -Dec. 1966).

[23] Reule, A. G., Appl. Optics 7_, 1023-1028 (1968) and J.

Res. Nat. Bur. Stand. (U.S.) 80A, No. 4, 609-624

(1976). See also NBS Technical Note 544, 17-34 (July

1967-June 1970); and 584_, 21-45 (July 1970-June 1971)

[24] Sloane, H. J. and Gallaway, W. S., Appl. Spectry. 51 , 25-30 (1977).

47
12. APPENDICES

For additional information and as a convenience to the

reader, reprints of papers which are considered particularly

relevant to the certification and use of SRM 935 have been

included in the Appendix of this publication together with a

copy of the certificate. These are:

1. Comments on Spectrometry Nomenclature.

2. Liquid Absorbance Standards.

3. Standard Reference Materials: Standard Quartz Cuvettes

for High—Accuracy Spectrophotometry.

4. Acidic Potassium Dichromate Solutions as Ultraviolet

Absorbance Standards.

5. Testing of Glass Volumetric Apparatus.

6. The Calibration of Small Volumetric Laboratory

Glassware

7. Certificate of Analysis, SRM 935, Crystalline Potassium

Dichromate for Use as an Ultraviolet Absorbance

Standard.

48
APPENDIX I

Reprinted from ANALYTICAL CHEMISTRY, Vol. 48, Page 1093, June 1976 Copyright 1976 by the American Chemical Society and reprinted by permission of the copyright owner

Reprinted by permission of the American Chemical Society

Comments on Spectrometry Nomenclature

Sir: As an optical physicist who collaborates with analytical chemists in the development and use of absorption and

luminescence spectrometers, I have become increasingly aware of the fact that our vocabularies are not always the same. Although some of these differences have been pointed out before (1) it seems worthwhile to familiarize the

readers of this journal with the currently prevailing terminology of physical optics. Perhaps the following comments

may be helpful to editors, authors, and nomenclature committees in establishing a common and consistent terminology that can be used in all branches of spectrometry and

would save readers a good deal of puzzlement and reading

between lines. The main purpose of these comments is to emphasize the need for interdisciplinary efforts in defining

spectrometry nomenclature without perpetuating the in- consistencies that exist at present. They do not represent an official position of the National Bureau of Standards.

The basic concept of modern optical terminology (2, 3) is to combine nouns and adjectives in order to describe quantities and properties as explicitly as necessary, rather than

using glossaries of short names for them. This "Chinese

Restaurant" method of nomenclature (4) offers flexibility

in introducing new terms, and has recently been extended

to include the photon quantities used in photochemistry

and similar disciplines dealing with the interaction of light

and matter (5). The basic list of nouns describing the transport of energy

according to the laws of geometrical optics is: Energy, Q.

Energy density, u = dQ/dV. Energy per unit volume.

Flux, 4> = dQ/df. Time rate of energy flow. Flux density, W = d4>/dA. Flux per unit area. Intensity, I = d<J>/dfi. Flux per unit solid angle.

Sterance, L = d

2 3>/d(Acos6)d£L Flux per unit projected

area and unit solid angle.

(A number of alternative and additional terms have been

proposed. For example: pointance to replace the continually misused term intensity; incidance and exitance, or inci- dent areance and (transmitted, emitted, etc.) areance for the flux densities arriving at and leaving a surface; steri- sent for the sterance generated per unit path length by

emission or scattering into the direction of propagation;

and fluence for the surface energy density incident upon a volume element irradiated from within a large solid angle

of rays (F. E. Nicodemus, private communication). As these

have not yet been accepted generally, they were not included here. However, the new term sterance was included as it avoids the misnomer photon radiance that has appeared in papers on luminescence spectrometry.)

These nouns are modified by the adjective radiant, and

the above symbols are written with a subscript 'e' (for energy), when radiometric units are used. The modifiers lumi- nous and photon are used in conjunction with subscripts

V (for visual) and 'p' (for photon) to indicate the use of

photometric and photon units, respectively. For example:

Radiant energy density, u e [J m

-3

]. Luminous intensity, 7v [lm sr -1

].

Photon sterance, LP [E m~2 sr-1s -1

]. (The basic unit for photon energy used here is the einstein

[E], defined as one mole of photons. It is not an SI unit, but

is used so extensively in photochemistry and photobiology

that its acceptance as a supplementary SI unit may be desirable.)

The additional modifier spectral and subscripts X and a

are used to denote derivatives of radiometric and photon

quantities with respect to wavelength and wavenumber, re- spectively. Thus:

Spectral radiant energy density, ue,x = due/dX [J m

-4

]. Radiant energy density per unit wavelength interval. Spectral photon intensity, 7Pi<r = dlp/da [E sr -1

s _1 m].

Photon intensity per unit wavenumber interval. Any of these can be modified further; such as: fluorescence

photon flux, transmitted spectral radiant flux density, or absorbed luminous energy. Usually, it is possible to drop

most of the modifying adjectives as well as the subscripts e,

v, or p, once the context has been clearly established or whenever a distinction is not necessary. The same method of nomenclature also provides a simple and logical way of specifying the precise meaning of the

quantities and material properties commonly measured in analytical spectrometry. Thus radiant absorptance ae and

photon absorptance ap should be used for the ratios of the

radiant or photon fluxes absorbed by a sample to those in- cident upon it, when measured with a large bandwidth so that these two ratios are not the same. On the other hand,

spectral absorptance a(\) is sufficiently accurate in the

case of measurements made with a small bandwidth. Similarly, luminescence yields (the ratios of the radiant or photon fluxes emitted by a sample to those absorbed by it) should be designated as radiant yield r] e or photon yield r] p (not energy yield or quantum yield). In this case, spectral

radiant yield, r) e ^ = dr)Jda, and spectral photon yield, j) p(T = dr/p/dc, are different quantities even in the limit of infi- nitely narrow bandwidths, and thus should be referred to by these names. [The dependence on wavelength or wavenumber is indi- cated by a subscript (7? Pi<r ) when the spectral distribution is defined as a derivative, but in functional form [a(\)] in the

case of spectral distributions that are not derivatives. This

is an important distinction that must also be borne in mind

in the presentation of spectra. The positions of the peaks

and valleys in derivative spectra depend on the units used,

so that these should not be published in the form of a single

graph with dual scales (such as wavelength and wavenumber, or spectral radiant and photon yield).] In addition to this general description of current optical

terminology, the following comments are made in direct

reference to the nomenclature list in the December 1975

issue of Analytical Chemistry (6). 1) This list cautions not to use optical density instead of absorbance, but defines the latter as "the logarithm to the

base ten of the reciprocal of transmittance"—which is the

definition of optical density. Internal transmittance

should have been substituted for transmittance in this def-
inition of absorbance. The International Lighting Vocabulary suggests transmission density and internal transmission density instead of optical density and absorbance.

This is more determinative, and also fits into a general

scheme where reflection density is defined as the negative

logarithm to the base ten of reflectance. On the other hand,

transmission and reflection density are operational quantities that merely express measured data on a logarithmic

scale, whereas absorbance is directly related to molecular

constants through the Lambert-Beer and Strickler-Berg

equations. Thus, in my opinion, it should be retained as a separate term. I believe that all confusion would be avoided

by defining:

Transmittance, r. Ratio of the flux transmitted by a sample to the flux incident upon it. Internal transmittance, r\. Transmittance exclusive of

losses at boundary surfaces and effects of interreflec- tions between them.

Transmittance density, D = —logini". Negative logarithm to base ten of transmittance.

Absorbance (Internal transmission density), A = —logioTj. Negative logarithm to base ten of internal

transmittance.

2) The definitions of absorptivity in chemistry and physics are not the same. In chemistry, it means absor6ance per

unit path length and unit concentration (A/bc), whereas

the International Lighting Vocabulary (2) defines it as in- ternal absorptance per unit path length (dajdb). Similarly,

it specifies transmissivity as internal transmittance per

unit path length and reflectivity as the reflectance of a

thick layer (so that a further increase in thickness will no

longer change its value). Generally, terms ending in -ance

represent sample properties, whereas terms ending in -ivity

denote material properties that are independent of sample

geometry. The quantity A/bc does not fall in this latter cat- egory as it is also independent of sample concentration.

The International Union for Pure and Applied Chemistry,

apparently aware of this discrepancy, has suggested calling

it absorption coefficient. However, as this name has been

given different and mutually inconsistent meanings in the

past, a less ambiguous word—perhaps specific absorbance —

would have been better. Therefore it is proposed to define: Absorptivity, a = d«j/dfe [m -1

]. Internal absorptance per

unit path length.

Specific absorbance, € = A/bc [kg-1 m2

}. Absorbance per

unit path length and unit concentration.

Specific molar absorbance, c m = A/bc m [mol -1 m2

]. Absorbance per unit path length and unit molar concentration.

3) The December 1975 nomenclature list also implies

that the Beer-Lambert laws are the same. The correct defi- nitions are: Beer's law: Absorbance is proportional to concentration.

Lambert's law: Absorbance is proportional to path

length. Also called Bouguer's law.

4) The definition of the angstrom unit in terms of the

red line of cadmium has been abrogated several years ago.

It is now defined as 10-10 m, exactly. However, the ang- strom is not an SI unit, and has been sanctioned by the In- ternational Committee on Weights and Measures only as a supplementary unit that will eventually be abandoned (7). Therefore, authors should be encouraged to use micrometers or nanometers.

5) The December 1975 list defines spectrometry as the

"measurement of spectra", but restricts the meaning of

j spectrometer to "instrument with an entrance slit, a dis- persing device, and with one or more exit slits . . .". As this excludes non-dispersive and slit-less instruments, such as Fourier and Girard spectrometers, it would be more consis- tent to call any instrument used for spectrometry a spectrometer. Therefore:

Spectrometer: Instrument for the measurement of spectra. The general term for instruments that measure spectral

distributions of radiometric quantities is spectroradiomet- j er. However, as this implies measurements in absolute , units, spectrometer is a better term for the simpler, usually

ratio-forming instruments used in analytical spectrometry.

Thus, a spectrophotometer could also be called absorption

spectrometer, and fluoroescence spectrometer would end

the controversy of spectrofluor imeter vs. spectrofluor ometer.(Strictly speaking, the word spectrophotometer is a mis- nomer. A photometer is an instrument that measures lumi- nous flux in lumens. Since the adjective luminous implies

the integral effect of visual radiation as perceived by the

human eye, the spectral analysis of luminous flux has no i physical meaning. However, in view of the firmly established meaning of spectrophotometer, it is not suggested to change it, although a scanning of the recent literature shows an increasing usage of absorption spectrometer.)

LITERATURE CITED

(1) G. A. W. Rutgers, in "Accuracy in Spectrophotometry and Luminescence

Measurements." Nat. Bur. Stand. (U.S.). Spec. Publ. 378, 53 (1973).

(2) "International Lighting Vocabulary", 3rd ed.. Publication CIE No. 17,

Paris, 1970.

(3) Editor's Page, J. Opt. Soc. Am., 57, 854 (1967).

(4) J. Geist and E. Zalewski, Appl. Opt., 12, 435 (1973).

(5) J. J. Murray, F. E. Nicodemus, and I. Wunderman, Appl. Opt.. 10, 1465

(1971).

(6) "Spectrometry Nomenclature", Anal. Chem., 47, 2526 (1975).

(7) "The International System of Units (SI)", Nat. Bur. Stand. (U.S.), Spec.

Publ. 330 (1971). Klaus D. Mielenz

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

RECEIVED for review September 26, 1975. Accepted December 23, 1975.

50
the

(7).

APPENDIX II

JOURNAL OF RESEARCH of the National Bureau of Standards— A. Physics and Chemistry

Vol. 76A, No. 5, September-October 1972

Liquid Absorbance Standards

R. W. Burke, E. R. Deardorff, and O. Menis

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(June 14, 1972)

Errors in the measurement of the absorbances of liquid filters result from instrumental and chemical uncertainties. This paper presents a systematic study of these variables on the absorbances of selected filters. Three types of liquid filters are discussed. These are (1) individual solutions of high

purity compounds, (2) composite mixtures and (3) aqueous solutions of organic dyes. The accuracy of the absorptivity data is established using NBS-calibrated glass filters. The magnitude of the errors

arising from spectral bandpass, beam geometry, stray light, internal multiple reflections, and refractive index are delineated. Finally, as a practical outgrowth of this study, the development and issuance

of NBS Standard Reference Material 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry, is described.

Key words: Absorptivity data; accuracy; liquid absorbance standards; Standard Reference Materials.

I. Introduction

In the use of filters for checking the accuracy of

le photometric scale of spectrophotometers, one

teeds materials which exhibit absolute spectral neurality. Such ideal filters are not available at present,

he use of the materials which have been suggested is mited by instrumental and sample variables. For this

eason, one must carefully specify conditions which

efine the other sources of errors. The first part of this paper reviews several of the nore important instrumental parameters and dis- cusses their effects on absorbance measurements.

Examples from the literature are given to illustrate

he magnitude of the errors arising from spectral

aandpass, stray light, nonparallel radiation, and

multiple reflections. The second part presents some

sxperimental studies of several materials which may

serve as calibration "filters" or standards.

II. Instrumental Parameters

A. Wavelength Accuracy

The absorbances of most samples are sufficiently

wavelength dependent that even in the most favorable

regions of maxima and minima a wavelength error of

1—2 nm can produce absorbance errors of a few

tenths of a percent. When performing measurements

on the slopes of absorption peaks, the wavelength

setting is obviously much more critical and errors are

typically several percent per nanometer. Wavelength accuracy, therefore, becomes especially

important when making absorbance measurements

at, for example, an isosbestic point.

A fine source provides the most definitive means for

establishing the accuracy of the wavelength scale.

The best single source is a mercury lamp which may

be used throughout the range of 200-1000 nm. For

calibration in the visible region, helium lines are

also useful.

Gibson [l] 1 has discussed the use of mercury and

helium sources and has listed those lines best suited

for wavelength calibration. Other sources which have

been used include neon, cadmium, cesium, and sodium.

The M.I.T. Wavelength Tables [2] summarizes the most

prominent emission fines of these elements. Not all wavelengths are given and care must be taken in

using any of these lines in order not to confuse closely

adjacent fines with the one being checked. For those

spectrophotometers having a hydrogen or deuterium

source, the emission lines at 486.13 and 656.28 nm

(H) or 485.99 and 656.10 nm (D) may provide con- venient checks at these wavelengths.

Calibrated holmium oxide and didymium glasses may

be useful secondary standards, particularly for checking recording spectrophotometers in which a dynamic

check of the instrument is often desirable. The

apparent absorption maxima of these filters, however,

may vary with spectral bandpass. Therefore, for the

highest accuracy, they should be calibrated for the

spectral bandpass at which they are to be used.

In general, prism instruments require more frequent

calibration than the grating type because their dispersion is temperature dependent. Corrections in the

visible region are of the order of 0.1 nm per degree

Celsius. Hysteresis effects must also be considered

1 Figures in brackets indicate the literature references at the end of this paper.

51
and the wavelength settings should always be approached from the same direction.

B. Finite Bandwidth

Spectral bandpass differences undoubtedly account

for a significant portion of the discrepancies in molar

absorptivity values reported in the literature. Some

have resulted from the employment of inadequate in- strumentation; others have resulted from improper use

of these instruments or complications arising from

solution equilibria.

Increasing the spectral bandpass at an absorption

maximum has two effects on apparent peak heights:

(1) the observed values are always less than the

true values and (2) the differences between the two

are proportionally greater at higher absorbances.

Thus, for a finite bandpass, a plot of absorbance

versus concentration or path length will always have

a smaller slope than it does in monochromatic radiation and, in addition, will be concave to the concentration axis. Figure 1 illustrates these effects. The reverse behavior will be observed for measure- ments at an absorption minimum. Meehan [3] has

given a simple example which verifies these two

effects mathematically.

1

CO MCE NITRATION (PATH LENGTH)

FIGURE 1. Effect of spectral bandpass on absorbance: (a) Monochromatic radiation (b) Finite bandwidth.

Consider that an absorbing system is illuminated

by two monochromatic radiations of wavelengths

\i and X-2 and that the Lambert-Beer law is obeyed

at each wavelength with absorptivities ai and a 2 . If the effective intensities of the two wavelengths are

1\ and It, respectively, the transmittance, T, is T= (7i-10- a i 6c +h 10- a2 bc )l(h + h) (1)

where 6 = path length and c— concentration. If A.i

is the wavelength of maximum absorption so that

a.\ > a 2 , then

(1 + r)

•10- a i" c [l + r -10(a i- a2)»c]

(2)

where r = Ii\I\. The absorbance, A, defined as — log

Tis

A = aibc + log (1 + r) - log [1 + r-10(a i- a 2>»c]

(3)

which may be differentiated with respect to be to give

dA/d(bc) = a, - [r(ai - a2 )10< a i-«2>f>c]/

[1 + r

• 10<«i-«2>6c] (4)

At low absorbances

dAld(bc)bc~o

ai + rat

1 + r (5)

whereas when be becomes large, 10Ql6c ^>10°26c and

the limiting slope is

dA/d(bc) bc ^ a0 = a2. (6)

Thus, the limiting absorptivity at low absorbances is is a weighted average of ai and at while, at higher

absorbances, it is equal to at, the smaller of the

individual absorptivities.

The theory for the correction of spectral bandpass

errors has been developed by Hardy and Young [4], Eberhardt [5], and Broderson [6]. Rigorous calculation

requires integration of the relation

^ot>s = k>g !

hS\dk

(7) hSk 10~A dk

where /\ is the incident intensity and Sx is the spectral

sensitivity of the detector. Equation (7) emphasizes

the fact that the observed absorbance depends not

only on the shape of the absorption curve, but also on

the wavelength distribution of the source and the

detector response. If I\S\ is constant over the wavelength interval used, approximate corrections for

spectral bandpass may be calculated by assuming a Gaussian shape for the absorption peak and a tri- angular slit function for the instrument. Some calculations are given in table 1 based on these assumptions.

The tabulated values of AobJA agree closely with

those observed experimentally for A up to 1. As

shown by Broderson [6] however, for values of RBW

larger than 0.5, AobJA also depends on A and the

above treatment is no longer applicable.

C. Stray Light

Stray light is defined as any light outside the spectral

region isolated by the monochromator that reaches the

detector. It is produced by scatter from the optics and

52
ill TABLE 1. Dependence of Aobs on spectral bandpass

at an absorption maximum under idealized

conditions (see text)

RBWa RBWa

0.0100 0.99995 0.0800 0.9970

.0200 .9998 .0900 .9962 .0300 .9995 .1000 .9954

.0400 .9992 .2000 .9819

.0500 .9988 .3000 .9604

.0600 .9983 .4000 .9321 .0700 .9977 .5000 .8987

a Relative bandwidth: Ratio of spectral bandwidth to half bandwidth of fully resolved peak.

Walls of the monochromator and is present in varying

imounts in all spectrophotometers,

ji Stray light can lead to varied problems in spectrophotometry. Spurious absorption bands may arise in

d |>ome cases. More frequently, however, deviations

iTom the Lambert-Beer law are produced. These

leviations are positive if the stray light is absorbed and

i) legative if it is not. Extensive literature exists on the detection, ineasures nent and mimization of stray light. Several of the

, r nore useful papers are those by Hogness, Zscheile

ind Sidwell [7], Perry [8], Slavin [9], and Poulson [10] n which additional references may be found.

The amount of stray light present is proportionally

| arge in those wavelength regions where the transB nission of the monochromator, the source intensity or he detector sensitivity are relatively low. These

egions, which should first be checked, are 200-230

lm and 600-700 nm. If a tungsten lamp must be used

n the range 350-400 nm, a visible cutoff filter should

|) ilways be employed.

The quantitative assessment of stray light requires

he use of extremely sharp cutoff filters. Slavin [9]

j ind Poulson [10] have described a number of liquid

i ind solid filters that may be used. Aqueous solutions of

|( he alkali halides, for example, are extremely useful

n n the ultraviolet. Care must be taken, however, that

ight leaks in the cuvet compartment are not contributng to the observed results. The apparent stray light

' vill not vary appreciably with slitwidth if only instru3 nental stray fight is present. In a quick test for light

I eaks, the apparent stray light will decrease rapidly as he slits are opened.

The stray light error encountered most frequently in

l! spectrophotometry produces a decrease in an absorpion peak and leads to negative deviations from the

.ambert-Beer law. If it is assumed that none of the

tray light is absorbed by the sample, the measured

ibsorbance is related to the true transmittance by the

expression [9]

^obs =log [(l-DS + n-1 (8)

I II » tfhere S is the instrumental stray fight expressed as a

j raction. The effects of several levels of unabsorbed

stray light on absorbance are given in table 2. Most

samples absorb sufficiently in other regions of the spectrum to filter out a proportion of the stray light. Thus,

indirect estimates of instrumental stray light based on

deviations from linearity are generally low.

Table 2. Effect of unabsorbed stray'light on absorbance

A

S= 0.0001 S = 0.001 S = 0.01

0.1 0.1000 0.0999 0.0989

.5 .4999 .4990 .4907

1.0 .9996 .9961 .9626

1.5 1.499 1.487 1.384

2.0 1.996 1.959 1.701

D. Nonparallel 1 ncident Ra diation

A perfectly parallel beam of radiation must come

from a point source and can carry only an infini- tesimal amount of energy. The light beam in a spectrophotometer always has some finite angular size. As a

result, the average light path is greater than the per- pendicular distance between the cuvet faces. Meehan

[3] has considered the case in which the incident

radiation is convergent or divergent in one dimension

such as corresponds, respectively, to radiation focused

by a cylindrical lens or leaving a narrow slit. If 0 and

R are the angles of incidence and refraction, re- spectively, the path length of the extreme ray (fig. 2)

is 6/cos R and the absorbance of this ray is A/cos R.

Other rays enter at smaller angles and the observed

absorbance is given by

b H

Figure 2. Nonparallel incident radiation [3]. Reprinted by permission of John Wiley & Sons, Incorporated.

f

ft max

^obs = ^/f?max I/COS RdR. (9)

For this relation to hold, the incident beam must be

uniform over its cross section. Evaluation of this

integral for Rmax of 2-10° (6max = 2.7°-13.4°) gives

the following percent errors in A (table 3). Thus, for

angles of incidence up to 5 degrees, the absorbance

error is less than 0.1 percent.

E. Multiple Reflections

On passage of light through a cuvet containing

solvent or sample solution, some radiation is reflected

53

470-063 OL - 72 - 7
Table 3. Dependence of absorbance error on nonparallelism of incident radiation

ft mM , degrees 0max , degrees Percent error in A

24

6

8

10

2.7

5.3

8.0

10.7 13.4

0.020

.081

.18

.33

.51

at each of the two air-glass and the two glass-liquid

surfaces. For perpendicular incidence, the fraction

/ reflected on passing from a medium of refractive

index nx to a second having retractive index m is given by the Fresnel expression

/= (10)

For an air-glass surface, /= 0.04 and for a glassliquid surface, /= 0.0035. When more than one surface

is involved, the effects of multiple reflections must be

considered. The essential question when dealing with

liquid filters is whether the solvent completely compensates for such reflections. According to Goldring

et al. [11], it does not. They consider the case where the

reflections from all surfaces perpendicular to the

fight beam on the two sides of the sample are grouped

together to form two effective reflection coefficients, ri and r2 , as shown in figure 3. Considering only first and

second order reflections, the observed absorbance is

%d-r,l

3*B,r, r,

P0T-P0Tr,

-T-Trj,

SOLVENT SAMPLE

FIGURE 3. Internal multiple reflections [11]. Reprinted by permission of the American Chemical Society.

Aobs = log—=log —— — — (11) P2 r07 (1 — rx + rir2 l

2

)

which, after transformation and series expansion, can be reduced to the following expression:

Aobs = A + 0.4343 (1-712

)

fir2 1-r,

(12)

Considering only the reflections from the two cuvet

faces and that ri = r2 = 0.05, the variation of ^ obs with

A isA 0.1 0.2 0.5 1.0 2.0

A^ 0.1004 0.2007 0.5010 1.0011 2.0011

Thus, because of internal multiple reflections, the

measured absorbance should always be larger than the

true absorbance with the percentage difference being

greatest at low absorbances. Verification of these

deviations is difficult experimentally. Goldring et aL

[11], however, have suggested ways of minimizing

reflection effects. These include positioning the cuvet

at a small angle to the beam and stopping down the

detector surface, immersing the cuvet in a fluid con- tained in a larger rectangular cuvet turned at an

angle to the beam, or by using a cuvet with prismatic

windows. In each case, the effective path length must

be determined separately. Other alternatives consist

of using cuvets constructed of absorbing materials or immersion of a glass filter into the sample. For a

limited wavelength range, reflection errors can also be

reduced by coating the external window surfaces

with anti-reflection layers. In addition to reflections from the cuvets, reflections

from other surfaces must also be considered. Gibson

[1] has pointed out that reflections may occur from the

surfaces of the slit and detector and has suggested a means for checking this source of error. A thin glass

plate is placed in its normal position at right angles

to the beam and then at a small angle from this position. In the second position the reflected energy is directed out of the beam while scarcely affecting the true absorbance.

F. Cuvets

The majority of absorbance measurements are performed on solutions and, in such instances, the

cuvet becomes an integral part of the measurement

system. Presently, the uncertainty in the length of the

light path can be a limiting factor in the determination

of molar absorptivity. Nonparallelism of the end

windows can be of even greater consequence. However, the effect can be minimized by using the same

cuvet orientation for all measurements. The uncertainty in path length is nevertheless greater.

Cuvets are available in a variety of shapes and : sizes and may be made of glass or silica. Construction remains largely an art and cuvets may vary con- siderably in their quality. Two methods of assembly

are commonly used. These are: (1) fusion using only

heat and (2) the use of intermediate, low-melting

glasses. At present, each has its disadvantage. The

first technique is the more desirable, but our ex- perience, primarily with rectangular cuvets, indicates

that the edges are not always completely fused.

These surfaces exhibit capillarity and may become

serious sources of contamination. Under such conditions, the cleaning of these cuvets is difficult, if not impossible. The use of low-melting fluxes on the other

hand can produce strains because of differences in coefficients of expansion and cuvets so constructed

are generally more fragile.

Cuvets are frequently offered in matched sets. This

may be considered more of a convenience than a necessity since this terminology is normally used onl>

to describe the transmission of the window material.

54
Path length and parallelism of end plates are of more

fundamental importance. Unfortunately, no generally

accepted tolerances have been established in this

country for the construction of cuvets. The British

Standards Institution, however, has published a set of specifications [12] which merit our consideration and

possible adoption. They recommend that the path

length be specified to ±0.02 mm and the end windows

be flat over a defined beam area to four Newton fringes

jper centimeter in mercury green light. It should be

feasible for greater accuracy, especially for the 10 mm

cuvets, to calibrate a limited number by appropriate

metrology techniques to ±0.01 mm.

