CO Q NBS SPECIAL PUBLICATION 260-53

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Standard Reference Materials:

Standardization

of pH Measurements
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901.

The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade,

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the Institute for Computer Sciences and Technology, and the Office for Information Programs.

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, DC. 20234.

2 Located at Boulder, Colorado 80302,
MATIOHAL IlUK.KA.VJ OV STANDARD*

fclBEABT

DEC 3

''

Standard Reference Materials:

Standardization of pH Measurements

; no . ZL(oQ - 5-3

I 9 75"

Richard A. Durst

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued December 1975
Library of Congress Cataloging in Publication Data

Durst, Richard A. 1937- Standardization of pH Measurements.

(Standard Reference Materials) (National Bureau of Standards Special Publication; 260-53)

Bibliography : p. Supt. of Docs. No.: C 13.10:260-53

1. Hydrogen- Ion Concentration—Measurement. I. Title. 11. Series: United States. National Bureau of Standards. Standard

Reference Materials. 111. Series: United States. National Bureau

of Standards. Special Publication; 260-53. QC100.U57 No. 260-53 [QD561] 602'.ls [54P.3728] 75-619344

National Bureau of Standards Special Publication 260-53

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-53, 48 pages (Dec. 1975)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

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

(Order by SD Catalog No. C13.10:260-53). Price $1.05 . (Add 25 percent additional for other than U.S. mailing).
PREFACE

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

National Bureau of Standards are "well-characterized 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

.

This 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

hoped 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

The certification of samples of buffer materials which

define points on the National pH scale has evolved over a

period of several decades primarily under the direction of

Roger G. Bates. While this process is still not complete

and the future will likely see pH standards developed for

specialized purposes, e.g., pH in sea water and biological

fluids such as blood, we have currently reached a point

where the primary standards available and the certification

techniques involved are relatively well established and few

changes are anticipated in terms of the basic technology and

philosophy of the NBS pH scale. Consequently, the time

seemed opportune to prepare this addition to the "260 Series"

of NBS Special Publications which was established for the

purpose of reporting technical work done at NBS in the area

of Standard Reference Materials.

This report is concerned primarily with a description of

the concept of pH, its definition, and the method and

facilities used at NBS for the assignment of pH values to

the standard buffer solutions. A brief discussion is also

included of pH instrumentation and some of the potential pitfalls associated with the measurement of pH and the use of

these standards. One additional point is worth noting. The

„ pH certificates reproduced, in the appendix of this publication refer to specific stocks of the Standard Reference

Materials and, since batch-to-batch pH variability exists,

the use of these values for other sources of these buffer

materials is not recommended. It is anticipated that in the

near future the International Union of Pure and Applied

Chemistry will recommend a revised pH scale defined by these

standards but of sufficiently reduced precision that analytically pure reagents may be used for routine pH calibration.

Finally, certain commercial materials and equipment

are identified in this report in order to adequately specify

the experimental procedure. In no case does such identification imply recommendation or endorsement by the National

Bureau of Standards, not does it imply that the material or

equipment identified is necessarily the best available for

the purpose.

Richard A. Durst

Analytical Chemistry

Divis ion

iv

i
CONTENTS

Page

1. Introduction 2

2. Definition of pH 2

3. Assignment of pH Values to Standards 5

4. NBS Facilities for pH Certification Measurements. . 7

5. Recommended Standards 15

6. pH Instrumentation and Electrodes 16

7. Conclusion 20

8. References 24

9. Appendix 2 5

LIST OF TABLES

Table No.

1. NBS pH Standards 21

2. Composition and Properties of Standard

Solutions 22

3. NBS pH and pD Standard Reference Materials ... 23

LIST OF FIGURES

Figure No.

1. Diagram of the temperature-controlled emf

measurement system 8

2. Emf measurement cell used for the certification of pH buffer solutions 10

v 3. Emf measurement cells in position in the

controlled- temperature water bath 11

4. View of the measurement facility including

the water bath, instrument rack and laboratory logic box interface (open

on rear wall) 13

5. Close-up view of the instrument rack with

thermostated standard cells on the

laboratory bench to the right 14

v
OTHER NBS PUBLICATIONS IN THIS SERIES

NBS Spec. Publ. 260, Standard Reference Materials: 1975 Catalog, June 1975, $1.50.*

NBS Misc. Publ. 260-1, Standard Reference Materials: Preparation of NBS White Cast Iron

Spectrochemical Standards, June 1964. COM

74-11061.**

NBS Misc. Publ. 260-2, Standard Reference Materials: Preparation of NBS Copper-Base

Spectrochemical Standards, October 1964.

COM 74-11063.**

NBS Misc. Publ. 260-3, Standard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard, October 1964. COM 74-11060.**

NBS Misc. Publ. 260-4, Standard Reference Materials: Sources of Information on Standard

Reference Materials, February 1965. COM

74-11059.**

NBS Misc. Publ. 260-5, Standard Reference Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys,

March 1965. 25 cents. (Out of print).

,NBS Misc. Publ. 260-6, Standard Reference Materials: Methods for the Chemical Analysis of

White Cast Iron Standards, July 1965. COM

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NBS Copper-Base Spectrochemical Standards, October 1965. COM 74-11067.**

NBS Misc. Publ. 260-8, Standard Reference Materials: Analysis of Uranium Concentrates at

the National Bureau of Standards, December

1965. COM 74-11066.**

NBS Misc. Publ. 260-9, 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, November

1965. COM 74-11065.**

NBS Misc. Publ. 260-10, Standard Reference

Materials: Homogeneity Characterization on

NBS Spectrometric Standards II: Cartridge

Brass and Low-Alloy Steel, December 1965.

COM 74-11064.**

NBS Misc. Publ. 260-11, Standard Reference

Materials: Viscosity of a Standard Lead-Silica

Glass, November' 1966. NBS Misc. Publ.

260-10.**

NBS Misc. Publ. 260-12, Standard Referenc

Materials: Homogeneity Characterization of

NBS Spectrometric Standards III: White Cast

Iron and Stainless Steel Powder Compact,

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

NBS Misc. Publ. 260-13, Standard Reference

Materials: Mossbauer Spectroscopy Standard

for the Chemical Shift of Iron Compounds,

July 1967. NBS Misc. Publ. 260-13.**

NBS Misc. Publ. 260-14, Standard Reference

Materials: Determination of Oxygen in Fer- rous Materials - SRM 1090, 1091, and 1092,

September 1966. NBS Misc. Publ. 260-14.**

NBS Misc. Publ. 260-15, Standard Reference

Materials: Recommended Method of Use of

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

Colorfastness to Light, June 1967. (Replaced

by Spec. Publ. 260-41.)

NBS Spec. Publ. 260-16, Standard Reference

Materials: Homogeneity Characterization oi NBS Spectrometric Standards IV: Preparation and Microprobe Characterization oi W-20% Mo Alloy Fabricated by Powdei

Metallurgical Methods, January 1969. COM

74-11062.**

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Materials: Boric Acid; Isotopic and Assay

Standard Reference Materials, February

1970. 65 cents.*

NBS Spec. Publ. 260-18, Standard Reference

Materials: Calibration of NBS Secondary

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

Amplitude Measurement "Process A,"

November 1969. (See Spec. Publ. 260-29.)

NBS Spec. Publ. 260-19, Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold

(Certification of Standard Reference Material

745), January 1970. PB 190071.**

NBS Spec. Publ. 260-21, Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and Silver, January 1971. COM

74-11359.**

NBS Spec. Publ. 260-22, Standard Reference

Materials: Homogeneity Characterization of

Fe-3Si Alloy, February 1971. COM

74-11357**

vi
NBS Spec. Publ. 260-23, Standard Reference NBS Spec. Publ. 260-35, Standard Reference

Materials: Viscosity of a Standard Borosili- Materials: Thermal Conductivity of Austencate Glass, December 1970. 25 cents* itic Stainless Steel, SRM 735 from 5 to

NBS Spec. Publ. 260-24, Standard Reference 280 K, April 1972. 35 cents.*

Materials: Comparison of Redox Standards, NBS Spec. Publ. 260-36, Standard Reference

January 1972. $1.* Materials: A Referee Method for the DeterNBS Spec. Publ. 260-25, Standard Reference mination of Calcium in Serum. SRM 915,

Materials: A standard Reference Material May 1972. COM 72-50527.**

Containing Nominally Four Percent Austen- NBS Spec. Publ. 260-37, Standard Reference

ite, February 1971. COM 74-11356.** Materials: Methods of Analysis of NBS Clay

NBS Spec. Publ. 260-26, Standard Reference Standards, June 1972. COM 72-50692.**

Materials: National Bureau of Standards-U.S. NBS Spec. Publ. 260-38, Standard Reference

Steel Corporation Joint Program for Deter- Materials: Preparation and Calibration of

mining Oxygen and Nitrogen in Steel, Febru- Standards of Spectral Specular Reflectance,

ary 1971. 50 cents* May 1972 . 60 cents.*

NBS Spec. Publ. 260-27, Standard Reference NBS Spec. Publ. 260-39, Standard Reference

Materials: Uranium Isotopic Standard Refer- Materials: The Eddy Current Decay Method

ence Materials, April 1971. COM f or Resistivity Characterization of High74-1 1358.** Purity Metals, May 1972. 55 cents.*

NBS Spec. Publ. 260-28, Standard Reference NBS Spec. Publ. 260-40, Standard Reference

Materials: Preparation and Evaluation of Materials: Selection of Thermal Analysis

SRM's 481 and 482 Gold-Silver and Gold- Temperature Standards Through a CooperaCopper Alloys for Microanalysis, August tive Study (SRM 758, 759, 760), August

1971. COM 71-50365.** 1972. 65 cents.*

NBS Spec. Publ. 260-29, Standard Reference NBS Spec. Publ. 260-41, Standard Reference

Materials: Calibration of NBS Secondary Materials: Use of Standard Light-Sensitive

Standard Magnetic Tape (Computer Ampli- Paper for Calibrating Carbon Arcs used in

tude Reference) Using the Reference Tape Testing Textiles for Colorfastness to Light,

Amplitude Measurement "Process A-Model August 1972. COM 72-50775.**

2," June 1971. COM 71-50282.** NBS Spec. Publ. 260-42, Standard Reference

NBS Spec. Publ. 260-30, Standard Reference Materials: The Characterization of Linear

Materials: Standard Samples Issued in the Polyethylene, SRM 1475, September 1972.

