NBS SPECIAL PUBLICATION 260-24
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 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 measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

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. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

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

- Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
|ed Bureau of Standards

. 2 4 UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Standard Reference Materials:

Comparison of Redox Standards

K. M. Sappenfield, G. Marinenko, J. L. Hague

Institute for Basic Standards

National Bureau of Standards Washington, D.C. 20234

0* or w

National Bureau of Standards Special Publication 260-24

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-24, 97 pages, (Jan. 1972) CODEN: XNBSA

Issued January 1972

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

(Order by SD Catalog No. C 13.10:260-24). Price $1.00.

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 compatability 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 Call, Chief

Office of Standard Reference Materials

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vi
TABLE OP CONTENTS

PAGE

I. INTRODUCTION . . . „ 1 II. SRM 83c, ARSENIC TRIOXIDE 3 A. Introduction 3 B. Sampling .,3 C. Homogeneity 4 D. Impurities 4 E. Effective Purity of SRM 83c, Arsenic Trioxide. 4 1. Comparison with SRM 83b, Arsenic Trioxide. 7 2. Comparison with SRM's 136b and 136c,

Potassium Dichromate 15

3. Coulometry 17

4. Comparison with Purified Iodine 22

5. Summary. 22 IIIo SRM 136c, POTASSIUM DICHROMATE 25

A. Introduction 25

B. Sampling 25

C. Homogeneity 26

D. Impurities 26

E. Effective Purity of SRM 136c, Potassium

Dichromate 26

1. Comparison with SRM 136b, Potassium

Dichromate 30

2. Comparison with SRM 83c, Arsenic Trioxide. 33

3. Coulometry 35

4. Summary 45 IV. SRM 4 Oh, SODIUM OXALATE 49 A. Introduction 49

B. Sampling 49

C. Homogeneity 49

D. Impurities 53

E. Effective Purity of SRM 40h, Sodium Oxalate. . 53

1. Comparison with SRM 40g, Sodium Oxalate. . 53

2. Determination of Impurities 55

3. Comparison with SRM 83c, Arsenic Trioxide. 6l

4. Comparison with SRM's 136b and 136c,

Potassium Dichromate 69

5. Summary 72

V. SUMMARY OP RESULTS 76 VI. REFERENCES . „ 80 vii
PAGE

APPENDIX 1 83 APPENDIX II 87 LIST OF TABLES

Table No.

1. Homogeneity and effective purity of SRM 83c,

Arsenic Trioxide, compared with SRM 136b,

Potassium Dichromate 5 2. SRM 83c, Arsenic Trioxide, ACS Specifications. . 7 3. Effective purity of SRM's 83b and 83c, Arsenic

Trioxide compared with purified iodine .... 14

4. Effective purity of SRM 83c, Arsenic Trioxide,

based on composites compared with SRM 136b,

Potassium Dichromate 16

5. Effective purity of SRM 83c, Arsenic Trioxide,

compared to SRM 136c, Potassium Dichromate . . 17

6. Effective purity of SRM 83c, Arsenic Trioxide,

by coulometry 21

7. Effective purity of SRM 83c, Arsenic Trioxide. . 24

8. Homogeneity and effective purity of SRM 136c,

Potassium Dichromate, compared with SRM 136b,

Potassium Dichromate 27

9. Effective purity of SRM 136c, Potassium

Dichromate, compared with SRM 136b,

Potassium Dichromate 32 10. Effective purity of SRM's 136c and 136b,

Potassium Dichromate, compared with SRM 83c,

Arsenic Trioxide 34

11. Effective purity of SRM 136b, Potassium

Dichromate, by coulometry 46

12. Effective purity of SRM 136c, Potassium

Dichromate, by Coulometry 47

13. Effective purity of SRM 136c, Potassium

Dichromate 48 viii
PAGE

14. Homogeneity of SRM 40h, Sodium Oxalate, and

comparison with SRM 40g, Sodium Oxalate. ... 50 15. Comparison of SRM 40g, Sodium Oxalate, with

composites of SRM 40h, Sodium Oxalate 56

16. Sodium hydrogen oxalate and sodium bicarbonate

in SRM 40h, Sodium Oxalate 57

17. Evolution of Carbon Dioxide on Boiling in Sodium

Oxalate Solution; Evolution of Carbon Dioxide

Without Heating in Sodium Oxalate Solution . . 59 18. Loss on heating of SRM 40h, Sodium Oxalate ... 61

19. Impurities in SRM 40h, Sodium Oxalate 6l

20. Effective purity of SRM 40h, Sodium Oxalate,

compared with SRM 83c, Arsenic Trioxide

(NaOH-HCl) 65

21. Effective purity of SRM 40h, Sodium Oxalate,

compared with SRM 83c, Arsenic Trioxide

(Na 2 C0 3 -H 2 S0if ) 70

22. Effective purity of SRM 40h, Sodium Oxalate,

compared with SRM 136b, Potassium Dichromate . 73

23. Effective purity of SRM 40h, Sodium Oxalate,

compared with SRM 136c, Potassium Dichromate . 74

24. Effective purity of SRM 40h, Sodium Oxalate. . . 75

25. Effective purity of oxidimetric standard

reference materials 77

26. Calibration curve 88

LIST OP FIGURES

Figure No.