B. Individual Solutions of High Purity Compounds

1. Potassium Dichromate. Numerous attempts

have been made to use chormium (VI) solutions

as ultraviolet absorbance standards. In the early

studies [7, 14, 15, 16, 17] alkaline solutions of potassium

chromate were preferred. More recently, potassium

dichromate in slightly acidic media has been described

[18, 19, 20]. The absorption spectra of these two sys- tems are quite different with the latter giving the more

desirable arrangement of maxima and minima (fig. 4). The approximate composition of chromium (VI)

solutions may be derived from the following equilibria:

III. Evaluation of Selected Liquid Filters

This section is a report of our efforts to develop

ivell characterized liquid absorbance standards. Three

general types of liquid filters are discussed. These are

1) individual solutions of high purity materials, (2) empirical mixtures and (3) aqueous solutions of organic

lyes. Much of the emphasis has been placed on determining the optimum conditions for preparation of these

liters and, once established, what the absorbing

species or ionic compositions of these systems are. iy combining this information with the instrumental

:onsiderations developed in the first part of this paper,

t is hoped that the resulting data represent a step

oward obtaining more meaningful absorptivity

neasurements.

A. Instrumentation

i Absorbance measurements at a fixed wavelength

yere performed manually on a high precision doubleleam spectrophotometer provided with a double

aonochromator. The accuracy of the photometric

cale of this instrument was established with the

IBS high-accuracy spectrophotometer described by

/lavrodineanu [13]. The wavelength scale was checked

nth a mercury lamp. Potassium halide solutions [9,

0] were used to assess stray light in the ultraviolet.

Room temperature was maintained between 24 to

'6 °C. The cuvet compartment and jacketed holders

vere thermostated by a circulating water bath. Copperonstantan thermocouples were used to measure

emerature differences between the bath and the

ample solution.

An electronic feeler gauge [13] was used to measure

he path length of the 10 mm rectangular cuvets

ormally used. Solutions, assumed to obey Beer's

aw, were used for calibrating the 1 mm cuvets. The accuracy of the microbalance was established

nth NBS-calibrated weights. Class A, 1 -liter volunetric flasks were checked gravimetically. All dilutions

'ere subsequently made by volume. To minimize

rrors from cell positioning, borosilicate, Pasteurype pipets were used to transfer solutions to and from

he cuvets.

220 250 300 350 400

WAVELENGTH, nm

FIGURE 4. Absorption spectra of 10-" M Cr (VI) in 0.05 N KOH

and 10-W HC104.

H2 Cr04 = tP + HCrOj (13)

HCrOf = H+ + CrO? (14)

2HCrC>4- = Cr2 Of + H2 0. (15)

Some values reported for the equilibrium constants

Ki, K2, and K3 at 25 °C are as follows:

A:, = 0.16 [21]

K2 = 3.2 x 10-7 [22] and 3.0 x 10~7 [23]

#3 = 43.7 [21], 35.5 [24] and 33.0 [25].

Thus, at a pH greater than 10, chromium (VI) exists

almost entirely as Cr0.r ions while, in weakly acidic

solutions, the predominant species is HCrOj which

partially dimerizes to Cr20f. The amount of dimer

will increase with increasing chromium concentration. The formation of H2Cr04 or CrOf can be es- sentially eliminated by maintaining the acidity near pH 3.

55
Much of the uncertainty that has arisen from the use

of acidic potassium dichromate solutions as potential

absorbance standards can be attributed to the dimerization reaction (eq (15)). It is primarily this equilibrium which leads to the observed deviations from

Beer's law. Because of the range of values reported

for K3 , one of the first objectives of the present study was to redetermine this constant. The spectrophotometric method was chosen and is described in some

detail below.

a. Spectrophotometry Determination of the

Dimerization Constant for: 2HCr04= = Cr2 07= + H2 0. If HCrOi" and Cr207= are the only Cr(VI)

species present with molar absorptivities ei and e-2 , respectively, the apparent molar absorptivity, em, will be given by

em = (1 — a)€i + 1/2 • ae2 (16)

where a is the fraction of total chromium in the

dichromate form. From eq (15) the thermodynamic

equilibrium constant of the dimerization reaction is

K3 [Cr2 Of] 72

[HCrOr? (17)

or, in terms of a

K3 2(l-a) 2Crr

' y\

(18)

where yi and 72 are the activity coefficients of HCrOf

and Cr20f\ respectively, and Crr is the total chromium concentration. Because the ionic strength never exceeded 0.01, the activity coefficients may be treated

by the Debye-Hiickel expression [25]

log yi

AZ'fpi2

(19)

where Z is the ionic charge, / is the ionic strength and

A has a value of 0.509 1/mol at 25 °C. Equations (16)

and (18) can be rewritten to give, respectively,

€m = a(l/2 e2 — 61) + €1 (20)

and

log K3 = log

a 2AP'2 2(l-a)2Crr 1+/1/2 (21)

Assuming a value of K3 , one can calculate from eq

(21) the a values at various total chromium concentrations. If the choice of K3 is correct, the measured

values of em should he on a straight line of slope (1/2

€2 — €1) and intercept €1, as seen from eq (20). The

best value of K3 is then determined by the method of

least squares.

1. Experimental Procedure

The chromium (VI) solutions used in the subsequent!

studies were prepared from NBS SRM-136c, potassium dichromate, which had been dried at 110 °C for two hours. The distilled water was shown to be free

of reducing impurities by titration with a dilute

potassium permanganate solution. Perchloric acid was used to maintain the pH at 3.0 ± 0.1. The optimum wavelengths for the determination of K3 were chosen as follows: Two solutions were pre- pared, one containing 391 mg K2Cr207/l and the other,

40.0 mg K2Cr20;/l. The former was transferred to a 1.023 mm cuvet and the latter to a 10.00 mm cuvet. A1 differential scan was then made on a recording spectrophotometer equipped with a 0-0.1 absorbance (full scale) slidewire, using the less concentrated solution- as the reference. The resulting spectrum is shown in figure 5. Relatively large differences in absorbance are seen in the wavelength regions near 275 and 385 nm

even though the total number of chromium atoms in each light path is the same. Thus, it is in these two

regions that the largest deviations from Beer's law

should be observed. Three wavelengths were subsequently selected in each region for the determination i of K3-390, 385 and 380 and 280, 275 and 270 nm ] respectively.

220 250 300 WAVELENGTH nm

350 40C

Figure 5. Difference spectrum of 391 mg K2Cr207/l in 1.023 mn

cuvet versus 40.0 mg KoCrsCVl in 10.00 mm cuvet; pH = 3.

2. Results

The apparent molar absorptivities calculate

from absorbance measurements on the four chromiur

concentrations used for the determination of K

are given in table 4. Using the method of least squares

these e m values were plotted against the a fraction

calculated from eq (21) for various assumed values 0 K3 . The molar absorptivities were weighted accordin

to the reproducibility of the absorbance measure

ments. A computer was used for all calculations

56
A typical graph of the residual standard deviation

of the experimental e m values from the best straight

jiline fit is shown in figure 6 for K3 values of 28 through

36. The minimum in this curve is the best estimate

of K3 and, for the case shown, is 32.6. The K3 values

at the six wavelengths are summarized in table 5.

Table 4. Apparent molar absorptivities, em, of K2Cr2 07 solutions at 25 °C; pH = 2.9 (HCIO4)

K2Cr207Conc. e m 1/mol" 1 cm~'

mg/1

100.06 400.48

699.86

1000.27

390 nm 385 nm 380 nm 280 nm 275 nm 270 nm

589.2 748.7 865.1 1579.3 1718.6 1881.0

714.1 878.6 997.8 1758.2 1914.4 2069.8

806.2 973.8 1094.0 1890.0 2053.5 2208.0

875.9 1045.6 1166.5 1988.2 2161.5 2313.6

2.0

o

I-<>

<o

z

<

0.8

0.4

FIGURE 6. Least squares determination of K3 .

Table 5. K3 values at 25 °C as determined at several

wavelengths

X, nm

390

385

380

280

275

270

32.4

33.3

33.8

33.5 32.6

31.6

Avg. 32.9

Std. Dev. 0.8

Rel. Std. Dev. 2.5%

b. Absorption Spectra of HCr04 and C^Oy.

The absorbances of three solutions containing nominally 20, 40, and 60 mg K2 Cr2 07/l (pH = 3.0 ±0.1) were measured in a 10.00 mm cuvet at 5 11m increments over the wavelength range of 220-400 nm. A

close approximation of e^CrOj) was obtained by

extrapolating the apparent molar absorptivities to zero chromium concentration. This extrapolation is simplified since the variation of a is nearly linear over

this concentration range. Similar absorbance measure- ments were performed on three solutions containing

1000 mg K2 Cr2 07/l using a 1.023 mm cuvet and the

corresponding a value was calculated from eq (21),

with K3 — 32.9. The first approximations of 1/2*

e(Cr2 Of) values were obtained by substituting the

extrapolated eCHCrO^) results and the above a

value into eq (20). The e(HCr04 ) and 1/2 • e(Cr2 07=)

values were then refined bv successive approximations.

The calculated absorption spectra of these two ions are shown in figure 7.

250 300 350

WAVELENGTH, nm 400

FIGURE 7. Absorption spectra of HCrO., and Cr2 Of.

c. Molar Absorptivities of HCrCf and Cr2Of at

350, 313, 257, and 235 nm. As shown previously

in figure 4, the absorption spectrum of weakly acidic

solutions of K2 Cr2 07 exhibit maxima at 350 and 257 nm

5
and minima at 313 and 235 nm. These maxima and

minima do not shift significantly over the concentration range of 20-100 mg K2Cr207/l. This is con- sidered a practical range for most applications since it yields absorbances of about 0.1-1.5 for a 10 mm cuvet. The molar absorptivities of HCrOj and C^Of at 350,

313, 257, and 235 nm were determined by the pro- cedure described above and are given in table 6. The

e(HCrOj) values are estimated to be accurate to ± 0.5 percent. Because of the imprecision of the K3 determination, the uncertainty of the dichromate

molar absorptivity values may be as large as ± 5

percent.

Table 6. Molar absorptivities, e, of HCrO^ and Cr20^

at 25 °C

e, 1 mol-1 cm-1

350 nm 313 nm 257 nm 235 nm

HCrO; 1576 711 2100 1804

1/2 Cr 0= 1788 823 3156 2688 ' 2 7

Table 7 shows the agreement between the experimental and calculated apparent molar absorptivities

for five concentrations of potassium dichromate in

dilute perchloric acid (pH = 2.92 ± 0.02). The experimental values are the averages obtained on five

individual samples measured at each concentration.

The 95 percent confidence limits for any given set of

measurements was 0.1 percent. The a values used are

also given. These are seen to vary nearly linearly over

the range of chromium concentrations employed.

d. Optimum pH and Choice of Acid. The

chromium (VI) solutions used in this study were prepared in dilute perchloric acid media having a pH of

about 3. This acidity was selected because the existing

equilibria data indicated this pH limited the formation

of either H2 Cr04 or CrOl to less than 0.1 percent of

the total Cr (VI) concentration. Perchloric acid was

chosen instead of sulfuric to prevent the formation of

chromate-sulfate complexes. Tong and King [24] and

Davies and Prue [25] noted slight differences in the

absorbances of dilute perchloric and sulfuric acid

solutions after correcting for differences in ionic

strength. They attributed these differences to mixed

complex formation. More recently, Haight et al. [26]

have shown that the conversion of HCrOj" to CrSOf is quantitative in 1 M HSO? solutions and that absorption

spectrum of this complex is significantly different from

the HCrOj spectrum.

Because most of the previous work reported in

literature has been done in sulfuric acid, it was con- sidered of interest to intercompare these two systems.

The results are shown in each acid between pH 2 and 3. At 350 nm the results obtained in perchloric acid are consistently 0.1-0.2 percent higher than in sulfuric;

at 235 nm, they are lower. The relatively large increase

in absorptivity at 235 nm at pH 1.90, together with the

above considerations, suggest the formation of additional chromium (VI) species in systems employing

dilute sulfuric acid solutions. (See table 8.) e. Effect of Temperature. The absorbances of a

solution containing 60 mg K2 Cr2 07/l in dilute perchloric

acid (pH = 3) were measured over the temperature

range of 17 to 37 °C. The percentage change in absorb- ance for wavelengths of 350, 313, 257, and 235 nm is shown in figure 8. The results are uncorrected for

Table 7. Experimental and calculated values (E, C) of the apparent molar absorptivity of K2 Cr207 at 25 °C;

PH = 2.9(HC104 ) e m , 1 mol _1 cm"1 K2 Cr2 07 Cone.

mg/1 a 350 nm 313 nm 257 nm 235 nm

20.22 0.0090 EC

Diff.

1577.6

1577.9 -0.3

713.5

711.9 + 1.6

2113.6

2108.7 + 4.9

1814.7

1812.1 + 2.6

40.09 0.0178 E

C

Diff.

1579.9 1579.8 + 0.1713.3 713.0 + 0.32120.8

2118.8 + 2.0

1820.2 1819.7 + 0.5

60.12 0.0262 E

C

Diff.

1581.4

1581.5 -0.1

713.8

713.8

0.0

2127.2 2126.7 + 0.51827.1 1827.2 -0.1

80.17 0.0344 E

C

Diff.

1584.0

1583.3 + 0.7

715.0

714.8 + 0.2

2137.6

2135.3 + 2.3

1835.8 1834.4 + 1.4

99.92 0.0423 E

C

Diff.

1585.5 1584.0 + 1.5715.6

715.0 + 0.6

2144.0

2143.7 + 0.3

1841.5 1841.4 + 0.1

58
Table 8. Apparent absorptivities, a, of sulfuric and

perchloric acid solutions of K2Cr207 at 25 °C; K2 Cr2 07 cone. = 0.050 g/1

a, 1 g

-1 cm-1

pH 350 nm 313 nm 257 nm 235 nm

H2S04 1.90 10.71o 4.83, 14.444 12.437 1.98 10.71 2 4.839 14.44o 12.41 8 2.20 10.722 4.839 14.429 12.402 3.00 10.733 4.844 14.427 12.38,

HC104 1.99 10.72, 4.848 14.448 12.396 3.08 10.740 4.845 14.434 12.383

Table 9. Apparent molar absorptivity of potassium

chromate in 0.05 M Na2 HP04 and 0.05 M

KOH at 25 °C

0.05 M Na2 HP04

€m, 1 mol-1 cm-1 K2 Cr04 Cone, ' M X 10s pH 373 nm 274 nm

7 9.2 4827 3703 14 9.1 4820 369,

21 9.1 4813 3692

0.05 M KOH

7 4830 3705 14 4824 3698 21 481 4 369,

0.6

-0.8

1 1 1

1

•v.

-V.

Vs. 31 3 nm

NT***

1 \

1 ^\350S257nm

N. \.

1 N

J 1 1 235 nm

1

15 20 25 30 35 40

TEMPERATURE, °C

FIGURE 8. Change in absorbance with temperature for acidic solutions of K2 Cr2 07 ; pH = 3 (HC104 ).

thermal expansion of the solvent. The absorption

minimum at 313 nm exhibits the smallest temperature

dependence (— 0.02 percent per °C) while the maxima

at 350 and 257 nm and the minimum at 235 nm have

temperature coefficients of — 0.05 percent per °C over

the range 20 to 30 °C.

f. K2O2O7 in 0.5 M Na2 HP04 and 0.05 M KOH.

To overcome some of the difficulties in handling

strongly alkaline chromate solutions, Johnson [27] has

recommended dissolving K2 Cr2 07 in 0.05 M Na2 HP04 at pH 9. The apparent molar absorptivities obtained in

this medium and in 0.05 M KOH are given in table 9. As seen from the data, both systems appear to exhibit

deviations from Beer's law of about three parts per

thousand over the concentration range studied. Although this deviation is the same magnitude and direction predicted from uncompensated internal multiple

reflections discussed previously, we have not been able

to demonstrate experimentally that such reflections are responsible for the observed results. Studies in

which the cuvet windows are coated with various antireflection layers are continuing however.

From a consideration of the equilibrium data, the

absorbances in the two systems should be nearly

identical. The ionic strength of the 0.05 M Na2 HP04

is 0.15 and, from Neuss and Rieman's work [22], the

ionization constant for the reaction HCrOi = H+ + CrO 4

is approximately 9X10-7 mol/liter. Thus, at pH 9.1,

only about 1 in 1000 chromium atoms is present as HCrO?. Since this species is less absorbing than

CrOj at 373 and 274 nm, the apparent molar absorptivities in 0.05 M Na2 HP04 should agree with those

obtained in 0.05 M KOH to within 0.1 percent.

The absorbance of alkaline chromate solutions de- creases with increasing temperature at both absorption

peaks. Between 17 and 37 °C, the temperature coefficient at 373 nm is — 0.09 percent per °C; at 274 nm,

it is -0.06 per °C.

2. Potassium Nitrate. Aqueous solutions of potassium nitrate exhibit a nearly symmetrical absorption

peak with \max at 302 nm. This system has been

studied extensively by Vandenbelt [14] and by the

British Photoelectric Spectrometry Group in two

collaborative tests [28, 29]. Molar absorptivities re- ported vary from 7.06 to 7.20 liter mol-1 cm-1

. This

range of values undoubtedly results in part from the

fact that KNO3 solutions deviate significantly from

Beer's law.

Careful reexamination of this system has yielded

the following apparent molar absorptivities (table 10).

These are seen to decrease rather markedly with

increasing potassium nitrate concentration. Several

factors conceivably contribute to. this behavior. In

addition to possible multiple reflection effects already

discussed some ionic interactions are undoubtedly

possible in this system because of the relatively high

concentrations of solute required coupled with the

fact that the N — O bond in the nitrate ion has some

polar character.

The potassium nitrate used in the above study was

analyzed for commonly encountered impurities. After

drying at 110 °C, the water content was less than 0.02

percent. Flame emission analyses [31] indicated the

following levels of alkali and alkaline earth impurities

59
(in parts-per-million): Li (< 0.2); Rb (13); Mg (0.2);

Ca (1); Sr(4) and Ba (< 2). At 302 nm, the absorbance of KN03 solutions de- creases with increasing temperature. Over the range

17 to 37 °C, the temperature coefficient is —0.14 percent per °C.

Solutions stored for up to six months showed no measurable change (=s 0.1 percent) in absorbance.

Edisbury [29] has cautioned, however, that some solutions may develop bacterial whiskers on standing.

Sterilization may be effected by prior boiling of the

distilled water.

Table 10. Apparent molar absorptivity, em , of aqueous

solutions of potassium nitrate at 25 °C

e m , 1 mol-1 cm-1

KN03 Conc.,M 302 nm

0.028 7.160 .056 7.142 .084 7.12 7 .112 7.106 .140 7.09,

a Spectral bandpass = 1.0 nm

i 3. Potassium Hydrogen Phthalate. High purity

potassium hydrogen phthalate (KHP) is readily available. It is presently issued by the National Bureau of

Standards as a primary acidimetric standard. Its use

in the preparation of buffer solutions of known pH is well established. It has also been used as a spectral

standard in the comparative evaluation of spectrophotometers [11, 32].

The absorbance of KHP solutions is dependent on

acidity, as shown in figure 9. In the present study,

dilute perchloric acid solutions were employed to minimize changes in ionic composition as a function

of pH. Acid concentrations much above 0.1 A' could

not be used because of the precipitation of potassium

perchlorate. Subsequently, all solutions were prepared

to contain 10 ml of perchloric acid per liter with a pH of 1.3. Ringbom [21] has given the first ionization constant

of phthalic acid as 1.6 X 10-3 at 25 °C at an ionic

strength of 0.1. At the pH used, the absorbing system

consisted of 97 percent phthalic acid and 3 percent

hydrogen phthalate ions. Table 11 summarizes the

absorptivity values obtained on these solutions.

Absorbance decreases at the 275.5 nm maximum and

increases at the 262 nm minimum with increasing

temperature. Over the range 17 to 37 °C the temperature coefficients are —0.05 percent and +0.05 percent

per °C, respectively.

Perchloric acid solutions of KHP (NBS-SRM 84g) were examined for possible fluorescence, using a high

sensitivity spectrofluorometer. With 280 nm excitation, a faint fluorescence was detected with A. max at approximately 350 nm. The possible effects of this fluorescence on the absorbance data remain to be established.

WAVELENGTH , nm

FIGURE 9. Absorption spectra of KHP solutions at different

acidities.

Table 11. Apparent absorptivity, a, of potassium hydrogen phthalate in dilute perchloric acid

at 25 °C; 10 ml HC104/liter

a, 1 g

_1 cm -1 NBS SRM 84g

Cone, g/1 275.5 nm 262 nm

0.034 6.332 4.494 .142 6.31 9 4.489

4. Cobalt and Nickel Sulfates and Perchlorates. Aquo cobalt (II) and nickel (II) ions have rela- tive broad absorption peaks with A. max at 512 and 394

nm, respectively (fig. 10). Solutions of either may serve ; .| as useful absorbance standards. Most of the attention , ^ to date, however, has been directed at cobalt sulfate

^ which was first recommended by Gibson [1]. In this study, the apparent molar absorptivities of ;, f Co(H2 0)^2 and Ni(H2 0)^2 were examined in sulfate and 1 perchlorate media. All solutions were prepared from i the high purity metals by dissolution in H2SO4-HNO3 [ or HCIO4-HNO3 acids. Nitrate was removed by I 1 repeated fuming until the acidity was reduced so that i on dilution to volume, a pH of 1 could be obtained. The I results obtained in the two acid media are given in i J

it table 12. The values in sulfate solution are signifij cantly higher than in perchlorate media for both I cobalt and nickel, suggesting that some complexation \ 1

of these ions by SO 4 or HSO7 has occurred. A differ- I >.

60
0.020

400 500

WAVELENGTH, nm 600

Figure 10. Absorption spectra of Ni(H2 0)+2 and Co(H20)+2

. 6 6

Table 12. Apparent molar absorptivities, em of acidic

solutions of cobalt and nickel ions

at 25 °C; pH = 1.0

Co Cone.

g/1

2.4

12.0

Ni Cone.

g/1

2.3 11.5

e m (512 nm)

1 mol~' cm-1

Sulfate Perchlorate

4.88, 4.869

4.803 4.79,

Cm (394 nm)

I moH cm-1

Sulfate Perchlorate

5.167 5.154

5.090 5.07,

ential spectrum of 0.2000 molar cobalt solution in

sulfate against the same cobalt concentration in

perchlorate media is shown in figure 11. In sulfate

solution, the formation of a second cobalt species with

\ max = 528 nm is indicated. Similar behavior was also

observed for nickel.

The absorbances of cobalt and nickel solutions at their maxima are temperature dependent and increase

with increasing temperature. Between 17 and 37 °C

the temperature coefficients are 0.18 and 0.14 percent

per °C at 512 and 394 nm, respectively. To explain

their nuclear magnetic resonance data on cobalt(II)

solutions, Swift and Connick [33] have suggested that

the octahedral Co(H2 0)6+2 complex is in equilibrium

with the tetrahedral Co(H2 0)|2 complex and that for- mation of the latter is favored by increasing temperature. By analogy with halide complexes, the tetrahedra 1

AA

0.010 -

400 500

WAVELENGTH, nm

600

FIGURE 11. Differential spectrum of cobalt sulfate versus cobalt perchlorate; 10 mm cuvets; pH = 1.

configuration would be expected to be approximately one hundred times more absorbing than the octahedral

complex. A very small shift in this equilibrium could

result in a relatively large change in absorbance and

thereby account for the large temperature coefficient

of this system. The same explanation may also be

valid for nickel solutions.

C. Liquid Filters of Improved Optical Neutrality

Two approaches have been used in an attempt to improve the optical neutrality and extend the wavelength range of liquid filters. One is based on the use

of empirical mixtures, while the other employs watersoluble, organic "black" dyes. In 1946, Thomson [34] described the preparation

of a grey inorganic solution consisting of chromic, cobaltous and cupric sulfates and potassium dichromate.

This solution, however, does not transmit below 300 nm. To extend the wavelength range farther into the

ultraviolet, a second exploratory solution (henceforth

referred to as the NBS composite) was prepared in

which the cupric and potassium dichromate components of the Thomson mixture were replaced by p- nitrophenol. The absorption spectra of these solutions are compared in figure 12 to a commonly used glass

filter (Chance ON-10). The NBS composite was pre- pared in dilute sulfuric acid and had a pH of 1. No

fluorescence was observed under these conditions.

Over the range 250-600 nm, the NBS composite

exhibits several broad maxima and minima whose

absorbances are less dependent on spectral bandpass

than the Chance ON-10 glass. Examples are given

in figure 13 at two wavelengths for each.

The possible use of either Thomson solution or the NBS composite as an absorbance standard is limited by two factors, both of which arise from the

presence of chromium (III) in these mixtures. First,

61
<O

COm

1.0

0.5

I CHANCE ON -10 THOMSON SOLUTION NBS COMPOSITE

\

300 400

WAVELENGTH , n m

500

Figure 12.

0.340

Absorption spectra of Thomas solution,

composite and a Chance ON-10 glass.

600

NBS

0.330

0.320

g 0.310 o

CO

CD< 0.300

0.290 -

0.280

"1 T

ON-10 (445 nm)

0N-I0(560 nm)

NBS (460 nm)

I I 10 20 30 40 50 60

SPECTRAL BANDPASS, nm

70 80

Figure 13. Variation of absorbance with spectral bandpass for NBS composite and Chance ON-10 glass.

an initial "aging" of 6-8 weeks period is needed in

order to achieve adequate spectral stability at room

temperature. Secondly, and even more critical for our purpose, is that the absorbances of the aged

solutions exhibit a nonreversible temperature dependence. They must therefore always be stored at the temperature at which they were aged.

Hall and Eyring [35] have studied the constitution

of chromic salts in aqueous solution and have suggested that during the aging process Cr(H2 0)J 3 ions

polymerize to yield species of the type

r

I (H 2 0)5 Cr—O—

H

Cr(H r 2 0)5 J

,

[(H2 0) 4 Cr

and

(H2 0)5 H

0—Cr(H2 0)4- H

r(H2 0)„] +4

-O—Cr(H2 0)5

in which the observed increase in acidity is explained

by the formation of —OH bridges. Recent studies [36]

have shown that temperature, initial pH and the type

of anion present significantly affect the equilibrium

composition. Although refluxing can reduce the aging

period from several months to several hours, this is of no practical advantage since the high temperature

products, on cooling, are no longer stable. In addition to the investigation of empirical mixtures,

several water-soluble organic dyes have also been

studied. Some typical absorption spectra are shown

in figure 14. All of these dyes have high absorptivities

and the concentrations used were of the order of 50

milligrams per liter.

<

CD

<Xo

00<

0.5 -

-- NEOLAN BLACK - CIBALAN BLACK — ALIZARIN LIGHT GREY \ LUXOL FAST BLACK L /

\ NIGROSINE

..

200 300 400

WAVELENGTH , nm

500 600

k

Figure 14. Absorption spectra of aqueous solutions of selected dyes.

Cibalan Black [37], Neolan Black [37] and Alizarin

Light Grey [38] were selected for further evaluation.

Aqueous solutions of these dyes were found insensitive

to pH over the range 2-9, not affected by heat and

relatively light fast. Solutions continuously exposed

to fluorescent light exhibited an absorbance change of about 1 percent over a period of two months. These

initial results appeared sufficiently promising to warrant purification and more extensive testing.