USSR (A Translation from the Russian), 45 cents.*

June 1971. COM 71-50283.** NBS Spec. Publ. 26043, Standard Reference

NBS Spec. Publ. 260-31, Standard Reference Materials: Preparation and Homogeneity

Materials: Thermal Conductivity of Electro- Characterization of an Austenitic Ironlytic Iron SRM 734 from 4 to 300 K, Novem- Chromium-Nickel Alloy, November 1972.

ber 1971. COM 71-50563.** 45 cents .* NBS Spec. Publ. 260-32, Standard Reference NBS Spec. Publ. 260-44, Standard Reference

Materials: Standard Quartz Cuvettes For Materials: Preparation and Use of SuperconHigh Accuracy Spectrophotometry. Decern- ductive Fixed Point Devices, SRM 767,

ber 1973. 55 cents.* December 1972. 75 cents.*

NBS Spec. Publ. 260-33, Standard Reference NRS Spec publ 260-45, Standard Reference

Materials: Comparison of Original and Sup- Materials: Powdered Lead-Based Paint, SRM

plemental SRM 705, Narrow Molecular 15?9 March 19?3 C0M 73.50226.**

Weight Distribution Polystyrene, H. L. Wag- NBS Spec publ 260-46, Standard Reference

ner, May 1972. COM 72-50526.** Materials: Thermal Conductivity and ElecNBS Spec. Publ. 260-34, Standard Reference trical Resistivity Standard Reference MateMaterials: Thermoelectric Voltage, April rials . Austenitic Stainless Steel, SRM's 735

1972. COM 72-50371.** and 798 , from 4 to 1200 K, March 1975.

$1.00.*

vii
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Materials: Electrical Resistivity of Electrolytic Iron, SRM 797, and Austenitic Stainless

Steel, SRM 798, from 5 to 280K, February

1974. 55 cents.*

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Materials: Description and Use of Precision

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SRM 933 and SRM 934, May 1974. 60

cents.*

NBS Spec. Publ. 26049, Standard Reference

Materials: Calibrated Glass Standards for Fission Track Use, November 1974. 75 cents.*

NBS Spec. Publ. 260-50, Standard Reference

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

from 4 to 1000 K, June 1975. $1.00.*

NBS Spec. Publ. 260-51, Standard Reference

Materials: Glass Filters As a Standard Reference Material for Spectrophotometry; Selection, Preparation, Certification, Use - SRM

930. (In press)

NBS Spec. Publ. 260-52, Standard Reference

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials 730 and 799, from 4 to 3000 K. $1.05*

NBS Spec. Publ. 260-53. Standard Reference

Materials: Standardization of pH Measurements. (In press).

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22151

viii
STANDARD REFERENCE MATERIALS:

STANDARDIZATION OF pH MEASUREMENTS

Richard A. Durst

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

One of the most widely performed analytical measurements in chemical laboratories is that of pH

using the glass electrode. In order to insure

the consistency of these measurements, the National

Bureau of Standards has adopted an operational

scale of pH defined in terms of the pH(S) of a

series of standard buffer solutions. Certified

samples of buffer materials, from which the

standard reference solutions of reproducible pH

can be prepared, are issued by the NBS. At

present, the operational scale is defined by

the pH of seven primary and three secondary

standard solutions.

This report is concerned primarily with a

discussion of the method used at the NBS for

the assignment of pH values to the standard

buffer solutions, a description of the NBS

measurement facilities, and a summary of the

characteristics of these standards. A brief

discussion of the types of electrodes used and

the calibration of pH instrumentation is also

presented.

Key words: Buffers; buffer solutions; glass

electrode; pH; pH buffers; pH electrode; pH

measurement; pH standards.

1
1. Introduction

Measurement and control of acidity and alkalinity are

frequently essential in both industrial and research work.

Commercial pH meters with electrodes of the glass type, some

of which are claimed to measure pH values with a reproducibility of 0.001 unit, are used in nearly every laboratory

where chemical analyses or control tests are performed.

These instruments are used to compare the pH of unknowns and

standard buffer solutions with which the electrode systems

are periodically calibrated. It is therefore very important

to have universal agreement on a standard scale of pH and to

adopt standard reference buffers to define fixed points on

this scale .

The National Bureau of Standards proposes the adoption

of an operational pH scale defined in terms of the pH(S) of

a series of standard buffer solutions whose assigned pH

values correspond to the negative of the common logarithm

of the hydrogen ion activity, -log an, in the respective

solutions. Certified samples of buffer materials, from which

standard reference solutions of reproducible pH can be prepared, are issued by the National Bureau of Standards. At

present, the operational scale is defined by the pH of seven

primary and three secondary standard solutions.

This report is concerned primarily with a description

of the method used at the National Bureau of Standards for

the assignment of pH values to the standard buffer solutions

and a summary of the characteristics of these standards.

The types of electrodes and calibration of pH assemblies are

discussed briefly, but for more details on these subjects,

specific monographs (see references) and the technical

literature of the equipment manufacturer should be consulted.

2. Definition of pH

The operational definition of the pH value is given by

equation 1 [ 1]

1

:

figures in brackets indicate the literature references at

the end of this paper.

2
CD

where E is the electromotive force of the cell:

s Pt; H 2 (g) | solution S || KC1 ( : 3 . 5M) | re fe rence electrode

and E x is the electromotive force, of the same cell when the

standard buffer solution S [pa H = pH(S)] is replaced by the

sample solution X; F is the faraday, 96 ,487 Omol" 1

; R is

the gas constant, 8.3143 JK^mol" 1

; and T is the absolute

temperature in kelvins, T = t(°C) + 273.15. The double

vertical line indicates a liquid junction between two different solutions and a single vertical line represents a phase

boundary

.

Although the hydrogen gas electrode is the primary

indicator for hydrogen ions, the glass electrode is much

more convenient and versatile and is usually used for practical pH determinations. However, corrections must be

applied to measurements made in strongly acid and strongly

alkaline solutions where the response of the glass electrode

no longer parallels that of the standard hydrogen electrode.

The interpretation of the pH values computed by equation

1 obviously depends to a large extent upon the significance

of the number assigned to the standard solution. If imperfections in this method of comparison could be ignored, the

pH unit would derive its character exclusively from that of

pH(S). A consideration of various possible standard scales

reveals that none can be thermodynamical ly exact if the pH

is to be a measured quantity.

The choice of a unit for expressing the degree of acidity

of the standard solutions is most properly made in the light

of (1) the nature of the response of physicochemical phenomena

to changes of acidity and (2) the techniques and methods of

measurement. The term pH was originally proposed by

S^rensen [2] and defined as:

pH = -log c H , (2)

where ch is the concentration of the hydrogen ion. Later,

as modern theories of electrolytic solutions developed, it

became clear that, except in special cases, it is not

possible to ascertain the concentration of hydrogen ions in

buffer solutions by direct electrode measurement and the

numerical values of acidity determined by the experimental

procedure of S0rensen bear no simple direct relation to

chemical equilibria formulated in terms of activities.

3
Inasmuch as the effective concentration or activity, a H , is fundamental in chemical equilibria, a definition of pH on

the activity basis has much to recommend it. Furthermore,

the well-established electrome tr ic method of pH measurement

furnishes a number related more closely to a H than to c H . For these reasons pH(S) is defined in terms of activity:

pH(S) = -log a H

= -log (m H

ry H ) (3)

where m is molality and y is the activity coefficient on the

molal scale. The fact that the activity of a single ionic

species is a concept lacking unique physical significance

does not preclude the establishment of a reasonable scale of

pa H , but this scale must be a conventional one. In other

words, the values of a H , although not unique hydrogen ion

activities, will nevertheless be numbers which, when inserted

into equations involving an, will furnish equilibrium data

consistent with those obtained by rigorous thermodynamic

methods [1]

.

Under ideal experimental conditions, the measured pH

is regarded as approaching the true -log in the sample

solution. These conditions are realized most closely for

'buffer solutions of concentrations between 0.01 and 0.1 molar

and pH between 3 and 11. Concentrated solutions, particularly

those containing strong acids and bases, display larger

deviations. Measurements in mixed solvents or nonaqueous

media must be treated as numbers only, since the interpretation of pH in solvents other than pure water is difficult and

a single pH scale encompassing all solvents, referred to the

aqueous standard state, cannot be realized at the present

time. However, significant advances have been made in recent

years in extending the pH concept and measurement methods to

mixed solvents of which water is the second component [3].

By a procedure analogous to that by which aqueous pH standards

have been established, pH*(S) values can be assigned to buffer

solutions in both nonaqueous and binary mixed solvents [3,

10].

The pH concept has been extended one more step by the

assignment of pD(S) values to the citrate, phosphate and

carbonate buffers in heavy water, deuterium oxide. The one

fundamental difference between the pH and pD scales is the

use of the deuterium gas electrode as the ultimate reference

basis for the deuterium ion activity scale. Otherwise, the

procedure used for assigning the standard reference values

is exactly analogous to the pH procedure [3], The NBS has

certified reference materials for the preparation of the

phosphate and carbonate buffers (see table 3 and the appendix)

.

4
3. Assignment of pH Values to Standards

The practical importance of the electrometric pH determination, together with the fact that the pH value lacks a

unique fundamental definition, have led to the adoption of

an operational pH scale, defined by equation 1. Accordingly,

pH is defined in terms of the pH(S) of one or more selected

standard buffer solutions together with the observed change

in emf when the standard is replaced by the sample in a

suitable cell. In order to endow the measured pH with as

much significance as possible, three measures have been

adopted [l] : first is the selection of a conventional pa H

scale for the numerical values of pH(S); second is the

selection of reference solutions with ionic strengths less

than 0.1; and third is the restriction of primary standards

to the pH range between the approximate limits 2.5 and 11.5.

The National Bureau of Standards issues certified samples

of buffer materials from which standard reference solutions of

reproducible pH can be prepared. In the National Bureau of

Standards' method of assignment of pH(S) values, the acidity

function, p(aH yC |), is first determined for a series of suitable buffer solutions containing added small concentrations

of a soluble chloride salt. The emf of hydrogen - silver

chloride cells of the type:

Pt; H 2 (g) | standard buffer, Cl"|AgCl, Ag

is measured for each solution under carefully controlled conditions. This cell is well suited for this type of measurement because: (1) the hydrogen electrode is the fundamental

indicator of hydrogen ion activity, (2) the hydrogen and

silver-silver chloride electrodes are well -characteri zed and

highly reproducible, and (3) this cell avoids the uncertainties

of the liquid- j unction potential [3] . From the known values

of the chloride ion concentration and the standard emf, E°,

the acidity function p(aH y Cj ) is readily obtained from the

measured emf, E, by the equation:

P(aH Yci) =

(-RT ~ln 10^ + log m o W

The limiting value of this acidity function p(a H y C |)° with

respect to the buffer solution without added chloride, i.e.,

the limit approached by p(aH Y C) ) as the concentration of

added chloride approaches zero, can be obtained by extrapolation. The pa H of the buffer is then computed from p(a H yc| )°

by the introduction of a conventional individual ionic

activity coefficient for chloride:

5
pa H

= p(a H Y C| )° + log YC| (5)

Inasmuch as this quantity, Yc i > nas no independent thermodynamic definition, the Bates - Guggenheim convention [4] has

been adopted for the evaluation of log YC[ :

-Alh log yc i 1 + 1. 5 Ps (6)

where A is the Debye-Hiickel slope and I is the ionic strength.

This equation is intended to apply to aqueous solutions at

ionic strengths not exceeding 0.1, and the coefficient 1.5

in the denominator is to be used over the temperature range

0 to 95 °C.

The final step in the assignment of pH values is the

identification of the pan of certain selected reference solutions with pH(S). The primary standards have pH values in the

range 3 to 11 and are chosen for their reproducibility,

stability, buffer capacity and ease of preparation [1]. For

these primary standard buffer solutions:

pH(S) = paH (7)

With the adoption of the Bates-Guggenheim convention, it has

become possible to assign pH(S) values to these standard solutions with third decimal significance. The assigned values

of pH(S) for the standard solutions at temperatures from 0 to

50 °C, and to 95 °C for certain buffers, are smoothed values

calculated by the equation:

pH(S) = « + B + CT + DT 2 (8)

where T is the temperature in kelvins e Inasmuch as each lot

of material is individually certified, the pH(S) values of

different lots may differ slightly in the third decimal

place

.

Originally, it was attempted to certify primary standards

suitable for calibrating any portion of the entire pH scale

from 0 to 14. However, it was soon recognized that the liquidjunction potential of the common pH cell displays considerably

greater variability outside of the intermediate pH range

from 2.5 to 11.5. In spite of these liquid- j unction potential

uncertainties, which could result in differences of several

hundreths of a pH unit, it is nevertheless desirable to

6
affirm the proper functioning of the glass electrode outside

of this intermediate pH region. For this purpose, additional

secondary pH standards have been designated: potassium

tetroxalate of pH(S) = 1.68 at 25 °C and a saturated solution

of calcium hydroxide of pH(S) = 12.45 at 25 °C.

Even within the intermediate pH range, secondary standards may have certain advantages over the primary standards.

For example, the secondary standard may be more easily prepared, more stable or chosen to match the composition and

pH of a type of sample on which repeated measurements are to

be made [1]. Recently, NBS certified a buffer solution of

tris (hydroxymethyl) aminomethane ("tris") and tris«hydrochloride. This buffer was selected as a physiological pH

standard because of its freedom from undesirable side reactions in biological fluids and the similarity of its pH

temperature coefficient to that of blood [5]. Although

intended as a primary standard, the tris buffer should be

regarded as a secondary standard for pH until consistency

with the primary standard scale is demonstrated.

Variations in the liquid- junction potential are never

entirely absent, even in the intermediate pH range. For

this reason, some inconsistency in pH exists among the primary standards when these are used to standardize cells in

which a liquid junction is involved [3]. The differences

may not be large, but they are such as to make it unrealistic

under normal conditions of measurement, to consider the

experimental pH(X) to have an uncertainty of much less than

0.01 unit. The actual magnitude of the inconsistency depends

on the structure of the liquid junction and the temperature.

The third decimal place assigned to the pH of standard solutions is therefore of limited significance for measurements

made with ordinary pH assemblies.

4. NBS Facilities for pH Certification Measurements

In 1972, the NBS emf measurement system was automated

in order to increase the reliability of the data acquisition and to reduce the time required for a pH certification

run from about five 8-hour days to two 24-hour days with

minimal operator attention. The primary purpose of this

system is to cause a constant temperature water bath to

cycle through a preprogrammed set of temperatures and to

record the data when the bath temperature and potentials read

from the measurement cells pass certain stability and control

requirements

.

The temperature-controlled emf measurement system is

diagrammed in figure 1. A centrally located computer, a

Univac Series 60, model 6135, is time-shared with about

7
Temp Set Point

Selector

Refrigeration Unit

Console Line Typewriter Printer

UNIVAC Series 60 Model 6135 Computer

Computer Interface

Digital Data Bus

Lab Interface

Teletype Lab Control Console

Temp

Sensor

Digital Barometer

Proportional Temp

Controller

Standard

Resistor

Heater

Unit

Platinum Resistance Thermometer

Constant Temp

Water Bath

Electrochemical

Cells

(Electrodes)

Disk Mag Tape

Archive

Fluke 8400A DVM

Input

Multiplexer

Electrometer Eppley

Standard Cells

Figure 1. Diagram of the temperature-controlled emf

measurement system.

8
a dozen other systems in the Analytical Chemistry Division.

The data communications system consists of four main parts:

(1) the digital data bus,. (2) the computer interface, (3) the laboratory logic box interface, and (4) the laboratory control console. The computer interface connects the computer

to the laboratory via the digital data bus and laboratory

interface. The laboratory instrument console provides control and the capability of entering auxiliary data for use

by the computer to service and/or control the interfaced

system. Two-party line digital data busses have been

installed in the Chemistry Building with outlets for each

laboratory. The communication between the laboratory and

the computer interfaces is accomplished by a multilevel

time-sharing scheme. The laboratory interface contains the

logic circuitry necessary to connect as many as four experimental systems to the digital data bus. Data that are

routinely collected by the system are stored on a randomaccess disc for a period of several days. If space on the

disc becomes scarce, the data files are read onto a master

archive magnetic tape for storage.

In performing a pH certification, the first requirement is to prepare the buffer solutions and measurement cells

with utmost care. As specified in the pH certificates (see

Appendix) , the carefully dried and weighed salts are dissolved in distilled water of sufficient purity to give a

conductivity of less than 2 x 10 6 S*cm _1 (ohm -1

•cm

-1

) at

25 °C. For buffers in the neutral and basic regions, carbon

dioxide must also be removed from the water prior to dissolution of the buffer salts. The hydrogen gas is purified of

oxygen by passage through a catalytic reduction tube or

through a palladium purifier.

The specially designed emf measurement cell shown in

figure 2 consists of a hydrogen gas electrode compartment,

a silver/silver chloride electrode compartment, and a series

of gas - dispersion compartments for humidifying the incoming

hydrogen gas by passage through the buffer solution. Details

of the preparation of the platinized (or palladized) platinum

and the silver/silver chloride electrodes can be found in

references [1] and [8].

The prepared measurement cells, usually consisting of

two cells containing each of the three levels of added

chloride, are placed in the controlled temperature water

bath as shown in figure 3. Also shown in this figure are

the coiled copper tubing hydrogen gas inlets and the two

platinum resistance thermometers.

The complete measurement system, with the laboratory

logic box interface (open) on the rear wall, is shown in

9
Figure 2. Emf measurement cell used for the certi

fication of pH buffer solutions.

10
Figure 3. Emf measurement cells in position in the

c on t r o 1 le d- t emp er a t ur e water bath.