Members of the Organic Chemistry Section have

assisted in this effort. The purification of these dyes

has proven extremely difficult, however, and yields of

only a few hundred milligrams of each have been

obtained. Much larger quantities are needed if these

dyes are to be used as Standard Reference Materials.

More efficient purification procedures are presently

being sought.

D. Standard Reference Material (SRM) 931

The first liquid absorbance standard to be issued by

the National Bureau of Standards is SRM 931. The I preparation and certification of this Standard Ref- erence Material are described below.

The filters were prepared by dissolving high purity

cobalt and nickel in a mixture of nitric and perchloric

acids. The weights of cobalt and nickel were chosen so that the absorbances of their aquo complexes were

approximately equal at their absorption maxima.

The absorbance of nitrate was adjusted to a comparable level by evaporation and subsequent addition

62
400 500

WAVELENGTH, nm

600 700

FlCURE 15. Typical absorption spectrum of SRM 931.

of small amounts of nitric acid. The final pH of these

mixtures was about 1. An absorption spectrum of a typical filter is shown

in figure 15. The peaks at 302 and 512 nm are due to absorption by NOj and Co(H2 0)^2

, respectively.

The peak at 395 nm and the plateau at 650-700 nm

are due to Ni(H2 0)+ 2

. The filters are supplied in 10 ml ampoules which

have been prescored for easy opening. An SRM unit

consists of three sets of filters, each set containing

a "Blank" (0.1N perchloric acid) and three absorbance

levels of the filter. Each set is packaged in an individual

tray for added convenience.

The filters were calibrated by randomly selecting

ampoules from each lot. The certified absorbances

are as follows:

Table 13. Certification of NBS SRM 931

Net absorbance3

Filter

Wavelength and (Bandpass), nm

302(1.0) 395(1.7) 512(2.0) 678(6.5)

"A' - 'Blank' 0.307 ±0.003 0.304 ±0.003 0.303 ±0.003 0.1 15 ±0.002

"B" -'Blank' 0.608 ±0.005 0.605 ±0.005 0.606 ±0.005 0.229 ±0.003

"C" - 'Blank' 0.906 ±0.007 0.907 ±0.007 0.911 ±0.007 0.345 ±0.003

a Net absorbances "A" — 'Blank,' "B" —'Blank,'

mm cuvets at 25.0 °C.

and "C" — 'Blank' were determined using 10.00

The uncertainties are given at the 95 percent confidence level and include a possible systematic error of

±0.5 percent.

Absorbances are certified at 25.0 °C. Absorbances

at other temperatures in the range 17 to 37 °C may be

r

' calculated using the formula:

At = A25[l + CA (t-2S)]

where: At —Absorbance at temperature t (°C)

A25 — Absorbance certified at 25.0 °C

Ca — Fractional change in absorbance per °C

.11

The values of Ca at the four wavelengths are given

below.

A, nm CA

302 -0.0014

395 + 0.0014

512 + 0.0018

678 + 0.0014

The absorbances of these filters will also depend on

the spectral bandpass used. To insure that the measured absorbances do not differ by more than ±0.1 percent from the true values, the effective spectral

bandpass should not exceed 1.5, 2.0, 3.3, and 8.5 nm at 302, 395, 512, and 678 nm, respectively.

IV. Conclusion

The availability of well characterized materials

from which the analyst prepares his own solutions is preferable to the issuance of ampouled liquids.

Although the former are not as convenient to use, they

offer potentially greater accuracy because of their

better stability.

The potassium dichromate and potassium hydrogen

phthalate used in this study are issued by NBS as primary oxidimetric and acidimetric standards (SRM

136c and 84g, respectively). The potassium nitrate is presently issued as a thermal analysis standard

(SRM 756). A product of comparable purity may be

63
obtained by repeated recrystallization of reagent-grade

material from distilled water. The cobalt and nickel

solutions are best prepared from the high purity metals

which are commercially available.

To certify absorptivities for these materials, one must resolve the question as to how well these systems

obey Beer's law. For acidic potassium dichromate, the

increased dimerization of HCrO^ to Cr2 Of with

increasing chromium concentration primarily accounts

for the relatively large deviations observed. With

potassium nitrate, one can only speculate that ionic

interactions are enhanced at the higher concentrations. In neither instance are these deviations suffi- ciently large to limit the usefulness of these materials

as liquid absorbance standards. Such deviations do

require that the absorptivity data be used with proper

precautions.

Additional studies are needed to explain the 0.2-

0.3 percent deviations from Beer's law obtained for

the various concentrations of alkaline chromate, potassium hydrogen phthalate, cobalt perchlorate and

sulfate, and nickel perchlorate and sulfate. Although

internal multiple reflection effects appear to be of the

proper magnitude to account for these differences,

initial attempts to measure such effects with magnesium fluoride-coated cuvets have not been successful. While extremely durable, magnesium fluoride

reduces reflections by about 50 percent and more

efficient coatings will probably be needed to resolve

this problem. If internal reflections are responsible

for the observed deviations, the absorptivity data

given should not be corrected when using liquid

absorbance standards for checking the accuracy of

the photometric scale since these reflections are included in every measurement. However, in the

determination of molar absorptivity, which is con- sidered a fundamental property of the material, appropriate corrections should be applied. Until this

question can be resolved, it is recommended that

these materials be used at concentrations which yield an absorbance of at least 0.5 when measured in a 10

mm cuvet. With this restriction, it is believed that the

uncertainty of the absorptivity data does not exceed

±0.5 percent at the 95 percent confidence level.

The authors wish to express their appreciation to Radu Mavrodineanu for providing the calibrated

Schott filters and to David S. Bright for performing

the computer calculations on the HCrOr/C^Of

equilibrium.

V. References

[1] Gibson, K. S., Nat. Bur. Stand. Circular 484 (1949).

[2] Harrison, G. R., M.I.T. Wavelength Tables (John Wiley & Sons,

Inc., New York, N.Y., 1969).

[3] Meehan, E. J., 'Treatise on Analytical Chemistry', Kolthoff,

I. M., and Elving, P. J., Editors, Part 1, Volume 5, Chapter 54

(John Wiley & Sons, Inc., New York, N.Y., 1964).

[4] Hardy, A. C, and Young, F. M., J. Opt. Soc. Am. 39 , 365

(1949).

[5] Eberhardt, W. H.J. Opt. Soc. Am. 40, 172 (1950).

[6] Broderson.S.J. Opt. Soc. Am. 44,22(1954).

[7] Hogness, T. R., Zscheile, F. P., and Sidwell, A. E.. J. Phys.

Chem. 41,379(1937).

|8] Perry, J. W., Photoelectric Spectrometry Group Bull. 3, 40

(1950).

[9] Slavin, W., Anal. Chem. 35,561 (1963).

[10] Poulson, R. E., Appl. Opt. 3,99(1964).

[11] Goldring, L. S., Hawes, R. C, Hare, G. H., Beckman, A. O., and Stickney, M. E., Anal. Chem. 25, 869 (1953).

[12] British Standard 3875, British Standards Institution (1965).

[13] Mavrodineanu, R.. Nat. Bur. Stand. (U.S.), Tech. Note 584, 178 pages (1971), O. Menis and J. I. Schultz, Eds.

[14] Vandenbelt, J. M., Forsyth, J., and Garrett, A., Anal. Chem. 17,

235 (1945).

[15] Haupt, G. W., J. Opt. Soc. Am. 42, 441 (1952).

[16] Haupt, G. W., J. Res. Nat. Bur. Stand. 48, 414 (1952).

[17] Vandenbelt, J. M., and Spurlock, C. H., J. Opt. Soc. Am. 45,

967 (1955).

[18] Gridgeman, N. T., Photoelectric Spectrometry Group Bull. 4, 67(1951).

[19] Ketelaar, J. A. A., Fahrenfort, J., Haas, C, Brinkman, G. A., Photoelectric Spectrometry Group Bull. 8, 176 (1955).

[201 Va.idenbelt, J. M., J. Opt. Soc. Am. 50,24(1960).

[21] Ringbom, A., Complexation in Analytical Chemistry, (John

Wiley & Sons, Inc., New York, N.Y., 1963), p. 294.

[22] Neuss, J. D., and Rieman, W., J. Am. Chem. Soc. 56, 2238

(1934).

[23] Howard, J. R., Nair, V. S. K., and Nancollas, G. H., Trans. Faraday Soc. 54, 1034(1958).

[24] Tong, J., and King, E. L., J. Am. Chem. Soc. 75, 6180 (1953).

[25] Davies, W. G., and Prue, J. E., Trans. Faraday Soc. 51, 1045

(1955).

[26] Haight, G. P., Richardson, D. C, and Coburn, N. H., J. Inorg. Chem. 3,1777(1964).

[27] Johnson, E. A., Photoelectric Spectrometry Group Bull. 17, 505 (1967).

[28] Edisbury, J. R., Photoelectric Spectrometry Group Bull. 1. 10(1949).

[29] Edisbury, J. R., Photoelectric Spectrometry Group Bull. 16,

441 (1965).

[30] Malitson, I., private communication.

[31] Rains, T. C, private communication.

[32] Ewing, G. W., and Parsons, T., Anal. Chem. 20, 423 (1948).

[33] Swift, T. J., and Connick, R. E., J. Chem. Phys. 37. 307 (1962).

[34] Thomson, L. C, Trans. Faraday Soc. 42 , 663 (1946).

[35] Hall, H. T., and Eyring, H., J. Am. Chem. Soc. 72 , 782 (1950).

[36] Burke, R. W., and Deardorff, E. R., Nat. Bur. Stand. (U.S.), Tech. Note 544, 153 pages (1970), O. Menis and J. I. Schultz, Eds.

[37] Ciba Chemical and Dye Company, Fair Lawn, New Jersey.

[38] Industrial Chemicals Division, Allied Chemical Corporation,

Buffalo, New York.

(Paper 76A5-733)

64
APPENDIX III

Standard Reference Materials:

Standard Quartz Cuvettes

For High Accuracy Spectrophotometry

R. Mavrodineanu

Analytical Chemistry Division

Institute for Materials Research

and

J. W. Lazar

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued December 1973

65
Library of Congress Catalog Number: 73-600200

National Bureau of Standards Special Publication 260-32

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-32, 26 pages (Dec. 1973)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1973

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402

(Order by SD Catalog No. CI 3. 10:260-32). Stock No. SN 003-003-0 121 3-1. Price 55 cents.

66
CONTENTS

PAGE

I. INTRODUCTION 1

II. EXPERIMENTAL 2

III. APPLICATIONS 6

IV. REFERENCES 17

LIST OF TABLES

Table No. PAGE

1. Radiation pathlength expressed in mm and

determined on 17 quartz cuvettes made at NBS. ... 5

2. Radiation pathlength expressed in mm and

determined on 10 quartz cuvettes made by a

commercial manufacturer 7

LIST OF FIGURES

Figure No. PAGE

1. Fused silica material and parts used for

constructing the cuvettes for high- accuracy

spectrophotometry . . . 11

2a. Standard instrument for measuring the thickness

of plates and length of quartz segments ...... 12

2b. Close-up of figure 2a 13

3. General shape and nominal dimensions (mm) of

NBS quartz standard cuvette for spectrophotometry 14

4a. General view of the standard instrument used to

measure the radiation pathlength of cuvettes. ... 15

4b. Close-up of figure 4a 16

67
STANDARD REFERENCE MATERIALS:

STANDARD QUARTZ CUVETTES FOR HIGH ACCURACY SPECTROPHOTOMETRY

R. Mavrodineanu

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

and

J. W. Lazar

Optical Physics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Accurate knowledge of lightpath and parallelism of

cuvettes used in spectrophotometry is one of the indispensable parameters which must be determined when

accurate transmittance measurements of liquid materials

is considered. A description is given of the design

and techniques developed at NBS for the production of

quartz cuvettes having a nominal radiation pathlength

of 10 mm +_ 0.03 mm. For each cuvette the pathlength and

parallelism are certified with an uncertainty of +_ 0. 0005

mm. The method and instrumentation used to determine

these parameters is also described in the paper.

Key words: Cuvette, spectrophotometry; lightpath;

pathlength; quartz, cuvette; radiation pathlength.

68
I. INTRODUCTION

True transmi ttance values can be obtained only when

using accurate measuring techniques and by taking into

consideration all factors which can affect and distort

the data [1]. Since transmittance is the ratio. of two

radiation flux intensities, it is necessary that the

photometric scale of the spectrophotometer used be

accurate. Other important conditions which must be

satisfied are: wavelength accuracy, use of adequate

spectral bandpass, use of collimated radiation, freedom

from reflections (interreflection) , fluorescence, polarization,

light scatter, optical interferences, particulate matter.

Surface conditions and the temperature at which the

measurements are performed must be defined, and the material

subjected to measurements must be homogeneous (freedom

of strain) and stable.

When liquid samples are measured the pathlength of

the radiation passing through the solution must be

accurately known. This last condition is particularly

important, not only when accurate transmittance measurements are contemplated, but also when the molar absorptivity

of a chemical species is sought. In these cases, an uncertainty of 0.1 mm in a nominal radiation pathlength of

10 mm for instance, will result in an error of 1 percent

in the absorbance measurements, and, similarly, an uncertainty of 0.01 mm will result in an error of 0.1 percent

absorbance. The object of the work discussed in this

paper is to describe the procedures developed and used

in the construction of quartz cuvettes having a nominal

radiation pathlength of 10 mm + 0.03 mm, parallelism

between the two transparent plates within 0.002 mm,

plate flatness of 2 to 3 fringes (mercury green line)

and parallelism of these plates within 0.002 mm over

the whole length of the plates.

69
Additional conditions to be met were free from

fluorescence and strain, and the use of fused silica of

optical grade.

The fulfillment of these specifications would

practically eliminate the radiation pathlength error when

absorbance measurements are performed at an uncertainty

level of 0.1 percent. Such an accuracy seems to be

at the limit obtainable *by the present state of the art

when measurements of absorbance are performed on liquid

samples, and include the errors of sample preparation and

handling, as well as the instrument uncertainty.

II. EXPERIMENTAL

Several designs were contemplated for constructing

quartz cuvettes which would satisfy the rather tight

specifications mentioned above. The technique illustrated

in figure 1 was finally selected, and was developed in

association with E. P. Muth of NBS Optical Shop, who also

made all the parts required for the cuvettes discussed in

this work. The final assembly was performed by E. I. Klein

from NBS Glassblowing Shop.

A rectangular- shaped fused-silica structure 48.0 mm

high, 13.5 mm wide and 300 mm long was used to make the body

of the cuvettes. Segments approximately 13 mm long were

cut from this material and ground and polished, using conventional optical shop techniques, to produce a rectangular

segment 10 mm +_ 0.03 mm long. The rectangular edges of

each segment were finished to a flatness of 1 to 2 fringes

(mercury green line) , and at one end a small orifice wasprovided to permit air expansion during the assembly

operation.

The length of every segment was checked for accuracy

with a standard measuring instrument, illustrated in

70
figure 2a and b, capable of determining this length with an

accuracy of +_ 0.5 ym.

Two plates made of non-fluorescent fused silica 51 mm

long, 16 mm wide and 2 mm thick were ground and polished

in a similar manner to a width of 12.5 mm, a thickness

of 1.1 mm with a parallelism of 0.002 mm and a flatness

of about 2 fringes (mercury green line) as shown in figure 1.

These plates were checked for thickness with the instrument illustrated in figure 2a, b and were attached to the

rectangular body by careful welding with city gas-oxygen

torch using a needle shape flame.

The top of the assembled unit was then cut to produce

a cuvette 43 mm high, and a rectangular fused silica block

of 6, mm was attached to the open end of the cuvette by

fusion. This block was provided with a standard tapered

opening with a diameter of 10 mm at the top. A standardtaper Teflon stopper was used to provide tight closure of

the cuvette (fig. 1) . The cuvette was finished by

grinding the two side walls and bevelling the edges. Its

nominal dimensions are given in figure 3. The unit was

stress-released by proper annealing.

The cuvette was then measured to determine the radiation

pathlength and the parallelism of the unit. For this purpose

the instrument illustrated in figure 4a and b was used, all

measurements being performed at a temperature of 20° +_

0 o 05 °C. The measurements were made as described below.

The cuvettes were lightly clamped in a fixture which

was bolted to the table of a Moore No. 3 Measuring Machine*

The identification of commercial instruments and products

is given only to permit reproduction of the work described in

this paper. In no instances does such identification imply

recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the particular equipment or

product is necessarily the best available for the purpose.

71
and were adjusted until the two plates which define the

radiation path length were vertical within the limits of

their parallelism. An electronic lever type gage, with

a resolution of 0.125 ym was mounted on the spindle. To

reach the bottom of the cuvette, a longer lever was

fabricated and mounted on the gage. Use of the longer

lever required that a correction factor be applied to all

measurements which decreased the resolution of the gaging

system to 0.312 ym.

As shown in Figure 4b, a combination of gage blocks

and end plates were wrung together so that the distance

between the inside faces of the end plates was 10.0000 +_ 0.0001 mm. The gage blocks were then positioned beneath

the center of the spindle and the gage lever was offset

from the center of rotation of the spindle so that when

the spindle was rotated 180° and the lever contacted

' each end plate the meter readout of the gaging system was

at its null (zero) position.

The machine table was then moved to position the

cuvette beneath the spindle. The spindle was lowered

until the gage lever was at its first measuring position

inside the cuvette. The table was moved until the lever

contacted the face of the cuvette and the meter was at its

zero position. The spindle was rotated 180° until the

gage lever contacted the opposite side of the cuvette. The

meter reading multiplied by the correction factor plus the

nominal 10 mm path length gave the true pathlength at

that point. Measurements were made approximately every 4 mm

by lowering the spindle.

Seventeen quartz cuvettes were made and measured at

NBS according to the procedures described in this paper,

and the values found are presented in table 1.

72
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73
From these data it can be concluded that only three

of the 17 cuvettes, or less than 18 percent of the NBS

cuvettes, were below the specifications established for

acceptance

The same measurements were performed on 10 quartz

cuvettes of commercial origin. The results assembled in

table 2 show that only 4 of the 10 cuvettes were acceptable

by the standards established in this paper, which indicate

a rejection of 60 percent. It must be noted, however, that

9 of 10 cuvettes were within the specifications of the

manufacturer which indicate a tolerance of nominal lightpath of 10 mm +_ 0.01 mm.

III. APPLICATIONS

As a result of these measurements, it was decided to

produce at NBS a number of quartz cuvettes, identical

to those described in this work, and to certify them

for radiation pathlength and parallelism. These cuvettes

would then constitute a Standard Reference Material (SRM)

in spectrophotometry, and be made available to the public

as SRM 932. Each cuvette will have a certificate of

calibration of which a reproduction is given here.

74
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75
National Bureau of Standards

CERTIFICATE

STANDARD REFERENCE MATERIAL 932

QUARTZ CUVETTE FOR SPECTROPHOTOMETRY

R. Mavrodineanu and J. W. Lazar

This Standard Reference Material is intended as a

reference source when production of accurate spectrophotometry data on liquid samples is considered. It

consists of an all-quartz rectangular cuvette designed

to fit the holder of conventional spectrophotometers. It

is provided with two optically transparent and parallel

windows defining a nominal radiation pathlength of 10 mm

+_ .03 mm. The inner surfaces of the windows are parallel

within .002 mm. The pathlength and parallelism are

certified with an uncertainty of +_ 0.0005 mm as determined

by measurements at positions equally spaced 2 mm from the

bottom to within 6 mm of the stopper top. These measurements are made at 20 °C, and the distance between each

position is about 4 mm.

Position of

Measurement

Radiation

Pathlength, mm

Bottom

Top

9

8

7

6

5

4

3

2

1

9.999

9.999

10.000

9.999

10.000

9.999

9.998

9.998

9.998

9.997

9.997

76
The general shape and nominal dimensions of the

cuvette are illustrated in the figure.*

The cuvette must be handled with great care and should

be held only by the frosted quartz side windows. When not

in use, it should be stored in the container provided for this

purpose. Extended exposure to laboratory atmosphere and

dusty surroundings is to be avoided.

The overall direction and coordination of the technical

measurements leading to certification were performed under

the chairmanship of 0. Menis and J. A. Simpson.

The technical and support aspects involved in the

preparation, certification and issuance of this Standard

Reference Material were coordinated through the Office

of Standard Reference Materials by T. W. Mears.

J. Paul Cali, Chief

Office of Standard Reference Materials

Washington, D. C. 20234

March 1, 1973

*Note : This figure is identical with figure 3 of this paper.

77
The cuvette was designed and produced at the National

Bureau of Standards by special techniques and from nonfluorescent optical -quality fused silica. The transparent

windows are attached to the body of the cuvette by direct

fusion, and the unit was strain-released by proper

annealing. The overall flatness of the transparent windows

is within two fringes (546-nm Hg line) , and their parallelism

is within two micrometers. The radiation pathlength measurements were performed before and after the assembly of the

cuvette using electronic feeler- gage type instruments

capable^ of a resolution of 5 parts in 10 . SRM 932 is a

result of the combined efforts of the Analytical Chemistry

Division, Institute for Materials Research, the Optical

Physics Division of the Institute for Basic Standards,

and the Instrument Shops Division.

E. P. Muth and E. I. Klein designed and assembled the

cuvette respectively. The radiation pathlength measurements were performed by E . G. Erber.

SRM 932 was developed to meet the needs expressed by

the Standards Committee of the American Association of

Clinical Chemists under the chairmanship of George N. Bowers,

Jr., M. D., Hartford Hospital, Hartford, Connecticut.

This work was initiated and performed with the approval

and encouragement of 0. Menis, Section Chief, Analytical

Chemistry Division.

78
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79
80

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I J

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RADIATION

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i!

TOP

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48

(FROSTED)

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(TRANSPARENT)

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6

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Figure 3. General shape and nominal dimensions (mm) of NBS

quartz standard cuvette for spectrophotometry.

82
Figure 4a. General view of the standard instrument used

to measure the radiation pathlength of cuvettes.

83

IV. REFERENCES

Mavrodineanu , R. , An Accurate Spectrophotometer for

Measuring the Transmittance of Solid and Liquid

Materials, J. Res. Nat. Bur. Stand. (U.S.), 76A,

405-425 (1972).

Clarke, F. J. J., High-Accuracy Spectrophotometry, J.

Res. Nat. Bur. Stand. (U.S.), 76A, 375-403 (1972).

Mielenz, K. D., Physical Parameters in High-Accuracy

Spectrophotometry, J. Res. Nat. Bur. Stand. (U.S.), 76A,

455-467 (1972)..

Burke, R. W. , Deardorff, E. R. and Menis, 0., Liquid

Absorbance Standards, J. Res. Nat. Bur. Stand. (U.S.), 76A , 469-482 (1972).

Burnett, R. W. , Accurate Measurement of Molar

Absorptivity, J. Res. Nat. Bur. Stand. (U.S.), 76A , 483-489 (1972).

a U. S. GOVERNMENT PRINTING OFFICE : 1973 —5^2-651/94

85
mm
APPENDIX IV

JOURNAL OF RESEA RCH of the National Bureau of Standards—A. Physics and Chemistry

Vol. 80A, No. 4, July-August 1976

Acidic Potassium Dichromate Solutions as

Ultraviolet Absorbance Standards*

R. W. Burke and R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 18, 1976)

The absorbanees of five concentrations of potassium dichromate in 0.001 M perchloric

acid have been determined at eight wavelengths in the ultraviolet on the National Bureau

of Standards Institute for Materials Research high-accuracy spectrophotometer. Four of the

wavelengths—235, 257, 313, and 350 nm—correspond to absorbance maxima or minima in the HCr04 ~ spectrum and are useful wavelengths for checking the accuracy of the absorbance

scale of narrow bandpass spectrophotometers. Although partial dimerization of HCr04 ~

to Cr2 07= produces small positive deviations from Beer's law at these wavelengths, the ap- parent absorptivities calculated for each concentration are reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are ±0.7 percent at 0.1 absorbance (A) and ±0.2 percent near A = l. These uncertainties include all known sources

of possible systematic error and the 95 percent confidence level for the mean. The remaining

four wavelengths used for measurement are near two predicted isosbestic points in the

HCr04 ~/Cr2 07~ spectra. The absorptivities at 345 nm are sufficiently independent of concentration that this wavelength can be used for checking absorbance linearity to one part

in a thousand over the range ^4 = 0.2-1. Key words: Absorbance linearity; accuracy; acidic potassium dichromate solutions; calibra- tion of ultraviolet spectrophotometers; liquid filters; transfer standards; ultraviolet absorb- ance standards.

I. Introduction

At present, there are no certified standards

variable from the National Bureau of Standards

S'BS) for checking the accuracy of the absorbance

;iale of spectrophotometers throughout the ultraviolet. The number of analytical applications in

lis important region of the spectrum, however,

mtinues to increase relatively sharply and the

Jed for such standards is becoming increasingly

lute. The area with perhaps the most serious need for

traviolet absorbance standards at the moment is inical chemistry. Workers in this field, for example,

equently use molar absorptivity as an index of

irity of their biological standards. Only recently,

urnett [1]

1 has discussed the importance of aclrate molar absorptivity measurements in the

inical laboratory. He especially emphasizes the

3ed for spectrophotometry accuracy in clinical

lzymology. Not only must the molar absorptivity

the substrate or enzyme-catalyzed reaction

"oduct be known accurately but also the individual

Jsorbance measurements on the test sample must

5 accurate. This dual requirement for accuracy

'This work was supported in part by the National Institute of General

tdical Sciences.

: Figures in brackets indicate the literature references at the end of this paper.

in this instance arises because high-purity and wellcharacterized enzyme preparations are not yet

routinely available for use as standards.

A program has been under way at NBS in the

Institute for Materials Research (IMR) since 1969

for the development and issuance of visible and

ultraviolet transfer standards for use as Standard

Reference Materials (SRM's). Two such standards

are currently available: (1) SRM 930, consisting of a

set of three neutral Schott NG glass filters and (2)

SRM 931, an empirical inorganic solution available

at three concentrations in 10-ml ampoules. Neither of

these SRM's meets the present needs for absorbance

standards in the ultraviolet. The glass filters do not

transmit below about 350 nm while the inorganic

solution is certified only at one wavelength in the

ultraviolet (302 nm) and with an uncertainty of ± 1 percent. Transfer standards capable of being certified throughout the range of 200-350 nm and with a smaller uncertainty are being investigated, however.

The most promising solid filter at the moment for

this purpose is the metal-on-quartz type. Some

recent experiences with this filter are discussed in the

paper immediately following [2]. Of the chemical or liquid-type filters that have been proposed, the

weakly' acidic potassium dichromate system is considered the best. A critical evaluation of this

S3r stem, as well as several other candidate materials,

was presented in a previous paper [3].