11
figure 4. Another view, figure 5, shows the entire electronics rack and the thermos tated standard cells on the

laboratory bench top. Briefly, the equipment in the electronics rack consists of (from the top): a digital barometer

which is automatically monitored during the certification

measurements is used to correct the partial pressure of

hydrogen for variations in the atmospheric pressure; the

autoranging and aut ofunct ioning digital voltmeter which

serves as the analog- to- digital converter for both resistance

and emf measurements; the proportional temperature controller

which, in conjunction with the temperature set point selector

(next panel down) and the bath refrigeration unit, maintains

preset bath temperatures to better than ± 0.01 °C by means of

immersion heaters; and the DVM input multiplexer which, under

computer control, switches the various measurement parameters

into the DVM for A-to-D conversion prior to transmission to

the computer for acquisition and storage. The functions

multiplexed include: temperature (platinum resistance),

standard resistor (calibration), standard emf cells (calibration), and up to ten measurement cells. The control console

permits communication with the computer via a series of

thumbwheel switches which enter all of the required input

parameters, e.g., temperature sequence, number of cells,

time delays, and program identification numbers. The control console also includes a set of pushbuttons which are

used to set-up, start (send), and terminate the experiment

as well as indicator lights which signal operations (data

acquisition, operate, auto, etc.) or problems (error, reject).

A vibrating- reed electrometer is located below the control

console and, in conjunction with the three -position pH

switch, provides a high- impedance input to the DVM for measurements with glass and high- res is tance ion- select ive

electrodes. The remainder of the equipment in the rack constitutes the Mueller bridge system for manual checking of the

water bath temperature via the second platinum resistance

thermometer shown in figure 3.

In operation, computer control begins by automatically

positioning the temperature set point selector at the starting sequence number. When the bath temperature achieves the

nominal value, within certain preset control limits as

indicated by the platinum resistance thermometer, the input

multiplexer automatically switches through the readout positions. After a programmed time delay, the multiplexer

recycles through the positions and compares the values to

the previous sets of data. After three cycles, if the data

agree. within the requisite control limits, the computer sets

the temperature control to the next sequence value and the

operation is repeated. If any of the values exceed the control limits, an out-of-range message is printed next to the

incorrect value and, following two more cycles, the computer

12
4. View of the measurement facility including

the water bath, instrument rack and laboratory

logic box interface (open on rear wall).

13
5. Close-up view of the instrument rack with

t he r mos t a t e d standard cells on the

laboratory bench to the right.

14
proceeds to the next sequence temperature after again flagging

the out-of -range value. Depending on the temperature value

or the direction of the temperature change, the computer

turns on the refrigeration unit or an auxiliary heater.

After the temperature sequence is completed, the routine is

automatically terminated. Reduction of the acquired data

is performed by batch operation on the NBS central computer,

a Univac 1108. The data reduction consists of a series of

operations including correcting the experimental emf values

to the standard partial pressure of hydrogen, calculating

the acidity functions, extrapolating to no added chloride

to obtain the limiting acidity functions, evaluating the

chloride ion activity coefficient using the Bates -Guggenheim

convention, and finally calculating the paH values. These

experimental values are then smoothed with respect to temperature (equation 8) by the method of least squares to give

the certified pH(S) values.

5. Recommended Standards

The National Bureau of Standards pH scale is defined in

terms of the pH(S) assigned to the seven primary and three

secondary reference solutions , listed in table 1. The composition and properties of the standard solutions are given

in table 2. The citrate, phthalate, phosphate, borax,

carbonate, tetroxalate, and tris buffer solutions are prepared by dissolving the indicated weights of the pure

reference materials in pure water and diluting to 1 dm 3 (1

liter) . The directions for the preparation of the molal

solutions are given on a volume basis, i.e., grams per liter

of solution, in order to simplify the preparation of the

buffer solutions. However, the weights of the salts indicated

in table 2 and in the NBS certificates for these pH standards

are corrected for solution density and air buoyancy and thus

correspond exactly to the nominal concentration on the molal

scale. The tartrate and calcium hydroxide solutions are made

by shaking an excess of the pure material with water in a

glass - stoppered bottle. The excess material should be

removed by filtration or decantation [1]. The calcium

hydroxide standard solution is best stored in a polyethylene

bottle. The tartrate solution is about 0.034m and the

calcium hydroxide solution about 0.021m.

Prior to weighing out the reference materials, several

should be oven dried: phthalate at 110 °C, phosphates (both)

at 110-130 °C, and the sodium carbonate at 275 °C; each

salt for two hours. The other materials do not require oven

drying but should not be exposed to excessively high humidity.

15
The weighed samples of reference materials should be

placed in a 1-liter volumetric flask, dissolved, and filled

to the mark with distilled water at 25 °C. The distilled

water should have a conductivity no greater than 2 x 10" 6 S*

cm

-1

. The phosphate, tris, borax and carbonate buffers

should also be prepared using water purged with a carbon

dioxide free gas or freshly boiled water of pH 6.7 to 7.3.

Boiled water should be protected from contamination by

atmospheric carbon dioxide with a tube of carbon dioxide

absorbant, e.g., soda lime, during cooling. Exposure of the

prepared buffer solutions to the atmosphere should also be

minimized. The more acidic buffers as well as the saturated

calcium hydroxide solution may be prepared from air-equilibrium water, pH 5.6 to 6.0.

It is recommended that the buffer solutions be replaced

about once a month, and sooner for the tartrate buffer which

is subject to mold growth accompanied by an increase in pH.

The addition of a small amount of thymol has been found to

be an effective preservative, although this treatment should

be avoided for measurements of the highest accuracy [1].

A complete list of the pH and pD Standard Reference

Materials presently available from the National Bureau of

-Standards is given in table 3. Although considerable work

has gone into the assignment of pH values to the citrate and

calcium hydroxide solutions, reference materials have not yet

been certified.

6. pH Instrumentation and Electrodes

A comprehensive discussion of pH instrumentation and

electrodes is beyond the scope of this report and, for more

details, the referenced monographs and the equipment manufacturer's literature should be consulted.

In general, the internal resistance of the pH cell will

determine to a large extent the type of equipment required

for the emf measurement. For low resistance cells, such as

the hydrogen electrode/silver-silver chloride electrode

cell used in the certification of pH buffers, the most

accurate method of measuring the emf is by means of a

potentiometer which "balances" or compares the unknown emf

with the emf of a standard cell. However, because of the

high resistance of the glass electrode, special highimpedance electrometers are required which do not draw an

appreciable current. As a result of the great progress made

16
in electronic instrumentation in recent years, there are now

many excellent pH meters capable of providing pH values with

a reproducibility of a few thousandths of a pH unit. However,

while modern digital pH meters can be read to the nearest

0.001 pH unit, the fundamental meaning of these measured

values is considerably less certain than the precision of

the measurement.

In making pH measurements it is important to remember

that the pH assembly is designed to indicate the difference

between a standard buffer and a test solution, both of which

should be at the same temperature. To affirm the proper

functioning of the glass electrode and the accuracy of the

instrument over the pH range of the test solutions, the

assembly should be calibrated with two standards which

bracket the pH of the samples. With this procedure, it is

assumed the emf of the cell varies linearly with the pH

within the range of the standards but that the pH electrode

response slope is not necessarily exactly Nernstian, i.e.,

(RT In 10)/F volts per pH unit. The pH of the test solution

X is then given by:

E E

pH(X) = pH(Si) + P

X

_ [pH(S 2 )

- pH(Si)] (9)

In actual practice, with modern direct - reading instruments

the above calibration is made by adjusting the standardization and sensitivity (slope) controls on the instrument.

Errors caused by fluctuations in the residual liquidjunction potential and' temperature are minimized by standardizing the assembly at a pH close to that of the test

solution

.

There are four principal types of pH indicator

electrodes

1. the hydrogen (gas) electrode

2. the quinhydrone electrode

3. the metal-oxide electrodes, and

4. the glass electrode.

The hydrogen electrode is the ultimate standard for the

determination of pH values but special precautions must be

taken to ensure the most reliable and theoretically correct

pH response. Owing to the experimental difficulties associated with it, other pH electrodes are commonly used for

routine measurements.

17
The quinhydrone electrode is a redox electrode which

produces an emf dependent upon the pH . It is subject to a

variety of limitations which greatly restricts its use.

For example, it is subject to an alkaline error which limits

its application to neutral and acidic media of pH less than

8. Of less importance is the salt error resulting from

unequal changes in the acitivity coefficients of hydroquinone

and quinone, but this error only becomes important at high

ionic strengths where the significance of pH is diminished.

Finally, this electrode is poisoned by proteins and cannot

be used in the presence of strong oxidizing or reducing

agents

.

The metallic electrodes, e.g., antimony, bismuth, and

tungsten, develop a potential by a redox reaction involving

the metal and a thin layer of oxide present on the surface

of the metal. In practice their pH response is variable (nonlinear) and careful calibration is critical. The electrode

response is sensitive to dissolved oxygen concentration, the

composition of the buffer and test solutions, and to stirring

rate. It is also affected by oxidizing and reducing agents,

certain complexing anions, and traces of some cations. The

main advantages of the metallic pH electrodes are rapid

response and rugged construction, and they can be used under

„ extreme pressure and/or temperature conditions.

The glass electrode is by far the most widely used pHindicator electrode. It is applicable over a wide pH range

and exhibits high stability, reliability and reproducibility.

It is the only one of the four types of pH electrodes that

is completely free of oxidation-reduction interferences. The

construction and response of this electrode has been discussed

in detail elsewhere [1,6]. Suffice it to say that the pHresponsive surface consists of a thin membrane formed from

a special glass which, after suitable conditioning, develops

a surface potential that is an accurate index of the hydrogen

ion activity of the test solution. The glass membrane is

usually made as thin as is consistent with adequate mechanical

strength. Even so, its electrical resistance is very high,

usually in the range from tens to hundreds of megohms, which

is the reason that electronic amplifiers must be used to

obtain adequate measurement accuracy. The exact mechanism of

operation of the glass electrode is still a matter of some

conjecture. However, there seems to be no doubt that during

the conditioning process of the glass membrane, there is an

exchange between alkali metal ions, such as sodium or lithium,

in the interstices of the glass matrix with hydrogen ions

(protons) from the solution. Then the exchange of labile

protons between the stable sites in the conditioned glass

surface (gel layer) and the solution phase appears to be the

mechanism by which the surface phase -boundary potential

mirrors changes in hydrogen ion activity in the solution.

18
The fact that this process is one of proton transfer rather

than electron transfer is the reason for the freedom from

redox perturbations mentioned above.

Although the glass electrode exhibits remarkable specificity for hydrogen ion, it is by no means perfect. Departures from ideal behavior can be found in alkaline solutions

containing high concentrations of alkali metal ions and also

in solutions of strong acids. By modifying the composition

of pH glasses, electrodes have been developed which show a

reduction in both the alkaline and acid errors. This should

be considered when selecting glass electrodes. That is, the

type of electrode chosen should be substantially errorfree

under the conditions of measurement. The glass electrode

has been fabricated into a wide variety of designs for

special applications. The most common type is the immersion

electrode, but capillary and microelectrodes are also used

rather widely, particularly in biological studies [6,7].

The electrical circuit of the pH cell is usually completed via a salt bridge and reference electrode. The three

main requirements for a satisfactory reference electrode are

reversibility, reproducibility and stability [8], The four

principal reference electrodes are the mercury-mercurous

chloride (calomel), the silver-silver chloride, the

mercury-mercurous sulfate, and the thallium amalgamthallous chloride electrodes. The most widely used reference electrode is probably the calomel electrode which is

highly reproducible but suffers from instability at elevated

temperatures (>80 °C) caused by disproportionation of the

mercurous chloride. The silver-silver chloride and thallium

amalgam- thallous chloride are less sensitive to temperature

effects and are recommended for measurements above 80 °C.

Probably the greatest source of difficulty in a pH cell

is the liquid junction which is formed at the area of contact

between the salt bridge and the test solution. The magnitude

of the liquid- junction potential depends on the design and

characteristics of the liquid junction, e.g., the composition

and concentration of the salt-bridge solution, and the type

of junction (capillary, porous plug, cracked bead, sleeve,

membrane, flowing). Unfortunately, measurements made with

the best reference electrodes and the most reproducible liquid

junctions are still subject to a residual liquid- j unction

potential error. This error results from a difference in

the liquid- junction potentials at the boundaries between the

salt bridge and the test and standard buffer solutions. In

general, the residual liquid- j unction potential can be

minimized by correct design of the liquid junction, properly

selected salt-bridge solution, ionic strengths of the test

and buffer solutions approximately equal and less than 0.1,

19
the pH between 3 and 11, and the exclusion of other phases such as colloids and suspensions.

The most commonly used salt-bridge solution is potassium chloride saturated at room temperature D It has been demon- strated experimentally and can also be shown by calculations using the Henderson equation for liquid- j unction potentials

[1], that the higher the concentration of KC1 in the salt- bridge solution, the more effective it is in reducing the liquid-junction potential. The saturated KC1 salt bridge

is not without its disadvantages, however. The most serious problem is the blockage of flow at the liquid junction by crystals of KC1 which form upon lowering of the temperature or

over longer periods, caused by evaporation of water from the salt-bridge solution. The blockage raises the resistance of

the cell and impairs the reproducibility of the liquid-junc- tion potential. For this reason, a slightly undersaturated

solution, such as 3.5 molar, is recommended [1].

7. Conclusion

The definition and measurement of pH, as we now know it,

is the result of a 65-year evolutionary process which was

„ greatly influenced by two facts of modern technology. One

is the very extensive, almost exclusive, use of commercial

glass electrodes for the routine determination of the acidity

of solutions. The other is the acknowledged fact that the

majority of practical pH measurements need only be reproducible, since the fundamental interpretation of these

numbers in terms of hydrogen ion activity would be sheer

fantasy [3] . These facts led to the operational definition

of pH (equation 1) which has now been adopted by the International Union of Pure and Applied Chemistry [9] and most

of the world. In its broadest sense, this acceptance means

that pH values can legitimately be assigned to a wide

variety of media, e.g., D 2 0 and many nonaqueous solutions,

in which a reproducible pH-related emf can be measured. The

NBS pH Standard Reference Materials provide a means for tying

the operational definition of pH to a consistent scale of pH

values assigned with as much thermodynamic significance as

is possible at the present state of this art.

The author especially wishes to acknowledge the contributions of Dr. Roger G. Bates whose efforts over many years

at NBS resulted in the national pH scale and forms the basis

of a large part of this report. Special thanks are also

expressed to Mr. J. Paul Cali and the Office of Standard

Reference Materials for their support of the pH certification

program

.

20
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22
Table 3

NBS pH and pD Standard Reference Materials

Material SRM designation

Potassium hydrogen phthalate 185e

Potassium dihydrogen phosphate 186-I-c

Disodium hydrogen phosphate 186-11 -c

Sodium tetraborate decahydrate 187b

Potassium hydrogen tartrate 188

Potassium tetroxalate 189

Sodium bicarbonate 191

Sodium carbonate 192

Tris (hydroxymethyl) aminome thane 92 2

Tris (hydroxymethyl) aminomethane 923

hydrochloride

Potassium dihydrogen phosphate (pD) 2186-1

Disodium hydrogen phosphate (pD) 2186-11

Sodium bicarbonate (pD) 2191

Sodium carbonate (pD) 2192

^Available from the Office of Standard Reference Materials,

National Bureau of Standards, Washington, D.C. 20234.

Request NBS Special Publication 260 for price list and

ordering information.

23
8. References

[1] Bates, R. G., Detevmination of pH3 Theory and Practice 3 2nd edition, John Wiley § Sons, New York (1973).

[2] S^rensen, S. P. L., Compt. rend, trav . lab. Carlsberg

8, 1 (1909).

[3] Bates, R. G., Anal. Chem. 40(6), 2 8A (1968).

[4] Bates, R. G. and E. A. Guggenheim, Pure Appl. Chem. 1_,

163 (1960).

[5] Durst, R. A. and B. R. Staples, Clin. Chem. 18_, 206

(1972) .

[6] Eisenman G., Ed., Glass Electrodes for Hydrogen and

Other Cations 3 Marcel Dekker , New York (1967).

[7] Lavallee, M. , 0. F. Schanne and N. C. Hebert, Eds.,

Glass Mioroeleotrodes 3 John Wiley § Sons, New York

(1969) .

[8] Ives, D. J. G. and G. J. Janz , Eds., Reference

Electrodes 3 Academic Press, New York (1961).

[9] IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units 3 But t erworths , London

(1970) .

[10] Paabo, M. , R. A. Robinson and R. G. Bates, J. Amer . Chem. Soc. 87, 415 (1965).

24
9. Appendix - Certificates for pH and pD Standard Reference Materials

U. S. Department of Commerce

Frederick B. Dent

Secretary

National Bureau of Standards

Richard W. Roberta, Director ^National ^ureatt of Standards

Certificate

Standard Reference Material 185e

Potassium Hydrogen Phthalate

pH Standard

E. S. Etz

This lot of potassium hydrogen phthalate (KHC 8 H4 04) was prepared to insure high purity and

uniformity. It meets the specifications of the American Chemical Society for reagent-grade material,

but should not be considered to be entirely free of impurities such as traces of occluded water, free acid or alkali, chlorides, sulfur compounds, and heavy metals.

The pH(S) values listed below correspond to log ( 1/

a, t) where aH is a conventional activity of the

hydrogen (hydronium) ion referred to the standard state on the scale of molality. The values were

derived from emf measurements of cells without liquid junction by the method of calculation

described in the Journal of Research of the National Bureau of Standards 66A, 179 (1962). The

uncertainty of the pll(S) of Standard Reference Material 185e is estimated not to exceed ±0.005

unit for the 0 to 60 °C temperature range and ±0.01 unit from 70 to 95 °C. (The certified values

listed below apply only to SRM 185e)

The 0.05-molal solution is recommended for the standardization of pll equipment. The pH(S) of

this solution as a function of temperature is given below:

°c pH(S) °C pH(S) °C pH(S) °C pH(S)

0 4.003 25 4.004 45 4.042 70 4.12

5 3.998 30 4.011 50 4.055 80 4.16

10 3.996 35 4.020 55 4.070 90 4.19

15 3.996 37 4.024 60 4.085 95 4.21

20 3.999 40 4.030

The potassium hydrogen phthalate was obtained from the J. T. Baker Chemical Company of

Phillipsburg, N. J.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of R. A. Durst.

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.

Washington, D. C. 20234

May 21, 1973

(over)

25

J. Paul Cali, Chief

Office of Standard Reference Materials
Directions for Use

Preparation of the 0.05-molal solution: Transfer 10.12 grams of SRM 185e to a 1-liter

volumetric flask. Add distilled water to dissolve the salt and fill to the mark with distilled

water at 25 °C. The distilled water should have a conductivity not greater than 2xl0 -6 ohm-1

cm

-1

. Mix thoroughly by shaking. The salt should by dried for 2 hours at 110 °C before use.

The water used in the preparation of this pH buffer standard need not be protected from

atmospheric carbon dioxide, and elaborate precautions for the exclusion of air from the solution are not necessary. The solution should, however, be protected against evaporation and contamination by molds. This standard buffer solution should be replaced at frequent intervals and

when mold is apparent.