214-427 0-76-6
In this paper, we present absorptivity values for

five concentrations of potassium dichromate in

0.001 M perchloric acid. These concentrations effectively span the absorbance range of A— 0.1-1.5

when measured in 10 mm cuvettes. The absorptivities were calculated from transmittance measure- ments at eight wavelengths in the ultraviolet on the

IMR high-accuracy spectrophotometer. Four of the

wavelengths—235, 257, 313 and 350 nm—correspond

to absorbance maxima and minima in the HCr04 - spectrum and are iiseful wavelengths for checking

the accuracy of the absorbance scale of narrow

bandpass spectrophotometers. Although partial dimerization of HCr04 ~ to Cr2 07 = produces small positive deviations from Beer's law at these wavelengths,

the apparent absorptivities calculated for each concentration are reproducible to one part in a thousand.

The estimated uncertainties in the absorptivity

values are ±0.7 percent at 0.1 absorbance (A) and

±0.2 percent near A=l. These uncertainties include

all known sources of possible systematic error and

the 95 percent confidence level for the mean. The

remaining four wavelengths used for measurement

are near two predicted isosbestic points in the

HCr04 _ /Cr2 07 = spectra. The absorptivities at 345 nm

are sufficiently independent of concentration that

this wavelength can be used for checking absorbance

linearity to one part in a thousand over the range

A=0.2-l.

II. Experimental Procedure

The potassium dichromate (K2 Cr2 07) used in this

study was a special lot of analytical reagent grade

material obtained from the J. T. Baker Chemical

Company.2 Drying studies performed at 105 °C

indicated that the surface moisture of this material

was less than 0.01 percent. No measurement of

possible occluded water was made. HoAvever, a recent paper [4] by Yoshimori and Sakaguchi has

shown that K2 Cr2 07 typically contains 0.01 to 0.02

percent occluded water which can be removed only

by crushing and drying at 350 °C.

Coulometric assay of the undried potassium

dichromate gave a purity, expressed as an oxidant,

of 99.972 ±0.005 percent at the 95 percent confidence level. Emission spectroscopy indicated that

the only significant elemental impurities present were sodium and rubidium. Their concentrations were estimated to be 0.02 and 0.03 percent,

respectively.

The concentrations of the K2 Cr2 07 solutions used

throughout this study are expressed on a weight/

weight basis. Milligram samples of K2 Cr2 07 were

weighed to the nearest microgram on a microbalance.

After dissolution in distilled water shown to be free

of reducing impurities (see Discussion), 1 ml of 1 M

perchloric acid was added and the solutions were

1 In order to describe materials and experimental procedures adequately, it is occasionally necessary to identify the sources of commercial products by 1he manufacturer's name. In no instance does such identification imply endorsement by the National Bureau of Standards, nor does it imply that the particular product is necessarily the best available for that purpose.

diluted approximately to volume in 1 -liter voir'

metric flasks. Each flask was fitted with a doubl<

ribbed Teflon stopper (Kontes Glass Company

Vineland, New Jersey) to prevent evaporation. Th

weight of each solution was determined on a single

pan top-loading balance having a sensitivity of 0.01 £ NBS-calibrated weights were used to establish th : accuracy of the balances. The concentrations of th solutions were then calculated after correcting a weights to vacuum. A solvent blank was prepared b

diluting 1 ml of 1 M perchloric acid to 1 liter witij

distilled water.

The Institute for Materials Research high

accuracy spectrophotometer was used for performin

the transmittance measurements. which, in turn, wei

converted to absorbance. The design and construt

tion of this instrument have been described in deta

by one of us (RM) in reference [5] and will not b] repeated here. Similarly, the quartz cuvettes use are also of NBS design and construction and ha\

been described previously [6]. These cuvettes ai currently available through the NBS Office < Standard Reference Materials as SRM 932. Eac

cuvette is certified for path length and parallelism t ±0.0005 mm.

Prior to use the cuvettes were cleaned by soakir

in concentrated (18 M) sulfuric acid for several hour

In order to minimize the heat of mixing, they wei

then transferred consecutively to 12, 6 and 3 i sulfuric acid before rinsing with distilled water. Afti

rinsing thoroughly, the cuvettes were air-dried undi an inverted Petri dish that served as a dust-protecti^ cover. For sample measurements, five calibrated cuvetti were placed in separate holders in the rotatin

sample compartment [5] of the IMR high-accurac

spectrophotometer and a reference filter (incone

on-quartz) was placed in a sixth position. Tl

cuvettes were left in their respective holders f( the duration of the experiment. All transmittanci!

absorbance measurements were made relative t air in a temperature-controlled room at 23.5 ±0

°C. The transfer of solvent blank and sample soh

tions to and from these cuvettes was made by meai

of borosilicate, Pasteur-type, disposable pipette

After being rinsed with the test solution, a fin transfer for measurement could be made in 10-1

seconds, after which time the cuvette was imm

diately stoppered with a snugly-fitting Teflc

stopper.

Six sets of solutions having nominal concentr;

tions of 20, 40, 60, 80 and 100 mg K2 Cr2 07/l were prepared. Each concentration within a si] was measured a minimum of six times at the eigl

wavelengths of interest. The absorbance, A= —icj (^sampie/TBiank), was computed for each wavelengl

and concentration from the average of the s transmittance measurements (T). Absorptiviti were then calculated after correcting the absorbanc

for systematic errors due to cuvette path lengt

beam geometry and internal multiple reflection

No correction was applied for the reflections di cussed by Mielenz and Mavrodineanu [7] fro

88
internal components such as the lenses and slit jaws because these are adequately compensated

for by the blank. A detailed account of these corrections and calculations will be given in an NBS 260

Special Publication which is now in preparation.

III. Discussion and Results

The ultraviolet absorbance spectrum of a 40 mg

kg-1 solution of potassium dichromate in 0.001 M

perchloric acid is shown in figure 1. The four wavelengths selected for certification of absorptivity of

this absorbing system, namely 235, 257, 313 and

350 nm, are also indicated. The maxima and minima

are sufficiently broad that serious restrictions are

not placed upon instrumental spectral bandwidth

requirements. The half bandwidths of the 257 and

1 350 nm peaks, for example, are approximately 60

1 nm so that an instrumental spectral bandwidth of

3 nm or less is sufficient for obtaining at least 0.999

'of the maximum peak intensities.

1.0 I i i

K

2 Cr2 0 y = 40 mg/kg

257

I

350

i

\ 313 / \

i i 250 300 350 WAVELENGTH, nm

Figure 1. Absorbance spectrum of K2 Cr2 07 in 0.001 M perchloric acid.

The first experiment performed on the IMR highaccuracy spectrophotometer was the determination

of the rinse behavior and reproducibility of the

transmittance/absorbance measurements of the solvent blank. In the initial studies, measurements

were made at 235 nm only, since any problems

associated with the transfer of solution were expected

to be the greatest at the shortest wavelength used.

The results obtained for the five cuvettes employed

are shown in figure 2. All exhibit a similar rinse

pattern, attaining a minimum and constant absorb- ance value after 5-6 rinses (the term rinse as used

here and throughout the remainder of this paper

refers to the exchange of one cuvette volume for a second one). Once the blank measurements were in

control, similar absorbance measurements were

then made on the first series of K2 Cr2 07 solutions.

Five concentrations having nominal concentrations

of 20, 40, 60, 80 and 100 mg K2 Cr2 07/kg were

measured. Table I summarizes the results of these

measurements together with the final solvent blank

values for 235 nm. The cycle of measurements shown

here was then extended to the other seven wave4.90 1

1 2 3 4 5 6 7

NO. OF RINSES

Figure 2. Apparent absorbances of the blank solvent in the five curvettes used—Nos. 14, 16, 18, 25, and 26 ref. [6].

Table I. Reproducibility of absorbance measurements at 235 nm for a cycle of solvent and sample runs

Solvent Sample Sample Solvent

Cuvette No.

After 5 rinses After 5 rinses After 7 rinses After 5 rinses

14 0. 05198 0. 30128 0. 30141 0. 05213

16 . 05099 . 55052 . 55057 . 05093

18 . 05310 . 80066 . 80074 . 05321

25 . 05002 1. 04863 1. 04855 . 05022

26 . 05064 1. 30351 1. 30333 . 05065

Ref. filter «

. 55913 . 55925 . 55936 . 55922

° Inconel-on-quartz.

89
lengths of interest. In all, six sets of K2 Cr2 07 solutions were measured. Typically, two volleys consisting of three transmittance/absorbance measurements

were made on each of the 30 solutions. The first volley was made after rinsing the cuvettes, initially

containing solvent, with five rinses of sample and was repeated again after two additional rinses.

Since no systematic increase in absorbance was ever observed when the results of the first volley of

measurements were compared to the second, the six

measurements were invariably averaged. The absorbances were obtained by subtracting the solvent

blank values from the sample readings. After applying appropriate corrections for beam geometry and

internal multiple reflections, the corrected absorbances C4 corr) were used to calculate the desired

absorptivities, using the relationship

Absorptivity =^4corr/6c,

where b =internal light path in cm and c = concentration of K2 Cr2 07 solution in g kg-1

. The absorptivities

computed for the five concentrations of K2 Cr2 07 solutions used are summarized in table II. The values

tabulated were determined from least-squares plots

of the experimental values and were subsequently

normalized to the concentrations shown. The

uncertainties given include all known sources of

possible systematic error and the 95 percent (2a)

confidence interval for the mean. The random

component of these uncertainties, based on standard

deviations computed from residuals resulting from

fitting the data to the various concentration levels

for each wavelength, is 0.07 percent at the 2a level. It is observed that all absorptivities in table II increase with increasing K2 Cr2 07 concentration.

These deviations from Beer's law are produced by the

fact that, in weakly acidic media, chromium (vi) ions

exist as two distinct absorbing species—HCrOr and

its dimerization product, Cr2 07 =

. The equilibrium between these two species ma,y be represented as

and the corresponding dimerization constant, KD , is given by

[Cr2 Q7 =]

D ~[HCr04 -] 2

• (2)

Although eq. (2) predicts that the formation of Cr2 07 =

is strictly a quadratic function of K2 Cr2 07 concentration, the value of i^D =32.9 (mol kg-1

) -1 obtained

previously [3] is of such magnitude that the percentage of total chromium present as the Cr2 07 = ion is very nearly a linear function of the K2 Cr2 07 con- centration for the range of solutions studied. Calculated values of HCr04 _/Cr2 07 = speciation in this

system are given in table III.

Table III. HCrO^/C^Or speciation in 0.001 M perchloric

acid solutions of K2 Cr2 07.

K2 Cr2 07 Percent Cr as Percent Cr as Cone, g kg-' HCr04 - Cr2 07 =

0. 020 99. 10 0. 90

. 040 98. 22 1. 78

. 060 97. 38 2. 62

. 080 96. 56 3. 44

. 100 95. 77 4. 23

2HCrOr =Cr207 =+H2 0 (1)

The direction and magnitude of the expected

deviations from Beer's law for the acidic potassium

dichromate system can be ascertained when the

speciation data in table III are combined with the

spectral characteristics of the two chromium (vi)

species. Figure 3 shows the relation of the absorbance

spectra of the HCrO\T and Cr2 07 = ions. At wavelengths of 235, 257, 313 and 350 nm, the Cr? 07 = ion

is shown always to have a larger absorptivity than

the HCrOr ion. Hence, the measured absorptivities

at these wavelengths should increase with increasing

K2 Cr2 07 concentration.

From the same argument, the data in figure 3 also

suggest that there are two wavelengths near 320 and

345 nm where Beer's law is obeyed. In an attempt to determine these isosbestic points experimentally,

absorbance measurements were also made on the

Table II. Absorptivities of K2 C2 07 in 0.001 M perchloric acid at 23.5 °C

Absorptivity, kg g-'em"1

K2 Cr2 07 Cone, g kg"1 235(1. 2)«nm 257(0.8)nm 313(0.8)nm 350(0.8) nm Uncertainty c

"0. 020 12. 243 14. 248 4. 797 10. 661 0. 034

. . 040 12. 291 14. 308 4. 804 10. 674 d

. 022

. 060 12. 340 14. 369 4. 811 10. 687 d

. 020

. 080 12. 388 14. 430 4. 818 10. 701 d

. 020

. 100 12. 436 14. 491 4. 825 10. 714 d

. 019

a Nominal concentration; all weights corrected to vacuum.

b Wavelength and, ( ), spectral bandwidth.

e Includes estimated systematic errors and the 95 percent confidence interval for the mean.

d For wavelength of 313 nm, the uncertainty is reduced to half of these values for K2 Cr2 07 concentrations of 0.040, 0.060,

0.080 and 0.100 g kg-'.

90
4000

> 2000

5

os 250 300 350 WAVELENGTH, nm

400

Figure 3. Absorbance spectra of the HCr04 _

ion and its dimerization product, Cr%0^

same solutions used above at 322, 323, 345 and 346 nm. The corresponding absorptivity values are given

in table IV. Although small systematic deviations

from Beer's law are still present, the absorptivities

calculated for 345 nm are sufficiently constant that

the acidic K2 Cr2 07 system can be used at this wavelength, over the concentration range shown, to check

the absorbance linearity of narrow bandpass spectrophotometers to one part in a thousand.

The variation of absorptivity of the acid K2 Cr2 07 system with temperature is the smallest that we

have observed for any liquid filter that we have

studied to date. For the four wavelengths recom- mended for checking the accuracy of the absorbance

scale—235, 257, 313, and 350 nm—the absorptivities

decrease with increasing temperature. Over the range

20-30 °C, the corrections are, respectively, —0.05,

—0.05, —0.02, and —0.05 percent per degree Celsius.

The —0.02 percent correction found at 313 nm cor- responds closely to the correction predicted from the

thermal expansion of the solvent. Until further evaluation can be made of the temperature dependence

of 345 nm wavelength recommended for checking

absorbance linearity, measurements should be restricted to 23.5 ±1 °C.

Several considerations were involved in the selection of perchloric acid for acidifying the K2Cr2 07 solutions. First, perchloric acid was preferred over

the sulfuric acid which has been used traditionally

because perchlorate ion, unlike sulfate, has no tendency to form mixed complexes with chromium (vi)

species. Secondly, the choice of 0.001 M acid rather

than 0.01 Mwas based on two factors : (1) the 0.001 M

acid represented a practical compromise for minimizing conversion of HCr04 ~ to either H2 Cr04 or Cr04 = and (2) the lower acid concentration substantially lowered the oxidation potential (~13 percent)

of the chromium (vi) ions and hence improved the

solution stability of the proposed K2 Cr2 07 standards.

A final point that needs to be emphasized concerns

the quality of the distilled water used for preparing

the standard solutions. Acidic potassium dichromate

is a potential oxidant and consequently the distilled

water used must be shown to be free of reducing

impurities in order that the absorptivity data given

in this paper be valid. A simple but yet definitive

test is outlined in figure 4. In this study a 500-fold

dilution of a 100 rag kg-1 standard solution of

K2 Cr2 07 was made with the distilled water in question and its absorbance measured at 350 nm. Not

only did the measured absorbance agree with the

predicted value, thus indicating no reduction of

chromium (vi), but also subsequent measurements

showed that this K2 Cr2 07 solution standard should

be stable for at least two months provided it is adequately protected against evaporation. In instances

where this test shows the distilled water to be sus- pect, it is recommended that the water be redistilled

from alkaline potassium permanganate in order to preoxidize the impurities.

Finally, the general use of solid and liquid transfer

standards for calibrating the absorbance scale of a

precision commercial spectrophotometer is illustrated in figure 5. The measurements on the glass

0.0025 r

0.0020

CD E £o Io 0.0015

vi —

m.

500—FOLD DILUTION OF 100 mg K

2Cr

2 0/kg

2 4 6 8 10 12 14 ^' I 30 DAYS AFTER PREPARATION

m

Figure 4. Test of the distilled water for reducing impurities.

Table IV. Absorptivities of K2O2O7 in 0.001 M perchloric acid near two predicted isosbestic wavelengths; temperature 23.5 °C

Absorptivity, kg g

_1 cm-1

K2 Cr2 07 322 (0.8) b nm 323 (0.8) nm 345 (0.8) nm 346 (0.8) nm

Cone, g kg"1

» 0. 020 5. 845 6. 065 10. 593 10. 615

. 040 5. 842 6. 062 10. 595 10. 621

. 060 5. 838 6. 059 10. 598 10. 627

. 080 5. 835 6. 056 10. 600 10. 632

. 100 5. 831 6. 053 10. 602 10. 638

"Nominal concentration; all weights corrected to vacuum.

b Wavelength and, ( ), spectral bandwidth.

91
8r

ABSORBANCE

Figure 5. Correction AA for the absorbance scale of a precision

commercial spectrophotometer.

filters were performed at 440 nm while the K2 Cr2 07 measurements were made at 350 nm. At both wavelengths the absorbances measured on the precision

spectrophotometer are high and should be corrected

by subtracting the appropriate AA values.

IV. Conclusion

Following widespread use in the collaborative

testing of spectrophotometers for more than 25 years,

the absorptivities of the acidic potassium dichromate

system have now been determined with a known

accuracy from measurements performed directly on

the NBS Institute for Materials Research highaccuracy spectrophotometer. Later this year, crystalline K2 Cr2 07 , together with detailed instructions on

preparing solutions from it, will be available from

NBS through its Office of Standard Reference

Materials as an SRM. In conjunction with the cali- brated quartz cuvettes previously issued (SRM 932),

it is believed that this material will provide a valid

check of the accuracy of the absorbance scale of narrow bandpass spectrophotometers in the ultraviolet from 235-350 nm.

The authors gratefully acknowledge the assistance

of the following persons at NBS during various

phases of this work: K. D. Mielenz, for helpful

discussions of fundamental spectrophotometric parameters; G. Marinenko, for performing the cou- lometric assay; J. A. Norris, for the the emission

spectrographic analyses; H. H. Ku, for the statistical

analysis of the data; and R. W. Seward, for supplying through the Office of Standard Reference Materials the samples of potassium dichromate.

V. References

[1] Burnett, R. W., J. Res. Nat. Bur. Stand. (U.S.), 76A

(Phys. and Chem.), No. 5, 483-489 (Sept.-Oct. 1972).

[2] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (1976), in

press.

[3] Burke, R. W., Deardorff, E. R., and Menis, O., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.),

No. 5, 469-482 (Sept.-Oct. 1972).

[4] Yoshimori, T. and Sakaguchi, N., Talanta 22, 233-238

(1975).

[5] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 76A

(Phys. and Chem.), No. 5, 405-425 (Sept.-Oct. 1972).

[6] Mavrodineanu, R., and Lazar, J. W., Nat. Bur. Stand.

(U.S.), Spec. Publ. 260-32, 26 pages (Dec. 1973), and

in Clin. Chem. 19, 1053-1057 (1973).

[7] Mielenz, K. D., and Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 77 A (Phys. and Chem.), No. 6, 699-703 (Nov.-Dec. 1973).

(Paper 80A4-908)

92
APPENDIX V

UNITED STATES DEPARTMENT OF COMMERCE • Lewis L. Strauss, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Testing of Glass Volumetric Apparatus

J. C. Hughes

National Bureau of Standards Circular 602

Issued April 1, 1959

[Supersedes Circular 434]

Available as COM 73-10504 from the National Technical Information Services

Springfield, VA 22161

93
Foreword

This Circular is a revision of NBS Circular 434, by Elmer L. Peffer (deceased) and Grace C. Mulligan. Its purpose is to bring up to date the specifications for glass volumetric apparatus of precision grade, the description of the

test methods used, and the directions for submitting apparatus for test. Specifications for newly developed items have been added, while others

have been changed to improve the usefulness of items or to take into consideration new manufacturing methods.

A. V. Astin, Director.

94
Contents

Foreword m

1. Introduction—purpose and scope 1

2. Standard specifications for glass volumetric apparatus 1 2.1. Types of apparatus which will be regularly admitted for test 1 2.2. General specifications 1 a. Units of capacity 1 b. Standard temperature 1

c. Material and annealing 1 d. Design and workmanship 1 e. Graduation lines 2

f. Inscriptions 2

2.3. Special requirements 2

a. Flasks 2

b. Cylindrical graduates 3

c. Transfer pipets 4

d. Burets and measuring pipets 4

e. Buret and pipet tips 4

2.4. Special Apparatus 4

a. Giles flasks 4

b. Specific gravity flasks ' 5

c. Sugar-testing flasks (Bates) 6

d. Babcock milk-test apparatus 6

e. Dilution (hemacytometer) pipets 6

f. Sahli hemoglobin pipets 7

g. Immunity unit cylinders and pipets 7

h. Special-purpose volumetric flasks 7

i. Other special apparatus 7

2.5. Tolerances 7

a. Flasks 7

b. Transfer pipets 7

c. Burets and measuring pipets 7

d. Cylindrical graduates 8

e. Delivery time 8

f. Special apparatus 8

3. Special rules for manipulation 8

3.1. Test liquid 8

3.2. Method of reading 8

a. Using water or other wetting liquid 8

b. Using mercury 9

3.3. Cleanliness of apparatus 9

3.4. Flasks and cylinders 9

a. Conical graduates 10

3.5. Burets 10

3.6. Measuring and transfer pipets 10

3.7. Special apparatus 10

4. Tests performed by the Bureau 11

4.1. Nature of tests 11

4.2. Precision stamp 11

4.3. Certificates and reports of capacity 11

4.4. Special tests 11

5. Directions for submitting apparatus for test 11

5.1. Application for test — 11

5.2. Shipping directions 11

5.3. Breakage 12

5.4. Address 12

5.5. Remittances 12

6. Appendix 12

6.1. Temperature corrections for volumetric solutions 12

6.2. Tables for determining the true capacities of glass vessels from the weight

of water in air 13

6.3. Change in capacity caused by change in temperature 13

95
Testing of Glass Volumetric Apparatus

J. G. Hughes

This Circular contains specifications and tolerances for glass volumetric apparatus of precision grade. Detailed information is given as to dimensions, graduations, inscriptions,

and tolerances for burets, pipets, flasks, cylindrical graduates, and certain kinds of special

apparatus.

A description of test methods used and the reports furnished and directions for submitting apparatus are included.

1. Introduction—Purpose and Scope

This Circular presents, to manufacturers and

users, information on the design, construction, and

verification of glass volumetric apparatus and

indicates the facilities available at the Bureau for

this verification.

The specifications and tolerances given herein

apply to burets, flasks, pipets, and other glass

volumetric apparatus of precision grade.

Information is given on the method of reading,

the test liquid used, and the methods of test. A

description of the tests performed and the reports

furnished and directions for submitting apparatus

are included.

2. Standard Specifications for Glass Volumetric Apparatus

The primary purpose of these specifications is to

define the requisite qualifications for precision

apparatus.

The Bureau aims to encourage excellence in

quality by cooperating with makers and users of

apparatus, and to this end endeavors to assist

manufacturers in establishing standards and perfecting their methods. In order that users of

standarized apparatus may fully benefit by the

facilities of the Bureau, it is necessary for them

when purchasing apparatus to be submitted for

test to . require that the apparatus shall comply

with the specifications of the Bureau. By admitting for test only apparatus conforming to

these standards, the work of testing is confined to

apparatus whose utility is sufficient to justify the

labor expended in its accurate calibration. Certain of the specifications, such as those regarding

quality of glass and process of annealing before

calibration, are largely dependent on the integrity

of the maker for their fulfillment. Only by

supporting conscientious makers, in giving consideration first to quality and second to cost, can

users of standardized apparatus secure a high

degree of excellence.

2.1. Types of Apparatus That Will Be

Regularly Admitted for Test

Precision grade apparatus of the following types

will be accepted for test on written request:

transfer pipets and capacity pipets, without subdivisions; burets, measuring pipets, and dilution

pipets, with partial or complete subdivisions;

volumetric flasks, cylindrical graduates, and specific gravity flasks.

Other types of apparatus should not be submitted without prior arrangement.

2.2. General Specifications 1

a. Units of Capacity

A liter is equal to the volume occupied by the

mass of 1 kg| of pure water at its maximum

density (at a temperature of 4° C, practically).

A milliliter (ml),2 is the one-thousandth part of

the liter.

b. Standard Temperature

Although 20° C has been almost universally

adopted as the standard temperature for glass

volumetric apparatus, many chemists are of the

opinion that 25° C more nearly approximates the

average laboratory temperature in the United

States. The Bureau therefore will calibrate glass

volumetric apparatus marked either 20° or 25° C.

c. Material and Annealing

The material should be of best quality glass,

transparent and free from striae, surface irregularities, and other defects which may distort the

appearance of the liquid surface or the portion of

the graduation line seen through the glass. All

apparatus should be thoroughly annealed before

being graduated.

d. Design and Workmanship

The cross section must be circular and the

shape must permit complete emptying and draining and thorough cleaning.

Instruments having a base or foot must stand

solidly on a level surface. For all instruments

1 The Bureau reserves the right to reject any apparatus on points affecting

its accuracy or utility not covered by these specifications.

2 The terms "milliliter" and "cubic centimeter" are sometimes used inter- changeably. While they are not exactly equal (1 ml=1.000028 cm'), the

difference is seldom of consequence in volumetric analysis.

96
except small flasks (below 25 ml) the base shall

be of such size that the instrument, empty and

without stopper, will stand on a plane inclined

15° to the horizontal. Flasks smaller than 25 ml

shall stand on a 10° incline.

Stoppers and stopcocks must be ground or fitted so as to work easily and prevent leakage.

Apparatus which is manifestly fragile or otherwise defective in construction will not be accepted.

The parts on which graduations are placed

must be cylindrical for at least 1 cm on each side

of every mark (except on certain small flasks and

special apparatus), but elsewhere may be enlarged

to provide the desired capacities in convenient

lengths.

Two scales are not permitted on the same piece

of apparatus. For example, apparatus should not

be graduated in both fluid ounces and milliliters.

In the case of two units, one of which is an exact

multiple of the other, such, for example, as drams

and fluid ounces, there is no objection to having

the 8-dr line, 16-dr line, etc., marked respectively,

1 fl oz, 2 fl oz, etc., provided that the two series of

numbers are placed on opposite sides of the

apparatus and the value of each subdivision is suitably indicated.

e. Graduation Lines

Graduation lines shall be fine, clean, permanent,

continuous, and of uniform width, perpendicular

to the axis and parallel to the base of the apparatus.

Line width should not exceed 0.3 mm for subdivided apparatus and 0.4 mm for single-line

apparatus.

All graduations must extend at least halfway

around; and on subdivided apparatus at least

every 10th mark, and on undivided apparatus,

all marks must extend completely around the

circumference. Subdivided apparatus must be

provided with a sufficient number of lines of suitable length to facilitate reading.

The clear space between two adjacent marks

must be not less than 1 mm wide. The spacing

of marks on subdivided apparatus must show no

apparent irregularities, and sufficient divisions

must be numbered to readily indicate the intended

capacity of any interval.

f. Inscriptions

Every instrument must bear in permanent

legible characters the capacity, the temperature

at which it is to be used, the method of use—that

is, whether to contain or to deliver—and on instruments which deliver through an outflow nozzle,

the time required to empty the total nominal

capacity with unrestricted outflow. Etching is preferred for inscriptions, although engraved or fused marking is acceptable provided such marking is neat and clear. Grit-blasted serial numbers

will be permitted with the same provision.