26
S. Department of Commerce

Maurice H. Stana

Secretary

<J S ' »

National Borvtiii of Standards

L. M. Bran>cg»to, Director

(dtxtxiitntt

Standard Reference Materials

Potassium Dihydrogen Phosphate (186-I-c)

Disodium Hydrogen Phosphate (186-II-c)

These standard reference materials are intended for use in defining the pH(S) scale. The lots of

potassium dihydrogen phosphate (KJI 2 P04 ) and disodium hydrogen phosphate (Na 2 HP04 ) were

prepared to ensure high purity and uniformity. They meet the specifications of the American

Chemical Society for reagent-grade materials, but should not be considered as entirely free from

impurities such as traces of water, free acid or alkali, carbon dioxide, chlorides, sulfur compounds

and heavy metals.

The pH(S) values listed below correspond to log(l/<JH ), where a\\ is a conventional activity of the

hydrogen (hydronium) ion referred to the standard state on the scale of molality. The values were

derived from the emf of cells without liquid junction by the method of calculation described in the

Journal of Research of the National Bureau of Standards, 66A, 179 (1962). The uncertainty of the

assigned values of pH(S) is estimated not to exceed ± 0.005 unit from 0 to 50 °C . The values listed

below applv only to the lots here certified. Minor variation of pH(S) (of the order of a few

thousandths of a unit) may be expected to occur between different lots. The solution 0.025 molal with respect to both KH 2 P04 and Na2 HP04

is recommended for the

calibration of pH equipment. The pll(S) of this solution as a function of temperature is given

below:

0

5

10

15

P H(S)

6.982

6.949

6.921

6.898

°C

20

25

30

35

pH(S)

6.878

6.863

6.851

6 842

°C

37

40

45

50

pH(S)

6.839

6.836

6.832

6 831

For pH measurements in the physiologically important range pH 7 to 8 a solution 0.008695

molal in KH 2 P04 and 0.03043 molal in Na2 HP04is also useful. The pH(S) values for this solution

as a function of a temperature follow: [See Journal of Research of the National Bureau of

Standards, 65A, 267(1961).]

0

5

10

15

pH(S)

7.534

7.501

7.472

7.449

°C

20

25

30

35

PH(S)

7.430

7.415

7.403

7.394

°C

37

40

45

50

P H(S)

7.392

7.388

7.385

7.384

The potassium dihydrogen phosphate was obtained from the Mallinkrodt Chemical Works of

St. Louis, Missouri; the disodium hydrogen phosphate from the J.T. Baker Chemical Co., of

Phillipsburg, New Jersey. The experimental work leading to the certification of this material was

carried out by M. Paabo and B. R. Staples.

The overall direction and coordination of technical measurements leading to the certification were performed under the chairmanship of R. A. Durst. 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.

Washington, D. C. 20234

September 1, 1970

J . Paul Cali, Acting Chief

Office of Standard Reference Materials

(over)

27
PREPARATION OF THE 0.025-MOLAL SOLUTION

Transfer 3.387 g of the potassium dihydrogen phosphate (186-I-c) and 3.533 g of the disodium

hydrogen phosphate (186-II-c) to a 1 -liter volumetric flask. Dissolve and fill to the mark with

distilled water at 25 °C. The distilled water should not contain dissolved carbon dioxide and should

have a conductivity no greater than 2 X 10" 6 ohm 1 cm 1

. Carbon dioxide-free water can be

prepared by boiling a good grade of distilled water for 10 minutes and guarding it with a soda-lime

tube while cooling. The salts should be dried for 2 hours at 1 10 °C to 130 °C before use. Although

elaborate precautions to prevent contamination of the buffer solution with atmospheric carbon

dioxide are usually unnecessary, the container should be kept tightly stoppered at all times when a sample is not actually being removed. (The figures given are weights in air.)

The solution should be replaced after a few weeks or sooner if molds or sediment appear, or if it has been exposed repeatedly to air containing carbon dioxide.

PREPARATION OF THE PHYSIOLOGICAL STANDARD SOLUTION

The physiological standard solution pH(S) = 7.415 at 25 °C, is prepared by transferring 1.179 g

of potassium dihydrogen phosphate (186-I-c) and 4.303 g of disodium hydrogen phosphate

(186-II-c) to a 1-liter volumetric flask, dissolving, and filling to the mark with water of the quality

described above. (The figures given are weights in air.)

This buffer solution is more sensitive to contamination with carbon dioxide than is the

0.025-molal solution. If the solution is to maintain the assigned pH(S) for a few weeks, exclusion of

carbon dioxide may be essential.

28
U. S. Departm^*' of Commerce

Maurice-H, Stans Secretary

National Bireap of) Standards

L. M. Brartac^WJ, Director

(Eerttftcate

Standard Reference Material 187b

Sodium Tetraborate Decahydrate

(Borax)

This standard reference material is intended for use in defining the pH(S) scale. This lot of

borax (Na 2 B4 07

• 10H 2 O) was prepared to ensure high purity and uniformity. It meets the

specifications of the American Chemical Society for reagent grade material. The water content of

this salt, stored under ordinary conditions, is less than theoretical. This does not affect the use

of this salt as a pH standard, but could lead to erroneous results were the partially desiccated

salt used as a boron or acidimetric standard.

The pH(S) values listed below correspond to log (l/a H ), where a H is a conventional activity

of the hydrogen (hydronium) ion referred to the standard state on the scale of molality. The

values were derived from emf measurements of cells without liquid junction by the method of

calculation described in the Journal of Research of the National Bureau of Standards, 66A, 179

(1962). The uncertainty of the pH(S) of Standard Reference Material 187b is estimated not to exceed ± 0.005 unit from 0 to 50 °C.

The 0.01-molal solution is recommended for the calibration of pH equipment. The pH(S) of this solution as a function of temperature is as follows:

°c pH(S) °C pH(S) °C pH(S)

0 9.460 20 9.227 37 9.093

5 9.392 25 9.183 40 9.074

10 9.331 30 9.143 45 9.044

15 9.276 35 9.107 50 9.017

The borax was obtained from the J. T. Baker Chemical Company of Phillipsburg, N. J. The

experimental work leading to the certification of this material was carried out by B. R. Staples.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of R. A. Durst.

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.

Washington, D. C. 20234 J. Paul Cali, Acting Chief

September 9, 1970 Office of Standard Reference Materials

(over)

29
Directions for Use

Preparation of the 0.01-molal solution: Crush gently any large lumps of salt (the salt must not he

dried in an oven before use). Transfer 3.80 g to a 1-liter volumetric flask, dissolve in distilled carbon

dioxide-free water, and fill to the mark with water at 25 °C. Water sufficiently carbon dioxide-free can be prepared by boiling for 10 min and cooling in a vessel guarded by a soda-lime tube. The

water should have a conductance less than 2 X 10 6 ohm 1 cm 1. To avoid contamination of the

buffer solution with atmospheric carbon dioxide, keep the stopper in place except when removing a portion of the solution. If desired, the solution may be protected with a soda-lime tube.

30
U.S. Department of commerce

National Bureau of Standards

Washington, D.C. 20234

Jlattonal bureau of gbtantiartig

Certificate

Standard Sample 188

Potassium Hydrogen Tartrate

(pH Standard)

Purity

This lot of potassium hydrogen tartrate (KHC4H4O8) was prepared to insure high purity and uniformity

and to assay close to 100 percent. However, it should not be considered as entirely free from impurities such

as traces of free acid or alkali, occluded water, chlorides, sulfur compounds, and heavy metals.

The values listed in this certificate are the assigned ^>H(S) numbers for solutions of Standard Sample 188.

These values correspond to log (l/aH ), where aB is a conventional activity of the hydrogen (hydronium ion

referred to the standard state on the scale of molality. They were derived from emf measurements of cells without liquid junction by the method of calculation described in the Journal of Research of the National

Bureau of Standards, 66A, 179 (1962). The uncertainty in the ^>H(S) listed is estimated not to exceed ±0.005

unit from 0 to 60 °C and ±0.01 unit from 70 to 95 °C.

A solution saturated with potassium hydrogen tartrate near 25 °C is recommended as a standard for the

calibration of pH equipment between 25 and 95 °C. The />H(S) of this solution as a function of temperature

is given below:

°c ^H(S) °C pU(S) °C *>H(S)

25 3.557 45 3.547 70 3.580

30 3.552 50 3.549 80 3.609

35 3.549 55 3.554 90 3.650

38 3.548 60 3.560 95 3.674

40 3.547

1 solution is also recommended as a standard for the range 0 to 60 °C.

ion of temperature is as follows:

°C ?H(S) °C ?H(S) °C *H(S)

0 3.711 20 3.647 40 3.632

5 3.689 25 3.639 45 3.635

10 3.671 30 3.635 50 3.639

15 3.657 35 3.632 55 3.644

38 3.631 60 3.651

The pH(S) of this

Directions for Use

Preparation of the saturated solution: Add an excess of Standard Sample 188 to distilled water contained

in a glass-stoppered bottle or flask and shake vigorously. With 100 percent excess of the salt, a few minutes of

shaking is sufficient for saturation. (One hundred ml of water at 25 °C will dissolve about 0.7 g of potassium

hydrogen tartrate.) Allow the solid to settle and decant the clear solution, or filter if necessary. Store the

solution in a glass-stoppered Pyrex bottle. For an accuracy of ±0.001 unit the temperature of saturation

must lie between 24 and 26 °C.