Every instrument must bear the name or trademark of the maker. Every instrument must bear

a permanent identification number, and detachable parts, such as stoppers, stopcocks, etc., belonging thereto, if not interchangeably ground,

must bear the same number. Interchangeable

ground-glass parts shall be marked on both members with the proper standard taper symbol and

the size designation, in accordance with Commercial Standard CS 21-57.

Figure 1 illustrates several arrangements of

designating marks that are considered suitable.

Marks may be placed elsewhere on apparatus if they are easily readable and do not interfere with

the proper use of the apparatus.

Figure 1. Examples of inscriptions suitable for volumetric

apparatus.

2.3. Special Requirements

a. Flasks

The limiting dimensions for precision-grade volumetric flasks are given in table 1. For flasks

smaller than 5-ml capacity, the limits on location

of the graduation line will be the same as shown

for a 5-ml flask.

Stopper sizes should be so chosen that the

smallest diameter of the ground zone in the flask

neck is approximately equal to the inside diameter

at the graduation line. The inside diameter of

the neck at the graduation line should never

exceed the smallest diameter of the ground zone by more than 1.5 mm. The shoulder at the base

of the ground zone should be smoothly rounded to

permit complete drainage.

97
Table 1. Limiting dimensions for volumetric flasks

(In millimeters)

Inside diameter of neck Minimum distance between

at graduation line graduation line and:

Capacity Top of flask Minimum Maximum Bulb » Stoppered Unstoptype pered type

ml

5 6 7 22 22 5 10 6 8 28 28 7 25 6 8 35 30 10

50_ 6 10 40 30 10 100 8 12 40 30 10

200 9 14 55 45 10 250 10 15 55 45 10 500. 12 18 60 60 20

1,000 14 20 70 60 20

2,000 18 25 70 60 20

• The distance between the graduation line and the bulb is measured to the point where the neck begins to expand into the bulb (on outside). A flask may be graduated both to contain and

to deliver, provided the intention of the different

marks is clearly indicated and provided the distance between the two marks is not less than

1 mm.

b. Cylindrical Graduates

Cylinders may be graduated either to contain

or to deliver, but a scale numbered both up and

down the length of the graduate is not permitted,

as it is obvious that the same graduate cannot be

correct both to contain and to deliver.

The relation of the height to the diameter must

be such that the graduation marks are not less

than 1 mm apart, and also that the graduated

height is at least five times the inside diameter.

In the case of the 10-ml cylinder subdivided to

0.1 ml, and the 25-ml cylinder subdivided to

0.2 ml, it will be found necessary to make the

graduated height considerably more than five

times the inside diameter in order to give a separation of 1 mm to the graduation marks. To

avoid excessive height, the subdivisions on 10-ml,

25-ml, and 50-ml cylinders may be 0.2 ml, 0.5

ml, and 1.0 ml, respectively.

Subdivision lines shall be omitted between the

base and the first numbered line. This will

eliminate readings near the base which are

difficult and not always accurate. (See fig. 2.)

The numbers indicating the capacity of the

graduate at its different points should be placed

immediately above the marks to which they refer.

Method of use—Graduates that are to be used

dry to receive and measure liquids should be calibrated to contain.

Graduates that are to be used to pour water into

other measures, and those which are to be used

wet to contain water from other measures, should

be calibrated to deliver. For example, a graduate

that is to be used in testing milk bottles, either by

pouring water from the graduate into the bottles or from the bottles into the graduate without drying the graduate between bottles, should be callNo 4 Maker T D 20°C

ml 500

450

400

350

300

250

200

1 50

100

50

Figure 2. Examples of cylindrical graduates.

brated to deliver. After having been wet the

graduate will, on successive fillings and emptyings,

deliver the same quantity that is poured into it. In ordering graduates, the purchaser should

consider the use to which they are to be put and

should specify accordingly whether they should be

calibrated to contain or to deliver.

Basis oj graduation—Graduates in customary

units should be graduated in accordance with the

following relations:

60 minims = 1 fl dr

8 fl dr= 1 fl oz 32 fl oz=l liq qt

4 liq qt= 1 U. S. gal

1 U. S. gal=231 in.3

For conversion to the metric system, the rela- tion is:

1 fl oz=29.5729 ml

Table 2. Limits of distance from highest graduation to top of graduate

Total Distance of highest nominal graduation from top Total nominal

capacity capacity

(customary (metric units) units) Minimum Maximum

Minims cm cm ml

60 2 4 5 120 2 4 10

fl oz

1_ _ 2 5 25

2. 3 5 50

4_ 3 6 100

8 4 8 250

16 5 10 500 32. 5 10 1,000

98
Table 3. Minimum neck diameter of cylinders with stoppers

Capacity ml.. 5 10 25 50 100 250 500 1,000 Minimum diameter

of neck mm.. 6 7 8 9 10 12 15 18

c. Transfer Pipets

Pipets for delivering a single volume are designated "transfer" pipets.

The suction tube of each transfer pipet must be

at least 16 cm long, and the delivery tube must be

not less than 3 cm nor more than 25 cm long. The

top of the suction tube must be finished with a smooth plane surface, at right angles to the axis.

The outside diameter of the suction and delivery

tubes of transfer pipets, exclusive of the tip, must

be not less than 5 mm. Limits of inside diameter

at the capacity mark are given in table 4. The capacity mark on transfer pipets must be

not more than 6 cm from the bulb.

The outlet of any transfer pipet must be of such

size that the free outflow shall last not more than

1 min and not less than the times shown in table 5

for the respective sizes.

Table 4. Inside diameter of transfer pipets at capacity

mark

Capacity of pipets (in milliliters) up to and

including .. .... .. 25 50 200 Diameter (in millimeters) : Max. 4 5 6 Min 2 2 2

Table 5. Minimum delivery times for transfer pipets

Capacity (in milliliters) up to and in- cluding... 5 10 50 100 200 Outflow time (in seconds) 15 20 30 40 50

d. Burets and Measuring Pipets

Only those burets emptying through a nozzle

permanently attached at the bottom are accepted

for test. Side tubes, unless provided with stopcocks, are not permitted on burets.

So-called Schellbach burets—that is, those having a milk-glass background with a colored center

line—will not be accepted for test on account of

possible errors resulting from parallax.

The distance between the extreme graduations

must not exceed 70 cm on burets nor 35 cm on

measuring pipets.

The rate of outflow of burets and measuring

pipets must be restricted by the size of the tip, and

for the graduated length the time of free outflow

must be not more than 3 min nor less than the

times shown in table 6. The upper end of any measuring pipet must

be not less than 10 cm from the uppermost mark

and the lower end not less than 4 cm from the

lowest mark.

On 50- and 100-ml burets, the highest graduation mark should be not less than 4 cm nor more

than 10 cm from the upper end of the buret.

Table 6. Minimum delivery times for burets and measuring

pipets

Length Time of out- Length Time of out- graduated flow not less graduated flow not less than— than—

cm sec cm sec 15 30 45 80

20 35 50 90 25 40 55 105 30 50 60 120 35 60 65 140 40 70 70 160

On burets having a capacity of 25 ml or less this

distance should be not less than 3 cm nor more

than 6 cm. (These requirements do not apply to

burets where the "zero" is at the end of an overflow tip.)

e. Buret and Pipet Tips

Buret and pipet tips should be made with

a gradual taper of from 2 to 3 cm, the taper at

the extreme end being slight.

A sudden contraction at the orifice is not permitted, and the end of the tip must be ground

perpendicular to the axis of the tube. The outside edge should be beveled slightly and all ground

surfaces polished.

In order to facilitate the removal of drops and

to avoid splashing, the tip of a buret may be bent

slightly.

Approved forms of tips for burets, measuring

pipets, and transfer pipets are shown in figure 3.

2.4. Special Apparatus

a. Giles Flasks

The~permissible error in the volume at the first

mark shall be the tolerance allowed for a flask

of that capacity. The permissible error in the

volume at the second mark shall be the tolerance

at the first mark plus the tolerance for a flask of

capacity equal to the difference between the two

marks, provided, however, that the error in the

volume between the two marks shall not exceed

the permissible error in the volume indicated by

the first mark.

At the capacity mark the inside diameter of

the neck shall be within the limits given in table 7. The neck shall be cylindrical on each side of

every graduation mark for at least the distances

given in table 8. On account of the bulb in the neck of a Giles

flask it is more unstable or topheavy than a flask

of the ordinary type. For that reason it has been

thought advisable to allow a somewhat shorter

minimum length for the cylindrical portion of the

neck, on each side of the graduation mark, than

is allowed on ordinary flasks. In other respects

the same general specifications apply to the Giles

flasks as to other volumetric apparatus.

99
Figure 3. Types of buret and pipet tips.

Table 7. Neck diameters of Giles flasks

Inside diameter of neck

Capacity

Minimum Maximum

ml mm 771771 25 to 27.5. _ 6 9

50 to 55 6 11 100 to 110 8 13 200 to 220.. 9 15 500 to 550 - 12 18

1,000 to 1,100 14 20

2,000 to 2,200 18 25

Table 8. Minimum length of cylindrical portions of neck on Giles flask

Above Between Between Below

upper upper lower lower Capacity graduation graduation graduation graduation

mark mark and mark and mark

bulb bulb

ml cm cm cm cm

25 to 27.5 3 0.8 1 1 50 to 55 3 .8 1 1 100 to 110 3 .8 I 1 200 to 220 4 .8 1.5 2 500 to 550 4 1.0 1.5 2 1,000 to 1,100 5 1.0 1.5 2 2,000 to 2,200 5 1.0 1.5 2

b. Specific Gravity Flasks

Material and annealing—The material from

which the flanks are made shall be glass of the best

quality, transparent, and free from striae. It shall adequately resist chemical action and have

small thermal hysteresis. The flasks shall be

thoroughly annealed before being graduated.

They shall be of sufficient thickness to insure

reasonable resistance to breakage.

Design—The cross section of the flask shall be

circular, and the shape and dimensions shall con- form to the diagram shown in figure 4. This

design is intended to insure complete drainage of

the flask on emptying and stability of standing on

a level surface, as well as accuracy and precision

of reading. There shall be a space of at least

1 cm between the highest graduation mark and

the lowest point of the grinding for the glass

stopper.

Capacity—The flask should contain approximately 250 ml when filled to the zero graduation

mark.

Graduations—The neck shall be graduated from

0 to 1 ml and from 18 to 24 ml into 0.1 -ml divisions.

There shall be two 0.1 -ml graduations below the 0

and two above the 1-ml graduation.

Standard temperature—The flasks shall be standard at 20° C. The indicated specific gravities will

then be at 20° referred to water at 4° as unity

that is, density at 20° in grams per milliliter.

Inscriptions—Each flask shall bear a permanent

identification number and the stopper, if not interchangeably ground, shall bear the same number.

The standard temperature shall be indicated and

the unit of capacity shall be shown by the letters

"ml" placed above the highest graduation mark.

Tolerance—The error of any indicated capacity

shall be not greater than 0.05 ml.

Interpretation of the specification—The foregoing

specification is intended to represent the most

desirable form of specific-gravity flask for use in

testing cements. Variations of a few millimeters

in such dimensions as total height of flask, diameter

of base, etc., are to be expected and will not be

considered sufficient cause for rejection. The

requirements in regard to tolerance, inscriptions,

and the length, spacing, and uniformity of graduations will, however, be rigidly observed.

56302S—60 2 100
GROUND GLASS STOPPER

HAVE TWO. 0.1 ml GRADUATIONS EXTEND ABOVE 1 ml AND BELOW

0 MARK

CAPACITY OF BULB APPR0X.250ml

Figure 4. Specific-gravity flask.

c. Sugar-Testing Flasks (Bates)

The flask shall have a height of 130 mm, and the

neck shall be 70 mm in length ; a tolerance of ± 5

mm is allowed for each. The internal diameter

shall be not less than 11.5 mm and not more than

12.5 mm. The upper end of the neck shall be

flared, and the graduation mark shall be not less

than 30 mm from the upper end and 15 mm from

the lower end of the neck. The flask shall be

standardized to contain 100 ml at 20° C with a

tolerance of 0.08 ml.

d. Babcock Milk Test Apparatus

Because most of the States now require that all Babcock glassware used in the State be approved

by officials of that State, the National Bureau of

Standards no longer accepts Babcock bottles and

pipets for calibration, except when specifically

requested by a State to settle a dispute. Specifications for Babcock glassware may be found in

official procedures of the Association of Official

Agricultural Chemists and certain dairy

associations.

e. Dilution (Hemacytometer) Pipets

The pipets should be of good quality glass, free

from bubbles and other defects, and sufficiently

strong to withstand normal usage. Tubing with

milk-glass backing is permissible. The capillary

should be of uniform diameter, except that the tip

of a white cell pipet may be tapered slightly. The

inside diameter at the tip should be between 0.3

and 0.5 mm. On all pipets the tip should be

ground smooth and at right angles to the axis of

the pipet, and beveled so that the external diameter at the end does not exceed 2.0 mm. All

ground surfaces should be polished to restore

transparency.

The bulb or mixing chamber should contain a

small, nonspherical glass bead to aid in mixing the

blood and diluting fluid. Pipets for red blood cells may be designated by a red bead, by a red stripe

running parallel to the axis, or both. White cell pipets should have white or clear beads.

The bulb should be So shaped that it can be

filled without entrapping air bubbles. The ca- pacity of the bulb is usually about 0.8 to 1 .2 ml for

the red cell pipet, and 0.2 to 0.4 ml for the white

cell pipet.

The pipet should be so graduated as to give

dilution ratios of 1 to 10, or 1 to 100, and may be

further graduated to give ratios of 0.1, 0.2, 0.5,

etc., to 10, and to 100; that is, the interval 0 to 1 may be subdivided to 0.1 or 0.5. The subdivisions, when present, should be uniformly

spaced along the capillary tube. The graduations

at 0.5, 1.0, 11, and 101 should be numbered.

All numbered graduation lines should extend at

least halfway around' the pipet, diametrically

opposite to the milk-glass backing. Etched lines are preferred, but engraved or fused lines are acceptable provided they meet the requirements

of paragraph 2.2. e. The "1" line shall be located

not less than 2 mm nor more than 5 mm below the

bulb, and the "11" (or "101") line shall be not

less than 2 mm nor more than 4 mm above the

bulb. Measurement is from the points where the

capillaries begin to expand into the bulb.

Dilution pipet tolerances—If V represents the

volume between the marks immediately above and

below the bulb, «i represents the volume of the

capillary between the tip and the line marked "1",

and v2 represents the volume of the capillary between the tip and the line marked "0.5", the toler- ances may be expressed as follows:

101
Red cell pipets—nominal ratio 100:1,

V/Vi must be not less than 95 and not more

than 105

V/v2 must be not less than 190 and not more

than 210.

White cell pipets—nominal ratio 10:1,

V/vi must be not less than 9.65 and not more

than 10.35

V/v2 must be not less than 19.30 and not more

than 20.70.

In an effort to give more exact information as to

accuracy, certain manufacturers have marked

dilution pipets with a so-called "correction factor".

This is usually written simply " +2", " — 1", etc. Because of the possibility that such a correction

will not be applied as the manufacturer intended,

and because mass-production test methods can- not always be relied upon to fix the corrections

within 1 percent, the Bureau does not recommend

this practice. In those cases where greater accu- racy is required than is provided by the application

of the prescribed tolerances (±5% for red cell pipets and ±3.5% for white cell pipets), it is suggested that the purchaser specify the desired

limits of error. This would provide the needed

accuracy without the application of corrections.

The Bureau does not accept for test pipets

bearing a "correction factor" except in very special

^cases and only in small quantities.

f. Sahli Hemoglobin Pipets

The capacity tolerance for Sahli pipets is ±2

percent of the nominal volume.

g. Immunity Unit Cylinders and Pipets

These instruments were designed by the Hygienic Laboratory of the U. S. Public Health

Service (USPHS) for use in the measurement of

toxin and serum dilutions. Specifications may be

found in USPHS Hygienic Laboratory Bulletin 21.

The instruments are made to conform in general

with the specifications contained in this Circular,

but with certain modifications necessary to adapt

them to the specific application.

The pipets are of three types: (1) Capacity or "wash-out" pipets, calibrated to contain the nominal volume; (2) delivery pipets, calibrated to deliver the nominal volume when the residue in

the tip after free outflow has ceased is blown out

and added to the initial delivery; (3) graduated

delivery pipets, called Ehrlich pipets, calibrated

to deliver the contents in the same manner as transfer pipets.

Tolerances for the first two types of pipets

named are the same for ordinary transfer pipets;

for the Ehrlich delivery pipets, the tolerance is 0.01 ml. For the cylinders, the tolerances are: For capacities less than and including 50 ml—0.05

ml; for capacities over 50 ml and including 100

ml—0.08 ml.

h. Special-Purpose Volumetric Flasks

Certain types of special-purpose volumetric

flasks such as the Engler viscosity flask and the

Kohlrausch and Stift sugar flasks, while not in

conformity with all the special requirements for

volumetric flasks listed in section 2. 3. a because of

their specific applications, will be calibrated and

marked with the precision stamp if they are in

satisfactory conformity with the general specifications in section 2.2 and the errors in capacity do

not exceed the tolerances for volumetric flasks

given in section 2.5.

i. Other Special Apparatus

Other types of special-purpose instruments may

be calibrated and marked with the precision stamp

if they conform with the general specifications

(section 2.2) and the errors in capacity do not

exceed the applicable tolerances in section 2.5.

Permanent identification numbers are required on all glass volumetric apparatus submitted for

certification.

Instruments obviously not capable of precision

measurements, as for example, centrifuge tubes, are not accepted because the time expended in

their calibration is not warranted by the degree of

accuracy required in their use.

2.5. Tolerances

a. Flasks

The capacity tolerances for flasks are given in

table 9.

Table 9. Capacity tolerances for volumetric flasks

Capacity (in milliliters) less than and including

Limit of error

If to contain— If to deliver—

ml ml

1 0.01

3 .015

5 .02

10 .02 0. 04

25 .03, .05

50 .05 .10

100 .08 HUB 200 .10 .20

300 .12 .25

500 .15 .30

1,000 .30 .50

2,000 • .50 1.00 Above 2,000 1 part in 4,000 1 part in 2,000

b. Transfer Pipets

The capacity tolerances for transfer pipets are given in table 10. These tolerances are applicable

to pipets of similar design made to contain the

nominal volumes.

c. Burets and Measuring Pipets

The capacity tolerances for burets and measuring pipets are given in table 1 1

102
Table 10. Capacity tolerances for transfer pipets Table 12. Capacity tolerances for cylindrical graduates

Capacity (in milliliters)

less than and including

Limit of

error

2 ml0.006

.01 .02

.03

.05

.08 .10

510

30

50

100

200._._

Table 11. Capacity tolerances for burets and measuring

pipets

Capacity (in milliliters) of total graduated por- tion less than and in- cluding—

Limit of error of total or partial capacity

Burets Measuring

pipets

ml ml

2... 0. 01

5 _ ... 0. 01 .02

10 .02 .03 30 .03 .05 50 .05 .08

100 .10 .15

d. Cylindrical Graduates

The capacity tolerances for cylindrical graduates are given in table 12.

e. Delivery Time

The actual delivery time of any instrument

must be within the limits prescribed in section 2.3,

and the error in the marked delivery time must

not exceed the limits given in table 13.

Metric units

Capacity of

total graduated

portion less than and

including—

Maxi- mum

diameter

ml

510.... 25

50....

100...

250...

500...

1,000. 2,000.

mm

1114

1923

29

40

5063

Limit of error

of total or

partial capacity

If to con- tain—

ml

0. 05

.08 .14

.20

.35

.65

1.1 2.0

3.5

If to de- liverml

0.06

.10 .18 .26

.40

1.3 2.5

5.0

Customary units

Capacity of

total graduated

portion less than and

including

Maxi- mum

diameter

Minims

60

120

fl OZ

1 (8 fl dr)

.

2

48

1632

Limit of error

of total or

partial capacity

in. %

Vi

in. %

11M

W«

l'Me

27A*

If to con- tain—

Minims

0.6

1.0

fl oz

0. 005

.008

.014

.021 .035

.060

If to de- liver—

Minims

0.7

1.3

fl oz

0. 006

.010

.017 .027

.045

.080

Table 13. Limits of error in marked delivery tit

Delivery time (in Limit of error seconds) less than in marked

and including— delivery time

sec 15 3 20 4 30 6 50 8 100 15 200 20

f. Special Apparatus

Capacity tolerances for the special purpose instruments described in section 2.4 are included in

the appropriate parts of that section.

3. Special Rules for Manipulation

These rules indicate the essential points in the

manipulation of volumetric apparatus which must

be observed in order that the conditions necessary

to obtain accurate measurements may be reproduced.

3.1. Test Liquid

Apparatus will ordinarily be calibrated with distilled water, and the capacity determined will

therefore be the volume of water contained or

delivered by an instrument at its standard temperature.

Certain special types of apparatus, such as pipets having a capacity of 0.01 ml or less, are more accurately calibrated with mercury. Instruments designed to deliver their contents should

always be calibrated with water.

3.2. Method of Reading

a. Using Water or Other Wetting Liquid

In all apparatus where the volume is limited by

a meniscus, the reading or setting is made on the

lowest point of the meniscus. In order that the

lowest point may be observed, it is necessary to

place a shade of some dark material immediately

below the meniscus, which renders the profile of

the meniscus dark and clearly visible against a

light background. A convenient device for this

purpose is a collar-shaped section of thick black

rubber tubing, cut open at one side and of such

size as to clasp the tube firmly.

The position of the lowest point of the meniscus

with reference to the graduation line is such that

it is in the plane of the middle of the graduation

line. This position of the meniscus is obtained by

making the setting in the center of the ellipse

formed by the graduation line on the front and the

back of the tube as observed by having the eye

slightly below the plane of the graduation line.

This is illustrated in figure 5. The setting is accurate if, as the eye is raised and the ellipse narrows,

the lowest point of the meniscus remains midway

between the front and rear portions of the graduation line. By this method it is possible to observe

the approach of the meniscus from either above or below the line to its proper setting.

103
L iquid

Figure 5. Method of setting water meniscus.

b. Using Mercury

In setting a mercury meniscus on a graduation

line, the instrument must be tapped sharply in

order that the meniscus may assume a normal

shape. The highest point of the meniscus is set on

the middle of the graduation line by employing

the principles outlined for water, but observing

from above the line.

3.3. Cleanliness of Apparatus

Apparatus must be sufficiently clean to permit

uniform wetting of the surface and to minimize

contamination of the liquid surface.

Of the various cleaning agents in common use, the Bureau prefers fuming sulfuric acid or a chromic-sulfuric acid mixture. For removal of

oil and grease, an organic solvent is used and the

cleaning completed with one of the above agents.

When drying is required before calibration, it is done with unheated compressed air which is cleaned and dried by passing through concentrated

sulfuric acid and a drying compound such as calcium chloride.

3.4. Flasks and Cylinders

In filling flasks, the entire interior of the flask

below the stopper will be wetted; in the case of

cylinders, the liquid is allowed to flow down one

side only. After filling to a point slightly below

the graduation line, these instruments are allowed

to drain for about 2 min. They are then placed

below a buret having a long delivery tube and a bent tip, and the filling is completed by discharging water from the buret against the wall of

the flask or cylinder about 1 cm above the graduation line, and rotating the receiving vessel to re-wet the wall uniformly.

Flasks and cylinders which are to be used to

deliver are filled approximately to the test point,

then emptied by gradually inclining them, avoiding as much as possible agitation of the contents

and re-wetting of the walls. Allow half a minute

for emptying. When the continuous outflow has

ceased, the vessel should be nearly vertical and

should be held in this position for another halfminute. The adhering drop is removed by contact with the wetted wall of the receiving vessel.

The rest of the calibration is performed in the same

manner as in calibrating to contain.

In calibrating a flask or cylinder to contain it is only necessary, after cleaning and drying the

apparatus, to weigh it empty, fill it accurately to the graduation mark, and again weigh. Table 16

or table 17 may be used to determine the capacity

at 20° C from the weight of the water contained at a known temperature.

In case flasks are to be tested in large numbers,

a volumetric method may be used to advantage.

The Bureau uses a series of volumetric standard

pipets (illustrated in fig. 6) each having a capacity

slightly less than that of the flask it is intended

to test. The water delivered from the appropriate standard pipet fills the flask nearly to the

graduation mark. After the 2-min drainage

period, the filling is completed from an accurately

calibrated buret. The capacity of the flask is then found from the known volume delivered by

the standard pipet and the additional volume

delivered by the buret. The standard pipets and

burets are themselves calibrated by weighing the

water delivered.

V

Figure 6. Standard pipet for flask calibrations.

104
a. Conical Graduates

The conical graduate is not classed as a precision

volumetric measure, and is not recommended

for use in checking the accuracy of other volumetric apparatus. The Bureau does not accept

these graduates for calibration except when re- quested by a State weights and measures official. Conical graduates are calibrated in the same

manner as cylindrical graduates, except that in

calibrating "to deliver", the emptying time is reduced from 30 sec to 10 sec and the drainage

time (after emptying) is also reduced to 10 sec, following customary methods of use.

3.5. Burets

Burets are calibrated in a vertical position.

After cleaning, they are filled with distilled water

to about 1 cm above the zero line. They are allowed to stand for about 30 min to check for

leakage, not only at the tip but also around the

stopcock plug. The water is then allowed to

discharge with unrestricted outflow and the

delivery time is noted and recorded. This

flushing serves to equalize the temperatures of

apparatus and water. The water temperature

is taken on the next filling by inserting a thermometer in the buret or, if it is not large enough to

receive a thermometer, in a plain glass tube of

larger diameter mounted near the buret.

If the temperature was taken in the buret, the

thermometer is then removed and enough water

added to raise the level to about 1 cm above the

zero line. Any water on the outside of the tip is removed with absorbent paper. The setting of

the meniscus on the zero line is made by allowing

the water to run out slowly. After the setting is made, any excess water on the tip is removed by

touching the tip with the wetted wall of a beaker

or other vessel. The very small quantity of water

which remains on the extreme end, or ground portion, of the tip is not removed, as the same quantity of water will remain after delivery into the

weighing flask.

Delivery of the water into the weighing flask

is made with the buret tip in contact with the

inside of the flask neck, the flask being inclined at an angle of about 20°. When delivery has been

completed, the weighing flask is immediately

removed horizontally from contact with the

instrument being calibrated. The flask is then

stoppered and weighed. The buret and thermometer tube (if used) are refilled preparatory to

measuring the next interval.

Buret stopcocks should be completely open

during delivery until the water level is only a few

millimeters above the test point, when the discharge must be slowed in order to make an ac- curate setting.

In making the weighings it is both convenient

and accurate to use the method of substitution.

By this method a constant tare is kept on one pan of the balance, while on the other pan is placed the object to be weighed and with it sufficient weights to secure equilibrium. In testing

volumetric apparatus, the receiving flask is first

weighed empty and then again after having delivered into it the water from the interval under

test. The difference between the two weights is the weight of the water delivered.