Solutions of the tartrate are very susceptible to mold growth which is usually accompanied by an increase

of a few hundredths in pH. For most accurate results, therefore, tartrate standards should be prepared fresh

each day. The distilled water used in preparation of these solutions should have a conductivity not greater

than 2 x 10-6 ohm-1 cm-1 at 25 °C, but dissolved carbon dioxide need not be removed.

Preparation of the 0.01-molal solution: Transfer 1.878 g (air weight) of Standard Sample 188 to a 1-liter volumetric flask. The salt need not be dried before use. Fill to the mark with water of the quality described

above and shake until all of the salt is dissolved and the solution thoroughly mixed. For most accurate

results, prepare the solution fresh each day.

A. V. Astin, Director

Washington, D.C. 20234

January 10, 1964 7IBHBi ,PO

31
U.S. Department of commerce

National Bureau of Standards

Washington, D.C. 20234

Jlattonal Ptireau of g^tantiarbsi

Certificate

Standard Sample 189

Potassium Tetroxalate

(pan Standard)

Purity

This lot of potassium tetroxalate dihydrate (KHC2CvH2 C2 Cv2 H2 0) was prepared to insure high purity

and uniformity and to assay close to 100 percent. However, it is certified only with respect to its paB value,

not as a pure substance.

paH Values

The values listed in this certificate are the conventional paB numbers for solutions of Standard Sample 189.

The values given correspond to log (\/aB), where aB is a conventional activity of the hydrogen (hydronium) ion

referred to the standard state on the scale of molality. They were derived from emf measurements of cells 'without liquid junction by the method of calculation described in the Journal of Research of the National

Bureau of Standards 66A, 179 (1962). The uncertainty is estimated not to exceed ±0.005 unit from 0 to 60 °C and ±0.01 unit from 60 to 95 °C.

The liquid-junction potential of the common pH cell displays a considerably greater variability in solutions

of pH less than 2.5 than in solutions of pH between 2.5 and 11.5. For this reason, solutions of potassium

tetroxalate are not recommended as primary standards of pH. They are useful, however, as secondary standards and for confirmatory purposes, with the expectation that the experimental pH may differ by 0.02 to 0.05

unit from the values of paB given above. They are also recommended when standards of paB are needed and

a liquid junction is not involved.

The 0.05-7n solution is recommended for use as a standard for paB. The paB values of this solution are given in the following table:

°c pam

0 1.666

5 1.668

10 1.670

15 1.672

20 1.675

25 1.679

°C pan

30 1.683

35 1.688

38 1.691

40 1.694

45 1 . 700

50 1 . 707

°c pan

55 1.715

60 1.723

70 1.743

80 1.766

90 1.792

95 1.806

Directions for Use

Preparation of the 0.05-molal solution: Transfer 12.61 g (air weight) to a 1-liter volumetric flask. Dissolve the salt and fill to the mark, with distilled water at 25 °C. It is not necessary to remove dissolved atmospheric carbon dioxide from the water and the salt should not be dried.

A. V. Astin, Director

Washington, D.C. 20234

January 10, 1964 „„„ „0

32
U. S. Depambfl^of Commerce

TSeflijtitarjr

National

A. V

Standards

rector Certificate of &nalpste

Standard Reference Materials

Sodium Bicarbonate (191)

Sodium Carbonate (192)

PU RITY

These lots of sodium bicarbonate (NallCO.-j) and sodium carbonate (Na2COs) were

prepared to insure high purity and uniformity . They meet the specifications of the

American Chemical Society for reagent-grade materials but should not Ik; considered as entirely free from impurities such as traces of water, free alkali, silica, chlorides, sulfur

compounds and heavy metals.

plI(S) Values

The pll(S) values listed Im:Iow correspond to log ( I /«n ), where a\\ is a conventional

activity of the hydrogen (hydronium) ion referred to the standard state on the scale of

molality. The values were derived from the emf of cells without liquid junction by the

method of calculation described in the Journal of Research of the National Bureau of

Standards, 66A, 179 (1962). The uncertainty of the assigned values of pll(S) is estimated

not to execeed + 0.005 unit from 0 to 50 °C. The values listed below apply only to the lots here certified. Minor variations of pH(S) (of the order of a few thousandths of a unit) may

he expected to occur between different lots.

The solution 0.025 molal with respect to both NallCO^ and INa2CO( is recommended for

the calibration of pll equipment. The pH(S) of this solution as a function of temperature is given below:

°C pH<S) °C p H(S) °C p H(S)

0 10.321 20 10.064 40 9.891

10.248 25 10.014 45 9.859

10 10.181 30 9.9BK 50 9.831

15 10.120 35 9.928

The sodium bicarbonate and sodium carbonate were obtained from the Mallinckrodt

Chemical Works of St. Louis, Mo. The experimental work leading lo the certification of this material was carried out by Bert R. Staples and Roger G. Bates.

The overall direction and coordination of technical measurements leading to the

certification were performed under the chairmanship of Roger G. Bates.

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 Thomas W. Mears.

Washington, 0. C. 20234

Julv 15, 1968

W. Wayne Meinke, Chief

Office of Standard Reference Materials

(ovct)

33
Directions for Use

Preparation of the ().025-molal solution. Transfer 2.092 g of sodium bicarbonate (191)

and 2.640 g of Na2CC>3 ( 192)( weights in air) to a 1-liter volumetric flask. Dissolve and fill to the mark with distillled water at 25 °C. The distilled water should contain no dissolved

carbon dioxide and should have a conductivity no greater than 2 x 10" 6

ft -1

cm1. Carbon dioxide free water can be prepared by boiling a good grade of distilled water for 10

min. and guarding it with a soda lime tube while cooling. The sodium bicarbonate should

not be dried by heating; the sodium carbonate should be dried for 2 hr. at 275 °C before

use. Although elaborate precautions to prevent contamination of the buffer solution wiOi

atmospheric carbon dioxide are usually unnecessary, the container should be kept tightly

stoppered at all times when a sample is not actually being removed.
U. S. Department of Commerce

Frederick B. Dent

Secretary

National Bureau of Standards

Richard W. Roberta, DirectorRational ^nttnn of jittantlartfe

Certificate

Standard Reference Materials

Tris( hydroxy methyl)aminomethane (922)

Tris(hydroxymethyl)aminomethane hydrochloride (923)

B. R. Staples

These Standard Reference Materials are intended primarily for use in preparing standard pH

buffer solutions in the physiological range. Such systems are used in the clinical laboratory in cases where phosphate buffers would cause undesirable side reactions or do not adequately approximate

the required variation of pH with temperature. Coulometric assay indicates the tris(hydroxymethyl)aminomethane to be 99.99 ± 0.02 mole percent after drying in a vacuum oven at 70 °C for

24 hours. Coulometric assay of the tris(hydroxymethyl)aminomethane hydrochloride as received was 99.69 ± 0.05 mole percent.

The pM(S) values listed below correspond to log(l/aH ), where aH

is a conventional activity of

the hydrogen (hydronium) ion referred to the standard state on the scale of molality. The values were derived from the emf of cells without liquid junction by the method of calculation described

by Bates [1]. The uncertainty of the assigned values of pH(S) is estimated not to exceed ± 0.005

unit. The values listed below apply only to the material herein certified.

The solution recommended for the calibration of pH equipment is 0.01667 molal with respect

to tris(hydroxymethy l)aminomethane and 0.0500 molal with respect to tris(hydroxymethyl)aminomethane hydrochloride. The pH(S) of this solution as a function of temperature is as follows:

°C pII(S)

0 8.471

5 8.303

10 8.142

15 7.988

°C

20

25

30

35

pH(S)

7.840

7.699

7.563

7.433

°C

37

40

45

50

PH(S)

7.382

7.307

7.186

7.070

The tris(hydroxymethyl)aminomethane and tris(hydroxymethyl)aminomethane hydrochloride were obtained from the Sigma Chemical Company of St. Louis, Missouri.

The overall direction and coordination of technical measurements leading to the certification was performed under the chairmanship of R. A. Durst.

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.

Washington, D. C. 20234

June 23, 1971

Revised December 13, 1973

J. Paul Cali, Chief

Office of Standard Reference Materials

(over)

3 5
The name "tris(hydroxymethyl)aminomcthane" is used preferentially in this certificate since it

is the name commonly used in clinical laboratories. Under the system of nomenclature recom- mended by the International Union of Pure and Applied Chemistry this material is named 2-amino2-(hydroxymethyl)-I ,3-propanediol. It is also known by several trivial and proprietary names, e.g., "tris", "THAM," "trizma," ' t tromethamine, ,, "tromethane,''' etc. However, the certified values

given herein apply only to the material supplied as an NBS-SRM.

These Standard Reference Materials (SRM's) are intended for "in vitro" diagnostic use only.

This material is for use in preparing standard buffer solutions for use in clinical chemistry. A

buffer solution having the pH values certified on the face of this certificate is 0.01667 molal with

respect to tris(hydroxymethyl)aminomethane and 0.0500 molal with respect to tris(hydroxymethyl)aminomethane hydrochloride. The preparation of this solution is described in the following

paragraph.

Transfer 7.822 g of tris(hydroxymethyl)aminomethane hydrochloride (SRM 923) and 2.005 g

of tris(hydroxymethyl)aminomethane (SRM 922) to a 1-liter volumetric flask. Dissolve and fill to

the mark with distilled water at 25 °C. The distilled water should not contain dissolved carbon

dioxide and should have a conductivity no greater than 2 x ]0 6 ohm"' cm 1

. Carbon dioxide-free

water can be prepared bv boiling a good grade of distilled water for 10 minutes and guarding it with

a tube of carbon dioxide absorbant. The certified pH(S) values apply for materials as received. No

particular drying procedures are necessary. The tris(hydroxymethyl)aminomethane should not be

heated above 80 °C and as a precaution the tris( hydroxy methyl)aminomethane hydrochloride

should not be exposed to temperatures above 40 °C. (The figures given are weights in air.)