Suppose, for example, that with a certain tare on the right-hand pan and the empty flask on

the left a load of 151.276 g is required in addition

to the flask to restore equilibrium, and that after

the water from the 10-ml interval of the buret

under test is delivered into the flask, only 141.310

g are required, and that the water is at a temperature of 23.4°C. The weight of the water added

is then 151.276 -141.310 =9.966 g. From table

16 (appendix), it is seen that to determine the

capacity of a 10-ml soft glass vessel at 20°C

from the weight of water delivered at 23.4°C,

0.035 must be added to the weight; that is, the

capacity of the 10-ml interval of the buret under

test at 20°C is 9.966 +0.035 = 10.001 ml.

3.6. Measuring and Transfer Pipets

Measuring and transfer pipets are calibrated in

much the same manner as burets, except that

the tip must be in contact with the wet wall of a beaker or other vessel when the setting is made

on the zero line of a measuring pipet or the ca- pacity mark of a transfer pipet. Also, the delivery time of a transfer pipet is measured with

the tip in contact with the vessel.

In calibrating transfer pipets, the water surface

must be observed closely as it approaches the tip

to make certain that outflow has ceased before

the weighing flask is removed . On the other hand,

if the flask removal is unduly delayed, afterdrainage will affect the result.

The water remaining in the tip of a pipet is

not blown out unless, as in the case of certain

types of special-purpose pipets, there is a wide

band or two narrower bands permanently marked

near the top of the instrument.

The water temperature is measured in the beaker

from which the pipet is filled (by suction).

3.7. Special Apparatus

The flasks and cylinders mentioned in section

2.4 are calibrated in the manner described in

section 3.4, except that the inside of the neck is wetted for a distance of only about 1 cm or less

above the test point.

A specific gravity flask is calibrated at the 1 ? 18, and 24 ml graduations.

Capacity pipets made to contain the nominal

volume are calibrated by weighing before and

after filling.

In calibrating "blow-out" pipets, the water

remaining in the tip after free outflow has ceased

is blown out, without waiting for drainage, and

added to the quantity delivered. The blow-out

is accomplished by a single strong puff, with the

pipet tip in contact with the neck of the weighing

flask.

105
4. Tests Performed by the Bureau

4.1. Nature of Tests

Apparatus submitted for test is first examined

as to its conformity with the specifications concerning quality and workmanship, design, markings, outflow time, etc. If the apparatus meets these requirements, it is calibrated either to ascertain whether the capacity

is correct within the prescribed limits of error or

to determine the correction for use in precise

measurements.

4.2. Precision Stamp

If the results of the preliminary examination

indicate a satisfactory conformity with the specifications, and the error in capacity is within the

prescribed tolerance, the official precision stamp,

consisting of the letters NBS and the year date,

surrounded by a circle, is etched on the instrument as shown below:

4.3. Certificates and Reports of Capacity

Burets will be calibrated for at least five intervals, and if found to conform with the specified

requirements will, in addition to the precision

stamp, be marked with an NBS identification

number as shown below:

NBS No. 1234

1958

A certificate will be issued giving the volumes

delivered from the intervals tested.

Only in special cases will certificates of capacity

be issued for apparatus other than burets. When

the measurements depending upon the use of the

volumetric glassware are required to be so precise

that a statement to the effect that the apparatus

is within the tolerances prescribed in this Circular

does not give sufficiently exact information, certificates may be issued. The request for certificates, when required, should be made at the

time the apparatus is submitted for test. Reports of capacity may be issued under the

following circumstances : (a) For burets when the error in capacity is greater than, but not more than twice, the

tolerance.

(b) When certificates are requested for other

apparatus, and the error in capacity exceeds, but

is not more than twice, the prescribed tolerance.

(c) For special types of volumetric apparatus

not covered by the specifications in this Circular.

When a report of capacity is issued, a number is applied as shown above, but the precision stamp

is omitted.

4.4. Special Tests

The Bureau will gladly cooperate with scientific

investigators, manufacturers of apparatus, and

others in the calibration of precision volumetric

apparatus not covered by the specifications in

this Circular, as far as the regular work of the

Bureau will permit. Tests of instruments not

included in the current fee schedules should be

arranged for by correspondence before shipment

of the apparatus; the application should state

fully the purpose for which the apparatus is to be

used, the need for the test, and the precision

desired.

5. Directions for Submitting Apparatus for Test

5.1. Application for Test

The request for test should be made in writing

and should include a complete list of the apparatus and a statement of the nature of the test desired. Representatives of state institutions entitled to tests free of charge must make application in writing for each test in order to avail

themselves of the privilege.

Unless otherwise requested, burets will be

examined and the capacity of five intervals tested.

If in satisfactory conformity with the requirements, the results of the test are certified. If

more than five intervals are to be tested, the

request must so state. Patrons should always examine apparatus care- fully before submitting it for test to ascertain if it complies with the construction specifications. Delay

and cost of transportation on apparatus not entitled to verification will thus be avoided.

The Bureau does not sell volumetric apparatus.

It may be purchased from manufacturers, importers, or jobbers, and submitted to the Bureau

for test. Purchasers of apparatus to be submitted to the

Bureau for test should so specify to the dealer in

order to avoid unnecessary delays and misunderstandings.

5.2. Shipping Directions

The apparatus should be securely packed in cases or packages which will not be broken in

transportation and which may be used in returning

the tested material to the owner. Great care should be taken in packing. Clean, dry excelsior

is a suitable packing material in most cases. Each

instrument should also be wrapped in strong paper

or other covering to prevent dust and excelsior

from getting into it,.

106
Transportation charges are payable by the party

requesting the test. The charges for shipment to the Bureau must be prepaid; and, unless otherwise

arranged, articles will.be returned or forwarded by

express "collect".

After the material is received at the Bureau, the

shipper will be notified of the test number assigned.

This number should be mentioned in any corre- spondence pertaining to the test.

5.3. Breakage

A considerable number of pieces of glassware are received broken, either on account of improper

packing or rough treatment in transportation, or both. Some are broken in return shipment. A

small percentage is broken in the testing laboratory.

There is no legal way in which the Bureau can make reimbursement for breakages of this kind.

It is therefore a matter of necessity, and not of

choice, that the Bureau makes those who send

apparatus for test assume all the risks involved.

Under the circumstances, all that the Bureau can do is to make every effort to reduce such breakage

to the absolute minimum.

5.4. Address

Articles and communications should be addressed, "National Bureau of Standards, Washington 25, D.C."; delays incident to other forms of

addresses will thus be avoided. Articles delivered

personally or by messenger should be left at the

receiving office of the Bureau, and should be ac- companied by a written request for test.

5.5. Remittances

Payment of test fee should be made promptly

upon receipt of bill. Remittances may be made

by money order or check drawn to the order of the

"National Bureau of Standards".

Copies of the current Test Fee Schedules may

be obtained from the Bureau upon request.

6. Appendix

Tables 14 and 16 are reprints of tables 38 and 43

from NBS Circular 19, 6th edition, Standard

Density and Volumetric Tables. They are included in this Circular for the convenience of users

of volumetric glassware. Tables 15 and 17 have

been added because of the increasing popularity of

borosilicate glass apparatus.

6.1. Temperature Corrections for Volumetric

Solutions

Table 14 gives the correction to various observed

volumes of water, measured at the designated

temperatures, to give the volume at the standard

Table 14. Temperature corrections for water

Measured in soft glass apparatus having a coefficient of cubical expansion

of 0.000025/°C

temperature, 20° C. Conversely, by subtracting

the corrections from the volume desired at 20° C,

the volume that must be measured at the designated temperatures in order to give the desired

volume at 20° C will be obtained. It is assumed

that the volumes are measured in glass apparatus

having a coefficient of cubical expansion of 0.000-

025/° C. The table is applicable to dilute aqueous

solutions having the same coefficient of expansion

as water.

Table 15 gives temperature corrections for

water, when measured in borosilicate glass apparatus having a coefficient of cubical expansion

of 0.000010/° C.

Table 15. Temperature corrections for water

Measured in borosilicate glass apparatus having a coefficient of cubical expansion of 0.000010/°C

Capacity of apparatus in milliliters at 20° C

Temperature of measure- 2,000 1,000 500 400 300 250 150 ment

Correction in milliliters to give volume of water at 20° C

0 C

15 +1.54 +0. 77 +0. 38 +0. 31 +0.23 +0. 19 +0. 12 16 _ +1.28 +.64 +.32 +.26 +.19 +.16 +. 10 17 +.99 +.50 +.25 +.20 +. 15 +.12 +.07

18 +.68 +.34 +. 17 +. 14 +. 10 +.08 +.05

19 +.35 +.18 +.09 +.07 +.05 +.04 +.03

21 -.37 -.18 -.09 -.07 -.06 -.05 -.03

22 -.77 -.38 -.19 -.15 -.12 -.10 -.06

23 -1.18 -.59 -.30 -.24 -. 18 -.15 -.09

24 -1.61 -.81 -.40 -.32 -.24 -.20 -.12

25 -2. 07 -1.03 -.52 -.41 -.31 -.26 -.15

26 -2.54 -1.27 -.64 -.51 -.38 -.32 -.19

27 -3.03 -1.52 -.76 -.61 -.46 -.38 -.23

28 -3. 55 -1.77 -.89 -.71 -.53 -.44 -.27

29 -4.08 -2.04 -1.02 -.82 -.61 -. 51 -.31

30 -4.62 -2. 31 -1. 16 -.92 -.69 -.58 -.35

Capacity of apparatus in milliliters at 20° C

Temperature of measure- 2,000 1,000 500 400 300 250 150 ment

Correction in milliliters to give volume of water at 20° C

° C

15 _ +1.69 +0.84 +0.42 +0. 34 +0. 25 +0.21 +0. 13

16 +1. 39 +.70 +.35 +.28 +• 21 +.17 +. 10

17 +1.08 +.54 +.27 +.22 +. 16 +. 13 +. 08

18 +0.74 +. 37 +.19 +. 15 +.11 +.09 +.06

19 +.38 +• 19 +. 10 +.08 +.06 +.05 +.03

21 -.40 -.20 -.10 -.08 -.06 -.05 -.03

22 -.83 -.41 -.21 -.17 -.12 -.10 -.06

23 -1.27 -.64 -.32 -.25 -. 19 -.16 -.10

24 -1. 73 -.87 -.43 -.35 -.26 -.22 -.13

25 -2.22 -1.11 -.55 -.44 -.33 -.28 -.17

26 -2.72 -1.36 -.68 -.54 -.41 -.34 -.20

27 -3.24 -1.62 -.81 -.65 -.49 -.41 -.24

28 -3. 79 -1.89 -.95 -.76 -.57 -.47 -.28

29 -4.34 -2. 17 -1.09 -.87 -.65 -.54 -.33

30 -4. 92 -2.46 -1.23 -.98 -.74 -.62 -.37

107
In using the above tables to correct the volume

of certain standard solutions to 20° C, more accurate results will be obtained if the numerical values

of the corrections are increased by the percentages

given below:

Solution Normality

N JV/2 N/W

HNOs- 50 25 6 H2 SO( 45 25 5 NaOH.__ 40 25 5 KOH 40 20 4

6.2. Tables of Corrections for Determining the

True Capacities of Glass Vessels From the

Weight of Water in Air

Table 16 gives, for a nominal capacity of 100 ml

and observed temperatures from 15° C to 32.9° C,

the amounts to be added to the apparent weight in

grams (in air against brass weights) of the water

contained in or delivered by a glass vessel to give

the capacity in milliliters at 20° C. It is calculated on the following data assumed as approximating

ordinary conditions: Observed barometric pres- sure—-760 mm; relative humidity—50 percent;

coefficient of expansion of glass—0.000025/° C.

> Example of use of table. Determination of ca- pacity of glass volumetric flask marked "To

contain 100 ml at 20° C".

Apparent weight of water at the

observed temperature, 24.5° C__ 99. 615 g From table 16, correction +0. 372

Actual capacity at 20° C 99. 987 ml

For capacities other than 100 ml, the corrections

given must be multiplied by the appropriate

factor. For example, the correction for 500 ml

would be five times the correction for 100 ml.

For borosilicate glass having a coefficient of

cubical expansion of approximately 0.000010/° C,

Table 17 should be used.

6.3.\Change in Capacity Caused by Change of

Temperature

Having determined the capacity at 20°"C, if it is desired to know what the capacity of the same

vessel will be at another temperature, the following

formula may be used

V,= V20[l +a(t-20)],

where

Vt =capacity at t° C,

V2o= capacity at 20° C,

a= coefficient of cubical expansion of material of

which the instrument is made. (For soft

glass 0.000025/° C)

This formula is applicable to instruments or measures made of any material of which the coefficient of cubical expansion is known. A few materials commonly used in volumetric apparatus and

their approximate coefficients are:

Coefficient of

cubical expansion 4 per deg C

Soft glass 0. 000025

Pyrex or KG-33 . 000010

Copper . 000050

Brass . 000054

Steel . 000035

Stainless steel . 000031

* Coefficients shown are averages, but are sufficiently accurate for volu- metric determinations.

Table 16. Table of corrections for determining the true capacities of glass vessels from the weight of water in air

(Soft glass, coefficient of cubical expansion 0.000025/° C)

Indicated capacity 100 ml

Tenths of degrees Temperature

0 1 2 3 4 5 6 7 , 8 9

° C

•

15_ 0.207 0.208 0. 210 0. 211 0. 212 0. 213 0. 215 0. 216 0.217 0. 219

16 .220 .221 .223 .224 .225 .227 .228 .230 .231 .232

17 .234 .235 .237 .238 .240 .241 .243 .244 .246 .247

18 _. .249 .250 .252 .253 .255 .257 .258 .260 .261 .263

19. .265 .266 .268 .270 .272 .273 .275 .277 .278 .280

" 20..- .282 .284 .285 .287 .289 .291 .293 .294 .296 .298

21 .300 .302 .304 .306 .308 .310 .312 .314 .315 .317

22 .319 .321 .323 .325 .327 .329 .331 .333 .336 .338

23 .340 .342 .344 .346 .348 .350 .352 .354 .357 .359

24.... .361 .363 .365 .368 .370 .372 .374 .376 .379 .381

25.... .383 .386 .388 .390 .392 .395 .397 .399 .402 .404

26. — .406 .409 .411 .414 .416 .418 .421 .423 .426 .428

27 .431 .433 .436 .438 .440 .443 .446 .448 .451 .453

28. .456 .458 .461 .463 .466 .469 .471 .474 .476 .479

29.. .482 .484 .487 .490 .492 .495 .498 .501 .503 .506

30 .509 .511 .514 .517 .520 .522 .525 .528 .531 .534

31 .536 .539 .542 . 545 .548 .551 .554 .556 .559 .562

32 .565 .568 .571 .574 .577 .580 .583 .586 .589 .592

108
Table 17. Table of corrections for determining the true capacities of glass vessels from the weight of water in air

(Borosilicate glass, coefficient of cubical expanson 0.000010/° C)

Indicated capacity 100 ml

Temperature

Tenths of degrees

0 1 2 3 4 5 O

7 Q Qy

° C

15... 0. 200 0. 201 0. 202 0. 204 0. 205 0. 207 0. 208 n oi n U. n Oil

U. Zi

1

n 9i 9 16 . 214 . 215 . 217 . 218 . 220 . Liu 99Q . ZZo . ZZO 99ft . ZZO 228

17 . 229 . 231 . 232 . 234 . 236 . 237 . zoy 9/11

. Z^l 9/19 9/1/1 18. . 246 . 247 . 249 . 251 . 253 . 254 ORft 9£Q . Zoo 9ftn 9ft 1 19 . 263 . 265 . 267 . 269 . £1 1 . Z7Z 97ft

. ZfO 97ft 9ftn

20 . 282 . 284 . 286 . 288 . 290 . 292 9Qft

. zyo

OQQ

. oUU

21_... . 302 . 304 . 306 . 308 . 310 . 312 . 314 . olO Q1 Q . olo Q9n

22 .322 .324 . 327 .329 .331 . 333 .335 .338 . o4U . O^tZ 23 .344 .346 .349 .351 .353 .355 .358 .360 .362 .365

24 .367 .369 .372 .374 .376 .379 .381 .383 .386 .388

25 .391 .393 .396 .398 .400 .403 .405 .408 .410 .413

26 .415 .418 .420 .423 .426 .428 .431 .433 .436 .438

27 .441 .444 .446 .449 .452 .454 .457 .460 .462 .465

28 .468 .470 .473 .476 .479 .481 .484 .487 .490 .492

29 .495 .498 .501 .504 .506 .509 .512 . 515 .518 .521

30 .524 . 526 . 529 .532 .535 .538 .541 .544 .547 .550

31 . 553 . 556 . 559 . 562 .565 .568 .571 . 574 .577 .580 32 . 583 . 586 .589 .592 . 595 .598 .602 .605 .608 .611

Washington, September 24, 1958.

109

U. S. GOVERNMENT PRINTING OFFICE : 1959

APPENDIX VI

NBSIR 74-461

THE CALIBRATION OF SMALL VOLUMETRIC

LABORATORY GLASSWARE

Josephine Lembeck

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1974

Final

Prepared for

Optical Physics Division

Institute for Basic Standards

Washington, D. C. 20234

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

111
The Calibration of Small Volumetric Laboratory Glassware

by

Josephine Lembeck

1. INTRODUCTION

This article describes an acceptable procedure for calibrating

burets, pipets and volumetric flasks of the types shown in figure

1. The physical characteristics of these items are indicated in

Interim Federal Specifications NNN-B-00789a (GSA-FSS) , NNN-P00395a (GSA-FSS} NNN-F-00289a (GSA-FSS), and NBS Handbook 105-2.

The general procedure is based upon a determination of the volume

of water either contained in or delivered by the vessel. This

volume of water is based upon knowledge of its measured mass and

its tabulated density. Inasmuch as these calibration volumes,

especially the delivery volume, may be altered by the manner in

which the vessels are used, it is imperative that the user of the

vessel employ the same techniques as those employed during

calibration. The techniques of importance here are:

1. reading and adjusting meniscus

2. filling the vessel

3. draining the vessel.

These three topics will be covered in detail.

2. CLEANING PROCEDURES

It is important that glassware be thoroughly cleaned before being

tested or used. Apparatus must be sufficiently clean to permit

uniform wetting of the surface. When clean, the walls will be

uniformly wetted and the water will adhere to the glass surface

in a continuous film. Imperfect wetting causes irregularities in

capacity by distorting the meniscus and allowing nonuniform

wetting of the interior surfaces to occur.

The liquids usually used for cleaning glassware are sodium

dichromate sulphuric acid cleaning solution (commercially

available from laboratory supply houses), nitric acid, fuming

sulphuric acid, alcohol and water. The choice of cleaning agent

to be used depends on the nature of the contaminant. After

cleaning with the cleaning solution and thoroughly rinsing with

tap water, the vessel should be rinsed with distilled water. If

112
the vessel is marked " TO CONTAINV it should additionally be

rinsed with ethyl alcohol and dried with clean air at room

temperature. NBS uses bottled breathing air and a drying rack

shown in figure 2. It is not necessary to dry any vessel marked

" TO DELIVER" . When cleaning small articles such as pipets, it is

usually easier to fill them with cleaning solution by suction,

vacuum, if available, or a small rubber bulb, but never by mouth.

The solution should be sucked through the pipet several times

until the entire inside surface is evenly coated. Rinse

thoroughly with tap water and then with distilled water.

For cleaning flasks, pour in enough cleaning solution while

rotating so that a film of solution will cover the entire

interior surface. A break in the film indicates a contaminated

area. For filling a buret with cleaning solution, it should be

held in a vertical position and filled by pouring into the top.

Open the stopcock to drain. Regardless of the type of vessel,

always rinse thoroughly, first with tap water and then with

distilled water.

3. FILLING AND DRAINING THE VESSEL

Pipets, burets and flasks have at least one index mark engraved

on their glass bodies which serves to define their reference

volume . Calibration consists of determining the volume of water

required to fill the vessel to the index mark (containment

volume), or the volume of water which can be drained from the

vessel under specified conditions after filling (delivery

volume)

.

The difference between containment volume and delivery volume is

significant. Since these volumes are determined in weighing

experiments, it is essential to avoid splashing droplets of water

on to supposedly dry surfaces and it is equally important to

assure that a surface is uniformly wetted when required by the

procedure.

In all cases, it is necessary to fill the reference volume of the

test vessel with water. This involves adjusting the height of

the liquid surface to coincidence with the horizontal plane

through the defining index mark.

This surface, or meniscus, is not flat; it is concave— that is,

its lowest point is in the center of the surface as shown in

figure 3. There are several criteria for deciding when

coincidence is achieved. Inasmuch as differing criteria result

in different surface heights, it is important that the same

criteria be employed during use of the vessel as was employed

during calibration. Experience has shown that the following

generally used technique is easy to apply and adequately

113
reproducible. In this procedure, the lowest point in the

meniscus is brought to the same horizontal level as the

horizontal plane through the engraved index line on the glass

vessel.

In order to observe the lowest point, it is necessary to place a

shade of some dark material immediately below the meniscus. This

renders its profile dark and clearly visible against a light

background. A method or device for this purpose is a collarshaped section of thick black rubber tubing, cut open at one side

and of such size as to clasp the tube firmly, as shown in figures

4 and 4A. The position of the lowest point of the meniscus with

reference to the graduation line is such that it is in the plane

of the middle of the graduation line. This position of the

meniscus is obtained by making the setting in the center of the

ellipse formed by the graduation line on the front and the back

of the tube, as observed by having the eye slightly below the

plane of the graduation line. The setting, the procedures for

which will be discussed later, is accurate if, as the eye is

raised and the ellipse narrows, the lowest point of the meniscus

remains midway between the front and rear portions of the

graduation line. By this method, it is possible to observe the

approach of the meniscus from either above or below the line to

its proper setting.

The foregoing procedure applies when the index line runs

completely around the neck of the vessel. On some vessels, such

as measuring flasks, figure 1, some of the lines go only one-half

way around the neck. Under these conditions the operator has

only one-half of an ellipse to assist him in defining the

horizontal plane through the index line. Experience has shown

that, although the 360° index line is most convenient for the

operator, there is insignificant difference between the

achievable reproducibility from this complete index, and one

which extends only 180° around the neck.

On some graduated neck vessels the lines are very short indeed,

extending less than one-fourth the way around the neck. Under

these circumstances, there is no reference line behind the

meniscus to help the operator in estimating the vertical position

of the horizontal index plane. When faced with this problem, it

is necessary for the operator to place his eye in his

best estimate of the horizontal index plane while observing the

meniscus. With experience, an operator should be able to judge

the position of this plane to within one or two centimeters. The

operator's eye will be (perhaps) twenty-five centimeters in front

of the short index line while the center of the meniscus will be

only one centimeter behind this line, (for a two centimeter neck

on the vessel) . The geometry of this situation dictates that the

vertical position error of the meniscus will be only one twenty114
fifth of the operator's error in estimating the position of the

horizontal index plane. Although an optical system (e.g. a

bubble level cathetometer) could reduce this error to

insignificance, there is little point to such a calibration

procedure unless a similar system were employed by the user of

the vessel.

Having filled the reference volume of the test vessel using the

meniscus observation technique procedure described above, the

problem is to determine the volume of water contained, or the

volume which can be drained out under specified conditions. This

volume is calculated from the density of the water from table 2

and its measured mass.

In the case of a flask "to contain," this mass is the difference

between the mass of the empty flask and its mass after filling as

described above. For the flask "to deliver" it is the difference

between the mass of the filled flask and the mass after the

specified drainage period, 30 seconds, has occurred but before

the walls have had a chance to become dry .

It is not necessary to weigh burets and pipets because they are

"delivery" instruments. Under these circumstances their delivery

volume is drained into a weighing flask. Weighing this flask,

before and after loading, provides the mass of water delivered by

the buret or pipet.

The detailed procedures in use at NBS for filling and draining

the various test vessels are given in the following paragraphs.

A. BURETS

Clamp the buret vertically on a support stand; also clamp a plain

glass tube, large enough to hold a thermometer, near the buret if

the buret is of such a size that is not large enough to insert a

thermometer in the top for observing the temperature of the

water. A typical setup is shown in figure 5. Fill the buret

from the top to the zero mark with distilled water from a

reservoir or storage bottle. Drain and record the delivery time.

Delivery time is determined by the unrestricted outflow of the

water from the zero mark to the lowest graduation mark with the

stopcock fully open. Refill the buret to approximately 1 cm

above the zero mark and fill the plain glass tube that holds the

thermometer; record the temperature. Set a meniscus on the zero

mark using the buret stopcock to lower the liquid level and touch

the tip with the wetted wall of a beaker to remove any excess

water. A weighing flask which has been lightly stoppered and

weighed empty is placed with the inside of the neck in contact

with the tip of the buret (the flask will be at a slight angle)

.

The stopcock must be fully open until the water is only a few

115
millimeters above the line being tested and then slowed so as to

make an accurate setting. When the setting has been completed,

move the flask horizontally, breaking contact with the buret.

The flask is then stoppered and weighed a second time, after

which the thermometer tube is refilled and the next interval

tested in the same manner as the first one - from the zero mark

to the next interval needed.

B. PIPETS

Fill pipet with distilled water by suction to the index mark and

measure delivery time with the tip in contact with the glass

surface or the internal side of a beaker as shown in figure 6.

Refill by suction slightly above index line. Record the

temperature of the distilled water in the beaker from which the

pipet is filled. Any water on the outside of the tip is removed

with filter paper after the filling is completed. The meniscus

is then slowly lowered to the index using either the stopcock or

hose clamp for "fine control." The tip must be in contact with

the wet wall of a beaker while the setting is made on the index

line. Any water remaining on the tip at this time is not

removed. The pipet is held in a vertical position while water is

delivered into a previously weighed weighing flask with the tip

in contact with the inside wall of the neck of the flask as

indicated in the figure 6A. Contact is broken immediately after

outflow ceases. The flask is now stoppered and weighed with its

contained load.

C. FLASKS (TO CONTAIN)

After cleaning and drying, weigh the empty flask including the

stopper. Place an appropriate sized funnel in the flask to

discharge the water below the stopper. Fill from beaker or

supply line to a point slightly below the graduation line,

maneuvering the funnel so as to wet the entire neck below the

stopper. Let stand for about two minutes to allow the walls to

drain. Place the flask under the buret and complete filling and

setting the meniscus and place stopper in neck to lessen

evaporation and weigh. A pipet or dropper with a finely drawn

tip may be used instead of the buret. Determine the temperature

by placing a thermometer in the filling beaker or in one which

has been filled from the supply line. The temperature may be

taken after final weighing by placing a thermometer directly in

the flask, provided the flask is of sufficient size to

accommodate it.

D. FLASKS (TO DELIVER)

Flasks calibrated to deliver are not dried prior to the test.

Fill slightly below the index line and empty in 30 seconds by

gradually inclining the flask so as to avoid splashing of the

116
walls as much as possible. When the main drainage stream has

ceased, the flask will be nearly vertical. Hold in this position

another 30 seconds and touch off the drop of water adhering to

the top of the flask. Place a stopper, or cap, on the flask to

reduce evaporation and weigh immediately. Take water temperature

and fill the flask, and make the meniscus setting on the index

line. Place a cap, or stopper, on the filled flask and weigh.

4. WEIGHING PROCEDURE AND CALCULATIONS

Most laboratories employ single-pan direct-reading balances for

their weighing work. Although the manufacturers' instructions

for us£ differ in minor detail from brand-to-brand, the major

operating features are essentially the same. Most instruments

are equipped with a screen upon which an image is projected of a

small reticle attached to the beam. Under - ordinary

circumstances, a slight "zero" adjustment is made to the screen

reading with the balance pan empty. When the pan is loaded by

the unknown object, the operator manipulates dials which

mechanically add or remove built-in weights from the internal

balance structure. On some balances, the screen readings will be

"off-scale," either high or low, depending upon whether the

operator should increase or decrease the dial-operated load.

When the screen is "on-scale" the balance is ready to read. Some

balances have colored lights to indicate whether an increase or

decrease of built-in load is called for. The more sensitive

direct-reading balances are constrained from swinging freely

until the proper dial-operated load is in place. This condition

is called "partial arrestment." On the other hand, the

relatively insensitive top-loading balances (used when the

physical size of the weighing flask is large) are never arrested

except when the balance is being moved.

When the manual adjustment of the dial-operated load is complete,

the numerical balance indication should be observed in accordance

with the manufacturer's instructions. The coarse decades are

usually observed on the weight manipulation dials, and the last

two (or three) decades are projected on the screen. There are

two such indications, Ig referring to the empty vessel, and IL , the loaded vessel. The indication referring to the water is

obtained by subtraction:

I = IT ~ It, w L E

If l£ and Ij^ are observed under the same conditions the zero

adjustment, called for in the operating instructions, may be

eliminated

.

These numerical indications are normally not the mass of the

unknown. They are a measure on an arbitrary scale called

117
Apparent Mass (see appendix) of the load of built-in weights

which exert the same force on the balance as the unknown load

under the ambient conditions existing at the instant of the

weighing. We can write an equation indicating the equality of

the forces exerted on the balance by the water and the built-in

weights associated with Iw

- After eliminating "g" from both

sides, we have

r

p a"

M

w

i - —

pw_ - "b P B

(1)

where MR and p B are the mass and density of the built-in weights,

m t he mass of water, and p. the air density at the instant of

measurement. If we rearrange terms and divide both sides by the

density of water, we obtain an expression for the sought after

volume of water, V.

V = M. Pw - p A

1 - PA

(2)

Although the volume, V, in equation (2) is the volume of water in

the weighing flask at the instant of the weighing, it is also the

calibrated volume of the vessel under test at temperature, T. To

obtain this volume at 20 °C (the usual reference temperature) requires multiplication by the thermal expansion factor, K, which

involves the test temperature, T, and the temperature coefficient

of cubical expansion, a, of the material from which the vessel is

made, thus,

K = 1 - a(T-20)

and

V = M

20 B P w

~ P A

1 - K (3)

As explained in the appendix, the mass of the balance weights,

Mb, which exert the same force as the water is given by the

product of the balance indication for the water, 1^, and the

apparent mass conversion factor, Q, where

w L E

Q

=

and

V

20

P B

D

2Q

- .0012p

B

P B

D

2Q

- .0012D20

"L Pw " p A

Pa

l - —

p B

K (4)

Equation (4) shows the numerical value of the calibration, V

to be the product of five easily evaluated terms.

20

118
TERM 1

I. - I,

is the difference obtained by subtracting the

balance indication in grams associated with the

empty weighing flask from that associated with the

loaded flask.

[

Q

]

TERM 2 is the apparent mass conversion factor. An acceptable value may be found in table 3 based upon the

density of the balance weights, Pg, and the apparent

mass scale to which they are adjusted, D20 . Both Pg

and D

2Q may be obtained from the manufacturer of the

balance if the information is not on the name plate.

This value need be calculated only once for each

balance

.

TERM 3

1

Pw - pa

TERM 4

i-!a

There are two density terms which require knowledge

of air density, p^, water density, p w

, and the

density of the balance weights, Pg. Air density may

be adequately estimated by table 1. Water density

is obtained from table 2. The density of the

balance weights, pg, is obtained from the balance

maker. His quoted value will be the density at

20°C. Inasmuch as the test temperature, T, will be

near 20° and the coefficient of expansion is small

the quoted value is adequate for this work.

TERM 5 The thermal expansion factor is obtainable from

table 4 based upon water temperature, T

w

, assumed

K equal to test temperature, T, and the coefficient of

J expansion of the vessel, a.

A sample calculation based on typical data is shown on the flow

diagram, figure 7.

Examination of these numerical values shows that the value of

V20 , equation 4, is primarily dependent upon the term showing the

two balance indications, II - Ig • The other four terms, all

near unity, are corrections to the balance term for the various

factors mentioned above. For calibration work which is satisfied

by an uncertainty of a percent or so, these four terms may be

ignored

.

Use of this complete format is a tedious task which requires the

use of four tables and the performance of a five-multiplier

product. This is because the format is designed to accommodate a

very wide range of input data. However, this format offers a number of advantages:

119
a. It satisfies the requirements for the calibration

of fused silica vessels of enormous thermal

stability as well as plastic vessels of extreme

thermal sensitivity.

b. It accepts data from the rather old direct-reading

balances which were equipped with brass weights as

well as the modern instruments with steel weights.

c. It is adaptable to balances having built-in weights

adjusted to the old apparent mass vs 8.3909 scale

and the new apparent mass vs 8.0 scale.

There are few, if any, calibration laboratories which work on

such a wide variety of vessels with so many types of instruments.

The data reduction may be grossly simplified to one table and the

performance of one multiplication by restricting its applicability to the work most frequently performed.

For example, the vast majority of vessels which are submitted to

NBS for calibration are made of borosilicate glass so we may

eliminate a as a variable and assign to it a constant value of

0.000 010/ °C so that K becomes a single-valued function of

temperature,

K = 1 - 0.000 010 (T-20).

Additionally, all balances in the glassware calibration

laboratory have built-in weights whose densities at 20 °C are

very nearly 7.78 g/cm3

, so we may eliminate as a variable,

assigning to it the constant value

p

fi = 7.78 g/cm3

.

The built-in weights of these balances have all been adjusted to

apparent mass vs 8.3909, so that we may eliminate Q as a

variable, assigning to it a constant value of

Q = 1.000 011 2.

Without significant error we may assign values for the density of

air and of water of

. 0.464554B - 40(.00252T - 0.020582)

P A 1000(T + 273.16)

where B = barometric pressure in millimeters of mercury and

the value of relative humidity is assumed to

be 40%. [1]*

* Numbers in brackets refer to similarly numbered references at

the end of this paper.

120
(T - 3.9863) 2 T + 288.9414"

508929.2 T + 68.12963 (0.999973)

which is the Tilton-Taylor formula [2] expressed in g/cm 3

.

The four correction terms given in equation 4 are now reduced to

experimental constants or quantities which may be expressed in

terms of measured barometric pressure, B, and/or measured

temperature, T. If Z is the product of these four terms

The right member of equation 5 may be put into the form of a

numerical table entered with measured values of temperature and

pressure from which a value of Z may be selected, then

The table of values of Z must necessarily span the temperature

range found in the calibration laboratory. Table 5 is an abbreviated form of the table used in NBS glassware calibration. For

the restricted conditions mentioned above (i.e. borosilicate

glass vessels calibrated by balances with weights of 7.78 g/cm3 density adjusted to apparent mass vs. 8.3909) table 5 and equation 6 may be used with no loss of accuracy.

5. THE USE OF EQUAL-ARM BALANCES

Laboratories which employ equal-arm balances in the calibration

of glassware may find adequate procedures in NBS Circular 602

"Testing of Glass Volumetric Apparatus" by J.C. Hughes.*

* This publication is out-of-print but is available for loan at

most Government Depository Libraries throughout the country.

- 0.000 010(T-20)

121
Appendix 1. Apparent Mass

Apparent mass scales (there are two in current use) are attempts

to simplify the calibration and use of weights. They are

discussed in detail by Pontius [3]. Their background is of no

interest here but the concept should be understood because the

built-in weights of direct-reading balances are adjusted to

nominal values of apparent mass. If water volumes are to be

calculated from knowledge of density and the results of weighing

tests, the weighing results must be in mass, not apparent mass.

The observed numerical indication, I, obtained from a measurement

on a direct-reading balance is the apparent mass of the effective

dial-operated load pertaining to that measurement. The effective

load is the sum of the active built-in weights plus a small

additional amount observed on the screen. It is necessary to

convert observed values of I from units of apparent mass into

units of mass. This conversion follows directly from the

definition of apparent mass.

DEFINITION : When, under specified ambient conditions,

an unknown object exerts the same force on a balance as

mass, M, of a specified hypothetical reference material,

the object is said to have an apparent mass versus the

reference material equal to M*, where M* is numerically

equal to M. The specified ambient conditions are (1)

temperature = 20°C, (2) air density = 0.0 01 2 g/cm3

. The

hypothetical reference material is completely specified

by its density at 20°C. For the older apparent mass

scale, this specified density at 20°C, D

2Q , is 8.3909

g/cm3

, and for the more recent scale, 8.0.

By convention M* is used when the apparent mass scale is referred

to density 8.3909 material, and M** when referred to 8.0 density

material.

In the conversion procedure, the "unknown object" referred to in

the definition is the effective dial-operated load whose mass, M,

is to be calculated. The defining equality between the forces

exerted by the effective dial-operated load and by the mass, I,

of the hypothetical reference material is

where p B

is the density of the built-in weights. The 0.0012 is

the specified air density which must exist during the defining

equality. The mass of the effective dial-operated load is then

122
M = I

P B

D

2Q

- .0012p

B

P B

D

2Q

- .0012D20

Values of Pg and D

20 > not snown ^n tne balance instruction

book, are available from the manufacturer.

If Q

=

P B

D

2Q - .0012p

B

P B

D

w

- .0012D2Q

then M = IQ

Values of Q for various values of p B

and D

2Q are given in table 3,

123
Appendix 2

The nominal values of the reference volumes of vessels are etched

on the glass walls, and may be in units of milliliters or cubic

centimeters. While, strictly speaking, milliliters are no longer

an acceptable unit, for the purposes of this report, they will be

considered as identical to cubic centimeters. The basic SI unit

of volume is the cubic meter, M . Due to its large size, it is

rarely employed in glassware calibration work.

Experience has shown that a competent operator should repeat

volumetric calibrations within the limits indicated below.

VESSEL NOMINAL SIZE REPRODUCIBILITY*

Transfer Pipet 1 cm3 0 002 3

c. .002

5 .002

10 .003

15 .005

25 .005

50 .007

100 .010

Flasks 10 3 cmJ 0.005 3 cmJ 25 .005

50 .007

100 .011

200 .014

250 .017

500 .021

1000 .042

Burets 10 cm3 0.003 cm3 25 .005

50 .007

100 .012

* The term "reproducibility" means the maximum deviation expected

between two independent determinations of the volume. To

determine the reproducibility of one's process one should

recalibrate one of his own vessels periodically (e.g. once a

month) to obtain a sequence of values V,, V 2 , V3

, V"n

. From

these values one can compute an average V and a standard

deviation s.d. = VS(V-V) 2 / (n-1) and use the quantity 3(/2)(s.d.)

as the reproducibility of his process. It is essential that the

recalibration be done far enough apart in time to be truly

independent and representative of the process and the random

errors which affect it.

124
The volumetric error associated with a

observation is a function of the magnitude of

error and the cross section area of the vessel's

Some typical values are given below.

Error

Meniscus

in

Position 0.5 cm

Typical Neck Diameters

1 cm 2 cm 3 cm

0.005 cm 0.001 cm 3 .004 cm 3 .016 cm 3 .035 cm 3

0.01 ti .002 ii .008 ii .031 " .071 "

0.05 ii .010 ii .039 ii .157 " .353 "

0.10 ii .020 it .078 ii .314 "

. 707 "

0.20 n .039 n .157 ti .628 " 1.414 "

REFERENCES

[1] Bowman, H.A. and Schoonover, R.M. , Procedure for High

Precision Density Determinations by Hydrostatic

Weighing, J. Res. Nat. Bur. Stand. (U.S.), 71C (Eng.

and Instr.), No. 3, pp. 179-198, (July-Aug. 1967).

[2] Tilton, L.W. and Taylor, J.K., Accurate Representation

of the Refractivity and Density of Distilled Water as a

Function of Temperature, J. Res. Nat. Bur. Stand.

(U.S.), 18, RP971, pp. 205-213 (Feb. 1937).

[3] Pontius, P.E., Mass and Mass Values, Nat. Bur. Stand.

(U.S.) Monogr. 133, 33 pages (1974).

faulty meniscus

the observation

neck.

USCOMM-NBS-DC

125
TABLE 1

Air Density

TABLE 1A.

Listed below in table 1A are the approximate average Winter

(January) and Summer (June-July) air densities for a group

of selected cities throughout the continental United States.

These densities are for an assumed temperature of 23°C and

are based on information about the barometric pressure and

relative humidity supplied by the Weather Bureau.

It should be remembered that these are average air densities

and that the actual air density at a given time and place

may differ from that given by as much as 3 percent in either

direction, which may result in errors in V 20 of 0.05%.

This data may be used as a guide in estimating values of

required for the calculation of V20 , equation 4.

NOTE CAREFULLY

in equation 4 must be expressed in g/cm3 and the values

tabulated below are in mg/cm3

. Therefore, tabulated values

must be multiplied by 0.001 (1.18 mg/cm3 becomes 0.00118

g/cm3

)

This table is taken from Appendix 1 in NBS Monograph 133,

Mass and Mass Values, by Paul E. Pontius.

TABLE IB.

This table provides values of air density calculated from

temperature, barometric pressure and assumed relative humidity of 40%. Errors in V20 which result from use of this

table are insignificant.

126
TABLE 1A

Place

City

Birmingham.. Montgomery..

Flagstaff.. Phoenix... Tucson... Yuma

Little Rock.

Los Angeles.... Oakland

San Diego San Francisco

.

Denver Grand Junction.. Pueblo

Hartford

Washington

.

Jacksonville.. Key West

Miami Tampa

Atlanta Augusta

(Aiken. S.C.) Savannah

Boise

Cairo Chicago

Moline

Springfield.

Fort Wayne..

Indianapolis..

Burlington Des Moines.. Dubuque

Sioux City...

Concordia... Dodge City. Topeka

Wichita

LouisviUe..

New Orleans. Shreveport

Eastport..

Boston

Alpena

Detroit Grand Rapids Marquette

Saull Ste. Marie .

Duluth Minneapolis

St. Paul (Airport)..

Vicksburg..

Kansas City.

St. LouU

Springfield...

Havre... Helena..

Kalispell

Air density

Winter Summer

Place

("if/t"')

1.16

1.18

0.92

1.15

1.08

1.19

1.18

1.17

1.19

1.18

1.18

0.98 . 1.00

1.00

1.18

1.19

1.19

1.19

1.19

1.19

LIS

1.18

1.19

1.07

1.18

1.16

1.17

1.16

1.15

1.15

1.16 LIS

1.16 1.15

1.13 1.08 LIS

1.14

1.17

1.19

1.18

1.18

1.18

1.16

1.16

1.15

1.13 1.16

1.14

1.15 LIS

1.18

LIS

1.17

1.14

1.03

1.02

1.03

(mx/cmJ

) 1.16 118

0.93

1.14

1.08

1.18

1.16

1.16

1.18

1.18

0.98

1.00 LOO

l.ia

1.1a

1.18

1.18

1.18

1.18

1.14

1.18

118

1.07

1.17

1.16

1.17

1.16

1.15

1.15

1.15 LIS

LIS

1.13

1.12

1.08 LIS

1.13

1.17

1.18

1.18

1.18

1.18

1.16 1.15 LIS

1.15

1.16

1.14 LIS LIS

1.18

LIS

1.16

1.13

1.08

1.02

1.03

Slate

Nebraska.

Nevada..

New Hampshire . New Jersey New Mexico New York

North Carolina..

North Dakota..

Ohio.

Oklahoma.

.

Oregon..

Pennsylvania. ... Rhode Island South Carolina

.

South Dakota...

Tenn

Texas..

Utah

Vermont..

Virginia...

Washington .

West Virginia..

Wisconsin

.

Wyoming..

City

Lincoln North Platte..

Ely Las V egas .

Concord Newark

Albuquerque.. Albany

Buffalo

Hatteras.

Raleigh...

Bismarck

Devils Lake.. Fargo

WiUiston

Cincinnati

.

Dayton

Cleveland..

Oklahoma City

.

Baker

Medford...

Portland... Roseburg..

Philadelphia..

Providence....

Charleston.... Columbia Huron

Rapid City....

Knoxville (Oak

Ridge). Memphis

Nashville

Abilene

Amarillo

Austin

Brownsville .. Fort Worth...

El Paso Houston San Antonio..

Salt Lake City.. Burlington Norfolk Richmond

Seattle Spokane

Walla Walla..

Elkins Parkersburg..

Green Bay.. Madison

Milwaukee..

Casper Cheyenne.

Sheridan...

Air density

Winter Summer

1.14

1.07

0.95 111

1.17

1.19

1.00

1.18

1.16

1.18

1.18

1.11

1.12

1.15

1.11

1.16 LIS

LIS

1.14

1.04

1.13 1.18 1.17

1.18

1.18

1.19

1.18 1.14

1.05

1.16

1.18 1.17

1.12 1.04

1.17 1.19

1.16

1.04

1.18

1.17

1.02

1.17 1.19 1.18

1.18

1.19

1.15

1.11 1.16

1.16 LIS

1.16

0.97 0.94

1.03

127
BAROMETRIC

PRESSURE

(MM OF HG) 16°C

C f\ f\ 600 0 .00096

C AC 605 0* 00097

610 0.00098

615 0.00098

co a 6tU n nnnc\t\ 0 .00099

COS. A AA 1 AA 0.00 100

630 0.00101

635 0.00102

C A A 640 A AA 1 AO 0 .001 02

C A R 645 A AA 1 A X 0 . 00 1 03

650 0.00104

655 0.00105

CC A 66 UA AA 1 A^ 0 .001 06

c c *» 665 a aa i nc 0 . 00 1 06

670 0.00107

675 0.00108

can A A A 1 An U .U01 09

6S5 A AA 1 1 A 0.001 10

690 0.001 10

695 0.0011 1 inn #00 0 .001 12

*? ac 705 0.001 13

710 0.001 14

715 0.001 15

720 0.001 15

725 0.001 16

730 0.001 17

735 0.00118

740 0.001 19

745 0.001 1

9

750 0.00120

755 0.00121

760 0.00122

765 0.00123

770 0.00123

775 0.00124

780 0.00125

785 0.00126

790 0.00127

795 0.00127

18°C 20 °C

0.00095 0.00095

0.00096 0.00095

0.00097 0.00096

0.00098 0.00097

0.00099 0.00098

0.00099 0.00099

0.00100 0.00099

0.00101 0.00100

0.00102 0.00101

0.00103 0.00102

0.00103 0.00103

0.00104 0.00103

0.00105 0.00104

0.00106 0.00105

0.00107 0.00106

0.00107 0.00107

0.00108 0.00107

0.00109 0.00108

0.001 10 0.00109

0.00110 0.00110

0.00111 0.00110

0.00112 0.00111

0.00113 0.00112

0.00114 0.00113

0.001 14 0.001 14

0.00115 0.00114

0.001 16 0.001 15

0.00117 0.00116

0.001 18 0.001 17

0.00118 0.00118

0.001 19 0.001 18

0.00120 0.00119

0.00121 0.00120

0.00122 0.00121

0.00122 0.00122

0.00123 0.00122

0.00124 0.00123

0.00125 0.00124

0.00126 0.00125

0.00126 0.00126

TABLE IB

22 °C 24 °C

0.00094 0.00093

0.00095 0.00094

0.00096 0.00095

0.00096 0.00096

0.00097 0.00096

0.00098 0.00097

0.00099 0.00098

0.00099 0.00099

0.00100 0.00100

0.00101 0.00100

0.00102 0.00101

0.00103 0.00102

0.00103 0.00103

0.00104 0.00103

0.00105 0.00104

0.00106 0.00105

0.00107 0.00106

0.00107 0.00107

0.00108 0.00107

0.00109 0.00108

0.00110 0.00109

0.00110 0.00110

0.00111 0.00110

0.00112 0.0011 I

0.00113 0.00112

0.00114 0.00113

0.00114 0.00114

0.00115 0.00114

0.00116 0.00115

0.00117 0.00116

0.001 18 0.001 17

0.00118 0.00118

0.001 19 0.001 18

0.00120 0.00119

0.00121 0.00120

0.00122 0.00121

0.00122 0.00121

0.00123 0.00122

0.00124 0.00123

0.00125 0.00124

26 °C 28 °C

0.00093 0.00092

0.00093 0.00093

0.00094 0.00093

0.00095 0.00094

0.00096 0.00095

0.00096 0.00096

0.00097 0.00097

0.00098 0.00097

0.00099 0.00098

0.00100 0.00099

0.00100 0.00100

0.00101 0.00100

0.00102 0.00101

0.00103 0.00102

0.00103 0.00103

0.00104 0.00103

0.00105 0.00104

0.00106 0.00105

0.00107 0.00106

0.00107 0.00107

0.00108 0.00107

0.00109 0.00108

0.00110 0.00109

0.00110 0.00110

0.00111 0.00110

0.00112 0.00111

0.00113 0.00112

0.00114 0.00113

0.00114 0.00113

0.00115 0.00114

0.00116 0.00115

0.00117 0.00116

0.00117 0.00117

0.00118 0.00117

0.00119 0.00118

0.00120 0.00119

0.00121 0.00120

0.00121 0.00120

0.00122 0.00121

0.00123 0.00122

128
j 0.0°C 0. 1°C 0.2^: 0 . 3 °C 0 .4°C 0.5°C 0.6°C 0. 7°C 0. 8°C 0. 9° C

.0 .999840 .999846 . 999853 .999859 .999865 . 999871 .999877 .999883 .999888 .999393

1.0 .999899 .999903 .999508 .955913 .9599 17 .999521 .999925 .999929 .999933 . 999937

c «v Q QQQ4

n

• 7 777 ** U . 99994

3

. 999945 . 999949 . 99995 2 . 999954 .999956 9 999959 .99996 i .999963

3.0 .999964 .999966 . 99996 7 # 999968 . 999959 . 999970 m 99997

l

. 99997

1

. 999972 . 999972

4. 0 • 9 999 72 , 999972 . 999972 . 999971 . 99 gg 7

1

. 999970 ,999969 . 999968 .999957 . 99996 5

5.0 .999964 .999962 .999960 .999958 .999956 . 999954 .99995

1

.999949 .999946 . 999943

6.0 •9S9940 .995937 . 999933 .999930 . 999926 .999922 .999918 .999914 .999910 . 999906

7 fi . g 9990

1

• 7 7 Q 7 O • cog 777 QQ1 O' tZ. . 99966 7 . 9996 81 . 909876 . 99 Q 8 7 1 . 99966 5 • 97 99J ' 860 <J VJ \J . 9996 54

fa n . 99984 8 m 999 2 • 9996 3 5 5 99629 .999822 . 9598 16 . 999 80 9 .999802 . 999795 . 9997 g 7 9.0 « 9 99780 oqq T 7 ~\ • 7 7 7 I 1 J . 999765 QQQ7 S 7 . 999749 . 999741 .999 73 3 . 99 97 25 .99971

6

. 999707

10.0 .999699 .999690 .999681 .959672 .999662 .999653 .999643 .999634 .999624 . 559614

11.0 .999604 .999594 .999583 .999573 .999562 .999552 .999541 •999530 .999519 . 595507

12 «0 . 999496 5594 85 m 999 47 3 .999461 • 999449 .999437 .99942 5 .99941

3

. 99940

1

. 9993 88 13 .0 .9 9937 6 .999363 . 999 350 . 999337 . 999324 . 9993 1

1

. 999297 . 999284 .999270 .999256

14.0 •999243 . 9592 29 . 99921

5

.999200 .999186 . 9991 72 .999 157 . 999142 . 9991 28 • 9991 I 3

15.0 .999098 .999083 .999067 .999052 .999036 .999021 .999005 .998989 .998973 .998957

16.0 .998941 .998925 .998908 .998892 .996375 .598858 .958841 .998824 .998807 .998790

17.0 . 998773 . 998755 . 998738 .99872 0 . 998"'02 . 998684 .998666 . 99 364 8 .998630 . 998612

18 .0 .998593 .998575 .998556 .998537 . 998519 . 998500 .998480 .998461 .998442 . 99842 2 19.0 . 998403 . 958383 . 998364 . 998344 .998324 . 998304 .998284 .993263 .998243 . 998222

20.0 .998202 .998181 .998160 .996139 . 998118 .998097 .998076 .998055 .998033 .998012

21.0 .997990 .997968 .997947 .997925 .997903 .997881 .997858 .997836 .957814 . 997791

22.0 •997768 .997746 • 997723 . 997700 .997677 . 997654 .997630 .997607 .597584 . 997560

23.0 .99753 6 •9975 13 .997 489 .9 97465 •997441 • 997417 • 997 39 2 . 997368 .997344 . 9973 1

9

24.0 •997294 • 9972 70 • 99724 5 .997220 • 997195 . 9971 70 • 997 14 5 .997 119 •997094 . 7 7 1 \J O O

25.0 .997043 .997017 . 996991 .996966 .996940 .996913 .996887 .996861 .996835 . 996808

26.0 .996782 .996755 .996723 .996702 .996675 . 996e48 .996621 .996593 .996566 .996539

C i . u Q Q AS 1 1 QC AZi AZ* oc A4 o a .77Q C OtU,'i 11 CQa7 7"2

. 7 7 O .3 / Z> C c A "X A. *\ • 7 7 O J lO • 7 70 C. O 0 . 7 7 O <L O U

? ft n . Q7 Q7 AD C.7 J^ i.3 • 770£ qqj: ITS 7701HO QQ A I 1 7 • 77 O UO U • 99603

1

QQAfifl 1 . QCi 7 7 SJ Q7 7/ 9C. 29 .0 • 995943 •9959 14 •995684 •995855 . 7 7 JO £ J . 995795 • 9 95 76 5 . QQS77, 7 7 JJ IOOa • 995 70 6 • 99 56 76

30.0 . 995645 . 99561

5

. 995 58 5 . 9 95554 . 995524 . 595493 .995463 .995432 •99540

1

. 995370

31.0 .995339 .995308 .995277 .995246 .995214 . 995183 .995151 .995120 .995088 .995056

32.0 .995024 . 994992 . 994560 .954528 .994896 .9948o4 .994831 .994799 '.994766 . 994734

33.0 .994701 .994668 .994635 .994602 .994569 .9945 36 .994503 •994470 .994436 • 994403

34.0 .994369 .994336 . 994302 .994268 .994234 . 994201 .994 167 . 994132 .994098 . 994064

35.0 .994030 .993995 . 9939ol .993926 . 993891 .993857 .993322 .993787 .993752 .993717

36.0 .993682 .993647 .99361

1

.993576 .993541 .993505 .993469 . 993434 .593398 .993362

37.0 .993326 .993290 .993254 .993218 .993182 .993146 .993109 .993073 .993036 . 993000

38.0 .992563 .992926 .992889 .992652 .992815 .592778 .992741 •992704 .992667 . 992629

39.0 .992592 .992554 .992517 .992479 . 992442 . 992404 .992366 .992328 .992290 •992252

TABLE 2: Density of Air Free Water in g/cm 3 as a Function of the Celsius

Temperature Scale, Based on the Work by H. Wagenbreth and W.