CAUTION: Reference electrodes containing linen-fiber junctions should not be used with this

buffer. Such junctions produce large liquid-junction potentials, drift, and long equilibrium times.

Standard Reference Material 922 should be stored in a well-closed container at room temperature. Exposure to high temperatures (above 50 °C) and direct sunlight should be avoided. Experience has shown this compound to be stable in storage for at least 12 years [2]. If the material

degrades such that the certified values are changed, the users will be notified by NBS. It is recom- mended that this material not be used after 5 years from the date of purchase.

Standard Reference Material 923 should be stored in a well-closed container at room temperature, preferably in a desiccator. Exposure to high temperatures (above 40 °C) and direct sunlight

should be avoided. SRM 923, tris(hydroxymethyl)aminomethane hydrochloride is quite hygroscopic even at relative humidities of 30 to 50 percent |3|. Experience at NBS has indicated that

properly stored this material is stable for at least 5 years. Weighing and other manipulations of SRM

923 should be avoided under conditions when the relative humidity exceeds 50 percent. If the

material degrades such that the certified values are changed, the users will be notified by NBS. It is recommended that this material not be used after 5 years from the date of purchase.

This buffer solution is sensitive to contamination by carbon dioxide [4]. If the solution is to

maintain the assigned pH(S) for a few weeks the exclusion of carbon dioxide is essential. The

solution should be replaced after a few weeks, or sooner if mold or sediment appear, or if it is repeatedly exposed to air containing carbon dioxide.

36
References:

[1] R. G. Bates, Revised standard values for pH measurements from 0 to 95 °C, J. Research NBS

66A, 179 (1962).

[2| J. A. Riddick, Amine buffers as aeidimetric standards, Ann. N.Y. Acad. Sci. 92,357 (1961).

[3] Sigma Tentative Technical Bulletin No. 106 B, Sigma Chemical Company, St. Louis, Mo.,

revised 1967.

[4] R. G. Bates and H. B. Hetzcr, Absorption of carbon dioxide by solutions of 2-amino-2-(hvdroxymethyl)-l,3-propanediol, Anal. Chem. 33,1285(1961).

These Standard Reference Materials have been measured and certified at the laboratories of the

National Bureau of Standards, Gaithersburg, Maryland. All inquiries should be addressed to:

Office of Standard Reference Materials

Room B31 1, Chemistry Building

National Bureau of Standards

Washington, D. C. 20234

The date of issuance and certification of these Standard Reference Materials was June 23. 1971.

3 7
U. S. Departfetff of Commerce

OR^Smith

National luf^*'oi

A. V.^preln

r

dards

Certificate of &nalpsrt£

Standard Reference Materials

Potassium Dihydrogen Phosphate (2186—1)

Disodium Hydrogen Phosphate (2186—11)

Purity

These lots of potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (Na2HP04) were

prepared to ensure high purity and uniformity. They meet the specifications of the American Chemical Society for reagent-grade materials but should not be considered as entirely free from impurities such as traces of water, free acid

or alkali, carbon dioxide, chlorides, sulfur compounds and heavy metals.

pD(S) Values

The pD(S) values listed below correspond to log(l/arj),where arj is the conventional activity of the deuterium ion

referred to the standard state on the scale of molality. The values were derived from the emf of cells without liquid

junction by a method of calculation analogous to that described for the assignment of pH(S) values [Journal of

Research of the National Bureau of Standards, 66A, 179 (1962)] . The uncertainty of the assigned values forpD(S) is estimated not to exceed 0.01 unit. The values listed below apply only to the lots here certified. Minor variations of

pD(S) (of the order of a few thousandths of a unit) may be expected to occur between different lots. The solution 0.025 molal with respect to both KD2PO4 and Na2DPC>4 is recommended for the calibration of pH

meters to be used for the measurement of pD in deuterium oxide. These compounds are prepared in situ by

hydrogen-deuterium exchange between the protium salts, KH2PO4 and Na2HPC»4 and the deuterium oxide solvent. The pD(S) of this solution as a function of temperature is given below:

t,°C pD(S) t,°C PD(S) t,°C PD(S)

5 7.539 25 7.428 40 7.387

10 7.504 30 7.411 45 7.381

15 7.475 35 7.397 50 7.377

20 7.449

DIRECTIONS FOR USE

The preparation of the 0.025 molal solution should be carried out by the addition of weighed quantities of the salts to weighed quantities of deuterium oxide in the following proportions (weights in vacuo): 0.003402g KH2PO4 and

0.003549g Na2HPC>4 per g of deuterium oxide. The deuterium oxide should have an isotopic composition of at least 99.5 mole percent D26. It should not contain dissolved carbon dioxide or other gases and should have a conductivity no greater than 2 x 10~ 6 ft -1 cm- 1 . The salts should be dried for 2 hr at 100 to 130 °C before use. Although

elaborate precautions to prevent contamination of the buffer solution with atmospheric carbon dioxide are usually unnecessary, the container should be kept tightly stoppered at all times when a sample is not actually being removed.

The development of the pD scale and the experimental work leading to the certification of these materials were performed by Maya Paabo and Roger G. Bates.

The overall direction and coordination of the technical measurements leading to certification were performed

under the chairmanship of Roger G. Bates.

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.

Washington, D. C. W. Wayne Meinke, Chief

May 28, 1968 Office of Standard Reference Materials

38
U. S. Departftsg^ of Commerce

C. RU&nith

"Be^pf^

National JJ A. V. s

ecto

n

r

dards Certificate of ^Inalpsts

Standard Reference Materials

Sodium Bicarbonate (2191)

Sodium Carbonate (2192)

Purity

These lots of sodium bicarbonate (NaHC03)and sodium carbonate (Na2CC>3) were prepared to ensure high purity

and uniformity. They meet the specifications of the American Chemical Society for reagent-grade materials but should not be considered as entirely free from impurities such as traces of water, free alkali, silica, chlorides, sulfur compounds

and heavy metals.

pD(S) Values

The pD(S) values listed below correspond to logO/flo), where up is the conventional activity of the deuterium ion

referred to the standard state on the scale of molality. The values were derived from the emf of cells without liquid

junction by a method of calculation analogous to that described for the assignment of pH(S) values [Journal of

Research of the National Bureau of Standards, 66A, 179 (1962)] . The uncertainty of the assigned values for pD(S) is estimated not to exceed 0.01 unit. The values listed below apply only to the lots here certified. Minor variations of

pD(S) (of the order of a few thousandths of a unit) may be expected to occur between different lots. This solution, 0.025 molal with respect to both NaDC03 and Na2CC»3, is recommended for the calibration of pH

meters to be used for the measurement of pD in deuterium oxide. The NaDC03 is prepared in situ by the hydrogen-deuterium exchange between the protium salt and the deuterium oxide solvent. The pD(S) of these solutions

as a function of temperature is given below:

t,°C pD(S) t,°C pD(S) t,°C PD(S)

5 10.998 25 10.736 40 10.597

10 10.923 30 10.685 45 10.559

15 10.855 35 10.638 50 10.527

20 10.793

DIRECTIONS FOR USE

The preparation of the 0.025 molal solution should be carried out by the addition of weighed quantities of the salts to weighed quantities of deuterium oxide in the following proportions (weights in vacuo): 0.002 lOOg NaHCC>3 and

0.002650g Na2C03 per g of deuterium oxide. The deuterium oxide should have an isotopic composition of at least 99.5 mole percent D2O. It should not contain dissolved carbon dioxide or other gases and should have a conductivity no greater than 2 x 10-6 fl~ 1 cm- 1

. The sodium bicarbonate should not be dried by heating; the sodium carbonate

should be dried for two hrs at 275 °C before use. Although elaborate precautions to prevent contamination of the buffer solution with atmospheric carbon dioxide are usually unnecessary, the container should be kept tightly

stoppered at all times when a sample is not actually being removed.

The development of the pD scale and the experimental work leading to the certification of these materials were performed by Maya Paabo and Roger G. Bates.

The overall direction and coordination of the technical measurements leading to certification were performed

under the chairmanship of Roger G. Bates.

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.

Washington, D. C. 20234 W. Wayne Meinke, Chief

May 28, 1968 Office of Standard Reference Materials

39
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

SP 260-53

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

STANDARD REFERENCE MATERIALS: Standardization

of pH Measurements

5. Publication Date December 1975

6. Performing Organization Code

7. AUTHOR(S)

Richard A. Durst

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 #9, above.

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 75- 619344

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.)

One of the most widely performed analytical measurements in chemical laboratories is that of pH using the glass

electrode. In order to insure the consistency of these measurements, the National Bureau of Standards has adopted an

operational scale of pH defined in terms of the pH(S) of a

series of standard buffer solutions. Certified samples of

buffer materials, from which the standard reference solutions

of reproducible pH can be prepared, are issued by the NBS

.

At present, the operational scale is defined by the pH of

seven primary and three secondary standard solutions.

This report is concerned primarily with a discussion of

the method used at the NBS for the assignment of pH values to

the standard buffer solutions, a description of the NBS measurement facilities, and a summary of the characteristics of

these standards. A brief discussion of the types of electrodes

used and the calibration of pH instrumentation is also

presented.

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) Buffers ; buffer solutions; glass electrode; pH;

pH buffers; pH electrode; pH measurement; pH standards.

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