Blanke, PTB-Mitteilungen 6-71.

129
P B

Q when 1^ wrien = o.UUOO g/cmJ 20 o.jyuy g/cm

7,70 1.0000058 1.0000128

7,72 1.0000054 1.0000124

7.74 1.0000050 1.0000120

7.76 1.0000046 1.00001 16

7.78 1.0000042 1.00001 12

7.80 1.0000038 1.0000108

7.82 1.0000035 1.0000104

7.84 1.0000031 1.0000101

7.86 1.0000027 1.0000097

7.88 1.0000023 1.0000093

7.90 1.0000019 1.0000089

7.92 1.0000015 1.0000085

7.94 1.0000011.0000081

7.96 1.0000008 1.0000077

7.98 1.0000004 1.0000074

8.00 1.0000000 1.0000070

8.02 0.9999996 1.0000066

8.04 0.9999993 1.0000062

8.06 0.9999989 1.0000059

8.08 0.9999985 1.0000055

8.10 0.9999981 1.0000051

8.12 0.9999978 1.0000048

8.14 0.9999974 1.0000044

8.16 0.9999971 1.0000040

8.18 0.9999967 1.000003 7

8.20 0.9999963 1.0000033

8.22 0.9999960 1.0000030

8.24 0.9999956 1.0000026

8.26 0.9999953 1.0000023

8.28 0.9999949 1.0000019

8.30 0.9999946 1.0000016

8.32 0.9999942 1.0000012

8.34 0.9999939 1.0000009

8.36 0.9999935 1.0000005

8.38 0.9999932 1.0000002

8.40 0.9999929 0.9999998

TABLE 3: Values of the apparent mass conversion factor,

Q, defined in the appendix as

P B

(D 20 - .0012)

Q

= D 20 (PB

- .0012)

when is the density of the balance weights

in g/cm-*, and D 20 is the apparent mass scale

to which the weights are adjusted.

130
Fused Boro- Poly- PolyWater Silica Silicate Soft propylene Carbonate

Temp (Quartz) Glass Glass Plastic Plastic

(°c) ot=l . 6 a=10 a=25 a=240 a=450

i s a 1i . aaaaax uuu uuo i nr\r\no c\

I . U UUUt

u

1

UUUUj

u

i n A r\ a a A 1 AAAQAD

11 a s O . J i aaaaa^ i a a a a i s l O AA A\

o

1 AAA ^£ A 1 AAA67 S

i o a 1

i • aaaaai> UUUUU6 1t . aaa UUUUa Ii n

u

1 AAAA9S 1 AAA94A 1 000450

i1 9y » 5 i aoaaai i aaaaas 1 AAA A 1 X 1 AAA 120 1 000225

20-0 1 .000000 • w VS w Is 1/ U 1 000000 1 000000 11 . 000000 w W w v W \J 1 .000000

20.5 >j»yyyyyy 0w. QQQ0Q5 y yy yy j 0 » QQOOBR y y y yo o 0 90QRBA 0 999775

21.0 0.999998 0.999990 0.999975 0.999760 0.999550

21.5 0.999998 0 .999985 0 . 999963 0. 999640 0.999325

22.0 0 .999997 0 . 999980 0.999950 0 . 999520 0.999 100

22.5 0 .999996 0.999975 0 . 99993R 0.999400 0 99RR75

23.0 0.999995 0.999970 0.99992 5 0 9992R0 0vy « 99R650 y yg vy v 23.5 0 . 999965 0 9999 1

\j*yyyy\*j \j0 * 999 y y y ivju 160 0 99B425

24.0 0 . 999Q94 0 0000RA A 0900AA A 000040 0 . 99B 200

24.5 a o oo o s

s

Au OOOSBB

.

yy? oo d A O OH 09 A 0\j » QQ7975 y y 1 y 1 ? 5 a A QQQQQ9 A O OO OS A A Q O o a 7 S A Q OB BAA 0 QQ7750

25 5 A Q OO O O

1

U

.

y y

y

BOO A QOBfiBA 0 QQ7525

9& a A QOQQQH A o o a D S A A<J O OB S£ A » y yo -'O u A 00 7 ^on

26.5 0.999990 0.999935 0.999838 0.998440 0.997075

27.0 0.999989 0.999930 0.999825 0.998320 0.996850

27.5 0.999988 0.999925 0.999813 0.998200 0.996625

28.0 0.999987 0.999920 0.999800 0.998080 0.996400

28.5 0.999986 0.999915 0.999788 0.997960 0.996175

29.0 0.999986 0.999910 0.999775 0.997840 0.995950

29.5 0.999985 0.999905 0.999763 0.997720 0.995725

30.0 0.999984 0.999900 0.999750 0.997600 0.995500

TABLE 4 : Values of the expansion factor K as a function of Cest

temperature for various materials from which laboratory ware

is made. The numerical values of a, the volumetric

coefficient of thermal expansion, given in the column

headings are parts per million per degree centigrade. In

plastics, columns 4 and 5, this coefficient may vary from

batch-to-batch, and the values stated are typical, mid-range

values. In the text K is defined as

K = 1 - a(T-20)

131
TEMP BAROMETRIC PRESSURE (MM OF HQ)

( °C) 620 640 660 680 700

16.5 1 .002390 1.002418 1.002446 1.002473 1.002501

19.0 1 .002480 1.002508 1.002536 1.002564 1.002592

19.5 1 .002573 1.002601 1.002629 1.002657 1 .002685

20.0 1.002669 1.002697 1.002725 1.002753 1.002780

20.5 1.002768 1.002795 1.002823 1 • 0028 5

1.002879

21 .0 1 .002869 1.002897 1.002924 1.002952 1.002980

21 .5 1.002973 1.003000 1.003028 1.003056 1 .003083

22.0 1 .003079 1.003107 1.003134 1.003162 1.003190

22.5 1.003188 1.003216 1.003243 1.003271 1.003298

23.0 1.003299 1.003327 1.003354 1.003382 1.003410

23.5 1.003413 1.003441 1.003468 1.003496 1 .003523

24.0 1 .003530 1.003557 1.003585 1.003612 1.003640

24.5 1.003649 1.003676 1.003704 1.003731 1.003758

25.0 1.003770 1.003797 1.003825 1.003852 1.003880

25.5 1 .003894 1.003921 1.003949 1.003976 1 .004003

26.0 1 .004020 1 .004047 1.004075 1 .004102 1.004129

26.5 1.004149 1.004176 1.004203 1.004231 1.004258

27.0 1.004280 1.004307 1.004334 1.004361 1.004389

27.5 1.004413 1.004440 1.004467 1.004495 1.004522

28.0 1 .004549 1.004576 1.004603 1.004630 1.004657

TEMP BAROMETRIC PRESSURE (MM OF HG)

(°C) 720 740 760 780 800

18.5 1 .002529 1 .002557 1.002585 1.002613 1.002641

19.0 1 .002620 1 .002647 1.002675 1.002703 1.002731

19.5 1 .002713 1 .002740 1.002768 1.002796 1.002824

20.0 1 .002808 1 .002836 1.002864 1 .002892 1.002919

20.5 1 .002906 1 .002934 1.002962 i .002990 1.003017

21.0 1 .003007 1 .003035 1 .003063 1.003091 1.003118

21,5 1 .00311 1 1.003139 1.003166 1.003194 1.003222

22.0 1 .003217 1 .003245 1.003272 1.003300 1.003328

22.5 1 .003326 1 .003353 1.003381 1.003409 1.003436

23.0 1 .003437 1 .003 465 1.003492 1.003520 1 .003547

23.5 1 .003551 1 .003578 1.003606 1.003633 1.003661

24.0 1 .003667 1 .003695 1.003 722 1.003 750 1.003777

24.5 1 .003 786 1 .003813 1.003841 1.003868 1.003896

25.0 1 .003 907 1.003934 1.003962 1.003989 1.004017

25.5 1 .00403

1 .004058 1.004085 1.004113 1.004140

26.0 1 .004157 1 .004184 1.00421

1.004239 1 .004266

26.5 1 .004285 1 .004312 1.004340 1.004367 1 .004394

27.0 1 .004416 1 .004443 1.004470 1.004498 1.004525

27.5 1 .004549 1 .004576 1.004603 1.004631 1.004658

28.0 1 .004685 1 .004712 1.004 739 1.004 766 1.004793

5: Values of Z (equation 6) as Explained in the Text.

132
n

INDEX

LINE

TO CONTAIN

NOMINAL

VOLUME

TO

DELIVER

TD

lzo° C

DELIVERY

TIME

IN

SECONDS

63

5*3

113 SEC #0

VOLUMETRIC

FLASK PIPET BURET

^57

SO.

100 • to*

10 SIC ONA<N

MEASURING

FLASK

10* c

10 tic DM* in

: 80

CYLINDRICAL

GRADUATE

FIGURE 1

SHOWN HERE ARE THE VARIOUS TYPES

OF VOLUMETRIC APPARATUS WHICH

MAY BE CALIBRATED BY -THE TECHNIQUES DESCRIBED IN THIS REPORT.

133
FIGURE 2

DRY AIR BOTTLES AND DRYING RACK

USED TO DRY VOLUMETRIC GLASSWARE

PRESSURE IS ABOUT 1 LB PER SQ IN.

DRYING USUALLY TAKES LESS THAN 10 MINUTES

134
MENISCUS SURFACE

BRIGHTLY ILLUMINATED

SCREEN —

FRONT SIDE OF

INDEX LINE

REAR SIDE OF

INDEX LINE

EXAGGERATED CURVATURE

MENISCUS SURFACE

CIRCUMFERENTIAL INDEX

LINE ON VESSEL

LINE - OF -SIGHT TO

FRONT SIDE OF INDEX

OBSERVERS EYE

LINE-OF- SIGHT TO

REAR SIDE OF INDEX

LINE-OF-SIGHT TO

MENISCUS

OBSERVER'S VIEW FROM SLIGHTLY

BELOW THE HORIZONTAL LEVEL OF

THE INDEX LINE.

BLACK RUBBER OPAQUE SCREEN

PLACED A FRACTION OF A

MILLIMETER BELOW THE INDEX

LINE.

SIDE VIEW

FIGURE 3

THESE DIAGRAMS ILLUSTRATE THE PROCEDURE FOR OBSERVING

THE MENISCUS POSITION DESCRIBED IN THE TEXT.

II

135
FIGURE 4

VARIOUS RUBBER COLLARS USED TO ASSIST

IN THE MENISCUS SETTING DURING THE

CALIBRATION OF BURETS, PIPETS AND FLASKS

136
I

FIGURE 4A

A FLASK WITH A COLLAR-SHAPED PIECE OF BLACK

RUBBER TUBING IN PLACE BELOW THE INDEX LINE

WHICH RENDERS THE PROFILE OF THE MENISCUS

CLEARLY VISIBLE AGAINST A LIGHT BACKGROUND

137
FIGURE 5

APPARATUS USED IN THE CALIBRATION OF A 50ML BURET. THE USHAPED TUBE AT THE TOP IS CONNECTED TO A DISTILLED WATER

SUPPLY LINE USED TO FILL THE BURET. THE WEIGHING FLASK

UNDER THE BURET IS IN POSITION, WITH ITS TIP IN CONTACT

WITH THE NECK OF THE FLASK TO RECEIVE THE MEASURED VOLUME.

THE BLACK COLLARS AT THE TOP OF THE BURET ARE AN AID IN

SETTING THE MENISCUS. THE THERMOMETER IS USED TO OBTAIN A

MEASURE OF THE TEMPERATURE, OF THE WATER IN THE BURET.

138
FIGURE 6

A

THE PIPET IS DELIVERING ITS MEASURED VOLUME INTO

A WEIGHING FLASK, WITH ITS TIP IN CONTACT WITH

THE NECK OF THE FLASK. WHEN THE OUTFLOW HAS

CEASED THE FLASK IS REMOVED, STOPPERED AND WEIGHED.

139
FIGURE 6

TRANSFER PIPET CLAMPED VERTICALLY ON A STAND IN FRONT OF A

FROSTED GLASS LIGHT BOX. THE TIP IS IN CONTACT WITH A PIECE

OF FROSTED GLASS WHILE THE SETTING OF THE MENISCUS IS MADE.

THE HOSE ON THE GLASS TUBE ABOVE THE STOPCOCK IS CONNECTED

TO A TRAP BOTTLE WHICH, IN TURN, IS CONNECTED TO THE VACUUM

LINE. THE HOSE CLAMP IS USED TO SLOW THE FLOW WHILE MAKING

THE MENISCUS SETTING. THE STOPWATCH IS FOR TIMING THE

DELIVERY OF THE PIPET. THE TEMPERATURE OF THE DISTILLED

WATER IS TAKEN IN THE BEAKER FROM WHICH THE PIPET IS FILLED.

140
INPUT DATA ITEMS

VOLUMETRIC EXPANSION

a COEFFICIENT OF VESSEL

UNDER TEST

T WATER TEMPERATURE

GEOGRAPHIC TEST SITE*

MANUFACTURER ESTIMATE

P B OF DENSITY OF BUILTIN BALANCE WEIGHTS

APPARENT MASS SCALE

D20 TO WHICH BALANCE

WEIGHTS ARE ADJUSTED

I BALANCE INDICATION FOR

LOADED WEIGHING FLASK

TYPICAL VALUES

I BALANCE INDICATION FOR

EMPTY WEIGHING FLASK

1

THE CALIBRATION VOLUME,

V

Q

, IS THE PRODUCT OF

THESE FIVE TERMS

.000010

T = 24.9 °C

TABLE

4

K = .999951 K = .999951

TABLE

2

P w

= .997068 g/cmWashington , D.C. TABLE

1A

PA= .00118 g/cro 3

'B = 7.78 g/cm ;

D

2 0 = 8.3909 g/cm3

P w

" P A

1-^

°B

= 1.004129

IL - 93.1056 g

TABLE Q = 1.000011

3 [ ]

IE

- 83.1513 g

T L~ h

* AIR DENSITY VALUES BASED UPON GEOGRAPHIC LOCATION

ARE QUITE ADEQUATE FOR THIS WORK. FOR A BETTER

VALUE, TABLE IB PROVIDES AN ESTIMATE BASED UPON

MEASURED TEMPERATURE AND BAROMETRIC PRESSURE AND

ASSUMED RELATIVE HUMIDITY.

cnr

= .999848

1.000011

= 9.9543 g

V

2Q

- 9.9935 cm 3

FIGURE 7

141

143
U.S. Department of Commerce a nnrMmv \/tt

Juanita M. Kreps ArrC.INDIA Vll

Secretary

National Bureau of Standards

Ernest Ambler, Acting Director Rational ££ttram of jltatulartte

Certificate

Standard Reference Material 935

Crystalline Potassium Dichromate

for Use as an Ultraviolet Absorbance Standard

R. W. Burke and R. Mavrodineanu

This Standard Reference Material consists of crystalline potassium dichromate of established purity. Solutions

of known concentrations of this SRM in 0.001 N perchloric acid are certified for their apparent* specific

absorbances**, Ca , at 23.5 °C.

This SRM is intended to be used as a reference standard for the verification of the accuracy and linearity of the

absorbance scale of absorption spectrometers that can provide an effective spectral bandpass of 1.2 nm or less

at 235 nm, and 0.8 nm or less at 257, 313, 345, and 350 nm. Such verification is accomplished by comparing the

measured apparent absorbances, Aa , to the Aa calculated from the certified €a values as described under

"Instructions for Use."

Table 1 gives the certified values of Ca in kgg -cm

1 for five concentrations of the SRM 935 potassium

dichromate in 0.001 N perchloric acid at 23.5 °C and the indicated wavelengths and spectral bandpasses for a 1-cm internal pathlength.

Table 1. 6a , Apparent Specific Absorbance,

3 kg g

' cm

Nominal Wavelength and (Bandpass) nm

Concentration

g'kg Uncertaii

- ' 235.0(1.2) 257.0(0.8) 313.0(0.8) 345.0(0.8)

b 350.0(0.8)

0.020 12.260 14.262 4.805 10.604 10.672 ± 0.034

.040 12.304 14.318 4.811 10.603 10.682 ± .020

d

.060 12.347 14.374 4.816 10.602 10.692 ± .020

d

.080 12.390 14.430 4.821 10.601 10.701 ± .020

d

.100 12.434 14.486 4.827 10.600 10.711 ± .020

d

6a values are given to the third decimal place to preserve the smooth variation of the data with concentration, although the

uncertainties are in the second decimal place. Wavelength 345.0 nm is near one of the two isosbestic points in HCrOJ/CnO? spectra. Because it is on the slope of the composite

spectrum, reproduction of the 6a values is dependent on wavelength accuracy. Measurements at this wavelength should be made only for

verification of the linearity of the absorbance scale. 6a values are not corrected for the effects of internal multiple reflections within the cuvette, nor have the weights been corrected to vacuum. With these two exceptions, the uncertainties given include all known systematic errors and the 95 percent confidence

interval of the mean.

At wavelength 313.0 nm, the uncertainty is reduced to ±0.010.

*The term "apparent" is used because no corrections have been applied to the data for the effects of internal multiple reflections within the

cuvette or for buoyancy, i.e., the weights used to express concentrations have not been corrected to vacuum. These combined corrections do not exceed 0.2 percent. The specific absorbances are given in reference 1. "The nomenclature used in this certificate is that recommended by K. D. Mielenz, Anal. Chem. 48, 1 093-1 094 ( 1976), which is reproduced

in the Appendix of NBS Special Publication 260-54.

Washington, D.C. 20234 J. paul Cali, Chief

June 1, 1977 Office of Standard Reference Materials

(over)

144
The overall direction and coordination of the technical measurements leading to this certificate were per- formed under the joint chairmanship of I. L. Barnes and J. R. DeVoe.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard

Reference Material were coordinated through the Office of Standard Reference Materials by R. W. Seward.

We wish to acknowledge: The contributions of K. D. Mielenz, NBS Analytical Chemistry Division, for his many

valuable discussions and suggestions, and H. H. Ku, NBS Applied Mathematics Division, for his statistical treatment of the data; the cooperation and early support of the National Institute of General Medical Sciences in

the research leading to this SRM; and the encouragement of George N. Bowers, Jr., M.D., Hartford Hospital,

Hartford, Conn., and Royden N. Rand, Ph.D., Eastman Kodak Co., Research Laboratories, Rochester, N.Y.

PREPARATION AND CERTIFICATION

The details of the preparation and certification of SRM 935 are provided in NBS Special Publication 260-54,

Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance Standard [2]. This publication should be referred to every time SRM 935 is to be used. Briefly, the transmittance, T, of the

solutions prepared from the undried, as received, material were measured with the NBS Institute for Materials

Research high-accuracy transmission spectrometer [3].

The €a values were calculated for each wavelength using the relation:

ea "TTT~"bTr (1)

where:

€a = apparent specific absorbance

Aa = apparent absorbance

Ds = transmittance density of the sample solution, -logioTs Db= transmittance density of the blank solution, -logioTb

b = internal cuvette pathlength, cm

c = concentration, by weight, of K.2Cr207 solution, g kg

The crystalline potassium dichromate used for SRM 935 is a special lot of analytical reagent grade material

obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J.

Assay: A coulometric assay of the purity of the undried material was performed by G. Marinenko of the NBS

Analytical Chemistry Division. The purity of SRM 935, expressed as an oxidant, was found to be 99.972 ± 0.005

percent where the uncertainty figure represents the 95 percent confidence interval for the mean based on 1

1

degrees of freedom. In addition, the material was examind by optical emission spectrometry for trace elemental

impurities by J. A. Norris of the NBS Analytical Chemistry Division. The only significant impurities detected were sodium and rubidium. Their concentrations were estimated to be in the range of 0.02 and 0.03 percent,

respectively. Drying at 105 °C for 12 hours showed that the surface moisture of this material was less than

0.01 percent.

Stability:

Solutions prepared from SRM 935 in the concentration range indicated in table 1 and made according to the

instructions given in NBS SP 260-54 have been found to be stable within the uncertainties given in table 1 for at

least six months when stored at room temperature and protected from evaporation and exposure to light.

INSTRUCTIONS FOR USE

The use of SRM 935 as an absorbance standard requires the careful preparation of a series of solutions of

known concentrations, c, of the potassium dichromate in 0.001 N perchloric acid. These solutions are transferred to a quartz cuvette of known pathlength, b, and their apparent absorbances measured at wavelengths

235, 257, 313, and 350 nm, using the spectral bandpass requirements given in table 1. The preparation and

measurement of these solutions are described in detail in Section 5 of NBS SP 260-54.

145
The accuracy of the absorbance scale of the spectrometer being tested is ascertained by comparing the measured

apparent absorbances, Aa , of a series of 0.001 N perchloric acid solutions containing 0.020 to 0.100 gram

K^C^Cb/kgto the Aa values calculated from the certified 6a values. Although the data in table 1 are given for

nominal concentrations of 0.020, 0.040, 0.060, 0.080, and 0. 100 g K2Cr2 07/kg, the €a values for concentrations

between these nominal concentrations can be determined by linear interpolation. Using the appropriate €a

values, the calculated Aa values at 23.5 °C are obtained from the expression:

Aa = 6a x b x c (2)

An example of the calculation of Aa for one concentration of KjCnCb under a specified set of conditions is shown below. Calculations of Aa for other concentrations and wavelengths are performed in a similar manner.

Conditions: Wavelength = 350 nm, spectral bandpass 0.8 nm or less b = 0.9982 cm

c = 0.04375 gkg 1

t = 23.5 °C

From column 6, table 1, the €a for concentrations of 0.040 and 0.060 g-kg"

1 are 10.682 and 10.692, respectively.

The corresponding €a for c = 0.04375 g-kg is:

fa = 10.682 + y(.0.692 -.0.682)

Ca = 10.682 + 0.0019

Ca = 10.684

The calculated apparent absorbance, Aa , from equation 2, is: Aa = 10.684 x 0.9982 x 0.04375

Aa = 0.4666

The uncertainty, AAa , in the calculated Aa is determined from the combined uncertainties in €a , b, and c in equation 2, provided no other systematic errors are present. Thus:

AAa = be | A€a | + eac | Ab | + €a b | Ac | (3)

To evaluate AAa , A€a is taken from column 7 of table 1 and the Ab and Ac values must be determined experimentally.

In the experiments performed to obtain the 6a values in table 1 , the uncertainties for b and c did not exceed 1 part

in 10 and 2 parts in 10 , respectively.

The solution of equation 3 gives:

AAa = 1(0.044) (0.020) + 10.7(0.044) (0.0001) + 10.7(1) (0.0000088) = 0.0010

Thus, the uncertainty of Aa, for the above set of conditions, is ±0.0010.

The correction of the absorbance scale of the absorption spectrometer under test is determined by plotting the

differences between Aa measured and Aa calculated as a function of absorbance. A typical plot of such a graph

is shown in figure 1. The apparent absorbances measured on this instrument at 350 nm are accurate when the

indicated correction is subtracted from the corresponding absorbance scale reading, provided that the

conditions of wavelength accuracy, spectral bandpass, and absence of stray light are fulfilled as specified in NBS SP 260-54. Correction curves for wavelengths 235, 257, and 313 nm are obtained in a similar manner.

146
Figure 1. Correction curve for the absorbance

scale of a precision spectrometer. The error bars

are the sum of the errors arising from the uncertainties in the certified apparent specific absorbances,

€a , cuvette pathlength, b, and concentration, c.

0.2 0.4 0.6

Absorbance

0.8 1.0

Temperature Correction:

Although ea values in table 1 are certified at 23.5 °C, SRM 935 can be used as an absorbance standard at other

temperatures in the range 20 to 30 °C provided corrections are made to the €a values. Over this range the

apparent specific absorbances decrease linearly with increasing temperature for all the wavelengths given in table 1. The corresponding temperature coefficients, k, for these wavelengths are given in table 2.

Table 2. Variation of €a with Temperature Over the Range 20 to 30 °C.

A, nm Temperature Coefficient, k

Percent per degree Celsius

235

257

313

345

350

-0.05

-0.05

-0.02

-0.08

-0.05

The value of 6a at any temperature in the range 20 to 30 °C can be calculated from the certified value and the

appropriate temperature coefficient using the relation:

t =23.5 Ca — Ca [

1+7o^- 23- 5

>]

where: Ca = apparent specific absorbance at temperature t (°C)

23 5 Ca

' = apparent specific absorbance certified at 23.5 °C.

k = temperature coefficient, percent per °C.

REFERENCES

1. Burke, R. W., and Mavrodineanu, R., Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance

Standards, J. Res. Nat. Bur. Stand. (U.S.), 80A (Phys. and Chem.), No. 4, 631-636 (July-Aug. 1976).

2. Burke, R. W., and Mavrodineanu, R., Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance Standard, NBS Spec. Publ. 260-54 (1977). Copies may be obtained from the

Office of Standard Reference Materials, National Bureau of Standards, Washington, D.C. 20234.

3. Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials, J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No. 5, 405-425 (1972).

147
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS SP-260-54

2. Gov 't Accession

No.

3. Recipient's Accession No.

4. title and subtitle standard Reference Materials:

Certification and Use of Acidic Potassium

Dichromate Solutions as an Ultraviolet

Absorbance Standard—,Sf?M 935

5. Publication Date August 1977

6. Performing Organization Code

7. AUTHOR(S)

R. W. Burke and R. Mavrodineanu

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as Item 9

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-600027

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The apparent specific absorbances of 0.001N perchloric acid

solutions of a high purity potassium dichromate salt, Standard

Reference Material (SRM) 935, have been certified at four wavelengths in

tlie ultraviolet—235 , 257, 313 and 350 nm. This publication describes in

detail the measurements leading to the certification, and discusses the

use of this SRM as a transfer standard for verifying the accuracy of the

absorbance scale of narrow bandpass absorption spectrometers in this

important wavelength region. Apparent specific absorbance data are also

reported near two predicted isosbestic wavelengths in the acidic

HCrCU /Cr 2 07 system. The apparent specific absorbances at 345 nm are

sufficiently independent of concentration that absorbance measurements at

this wavelength can be used for verifying absorbance linearity to about

one part in a thousand.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Absorbance linearity; accuracy of transmittance or

absorbance scale; acidic potassium dichromate solutions; calibration of

ultraviolet spectrometers; certification of apparent specific absorbances

isosbestic wavelengths; liquid filters; Standard Reference Material;

18. AVAILABILITY [^Unlimited transfer standards

I For Official Distribution. Do Not Release to NTIS

p? Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SD Cat. No. CI \ 10: 2bQ- t)4

1 ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

UNCLASSIFIED

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157

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