1. Homogeneity of SRM 83c, Arsenic Trioxide .... 6 2. High accuracy constant-current and timing

circuit 18

3. Effect of pH on the st oichiometry of arseniteiodine reaction 19 ix
PAGE

4. Homogeneity of SRM 136c, Potassium Dichromate . . 27

5. Coulometric determination of the homogeneity of

SRM 136c, Potassium Dichromate 29

6. Current, potential curves for oxidation of

ferrous ion at a polarized platinum electrode. 37

7. Typical end-point determination 43

8. Homogeneity of SRM 40h, Sodium Oxalate, KMnCU

solution no. 3 51

9. Homogeneity of SRM 40h, Sodium Oxalate, KMnO

solution no. 4 52

10. Intercomparison of redox Standard Reference

Materials 77

11. Photometric calibration curve for excess iodine. 89

x
STANDARD REFERENCE MATERIALS:

COMPARISON OP REDOX STANDARDS

K. M. Sappenfield, G. Marinenko and J. L. Hague

Institute for Materials Research

National Bureau of Standards

The methods described in this publication are those that have been used at the National Bureau

of Standards for the comparison of the current

oxidimetric standard reference materials with each

other and for the determination of the effective

purity of these standard reference materials. The

methods, which are modifications of well-established

and previously published methods, were selected or designed for their accuracy and dependability. An accurate method was devised for the determination

of the effective purity of the arsenic trioxide

standard SRM 83c by adding solid iodine and determining the excess iodine photometrically.

Key words: Analytical procedures, arsenic-iodine

method, comparison, effective purity,

oxidimetric standards.

I. INTRODUCTION

The National Bureau of Standards issued its first oxidimetric Standard Reference Material, SRM 40, Sodium Oxalate,

in 1912. Before the issuance of this standard, analytical

chemists had raised objections that the commercially obtainable

material was variable and untrustworthy. In 1927 arsenic

trioxide standard SRM 83 was added to the list of standard

reference materials available from NBS and in 19^4 potassium

dichromate standard SRM 136 was issued.

The materials for standard reference materials are purchased under special specifications to insure uniformity in

size and composition of the crystals so that an entire lot

of each standard reference material can be issued under one certificate of analysis. Such materials are specially prepared

by the manufacturers for NBS.

1
This publication gives in detail the methods that have

been used at NBS for the certification of the current oxidimetric standard reference materials. Also the methods for

the intercomparison of the oxidimetric standard reference

materials and for assay by coulometry are described.

The methods described are essentialy the classical ones with modifications to improve the accuracy and precision of

the assay of standard reference materials. Weight burets

were used instead of volumetric burets, all weighings were corrected to vacuum, and oxygen and carbon dioxide were removed

from solutions when they would interfere with the reactants.

The detection of the end points was studied and improved and excess reagent, needed for end point detection, was more accurately determined. Also included are the coulometric

procedures for assay of arsenic trioxide and potassium

dichromate

All of these standard reference materials have been

issued with a provisional certificate but in the near future

SRM 40h, Sodium Oxalate; SRM 83c, Arsenic Trioxide; and SRM

136c, Potassium Dichromate, will be assigned a final certificate

value. The current certificates are shown in Appendix I. The final section gives a complete summary of the intercomparisons of the oxidimetric standard reference materials

now being issued by the National Bureau of Standards. SRM

136b, Potassium Dichromate, was included in this discussion

since its effective purity had been determined by an independent method, coulometry.

2
II. SRM 83c, ARSENIC TRIOXIDE

A. Introduction

Arsenic trioxide solution was first applied in titrimetric

analysis by Gay Lussac in 1815 [1]. He used it in chlorimetry

with an indigo indicator. In improving the method Penot [2]

dissolved the arsenic trioxide in a sodium hydroxide solution

instead of hydrochloric acid and used strips of starch iodide

paper as the indicator. Mohr [3] made further improvements

and it is essentially his procedure which is used at the

present time. This procedure entails the use of a standard

iodine solution instead of chlorine water as the titrant and

the starch solution indicator is added directly to the analate

In 1918, Chapin [4] proposed the use of arsenic trioxide

as a primary standard in iodimetry. In a series of experiments, he showed that arsenic trioxide meets the criteria for

a primary standard. The National Bureau of Standards first

issued arsenic trioxide as a standard reference material for

iodimetry in 1927. Since then three renewals have been issued,

SRM 83c, Arsenic Trioxide, being the current standard reference

material.

The arsenic trioxide, issued as SRM 83c, was produced

commercially to meet special specifications of high homogeneity

and purity so that the entire lot could be issued under one certificate of analysis.

B. Sampling

The material was received from the supplier in thirty

glass jars containing 25 pounds each of arsenic trioxide.

After the jars were thoroughly mixed by rolling, a representative sample of 30 g was removed from each. Five gram

portions from each of the above representative samples were combined into two composite samples. One composite was made

from samples obtained from the odd numbered jars and one from the even numbered jars.

3
C . Homogeneity

The individual representative samples were compared,

using SRM 136b, Potassium Dichromate, to determine whether

the material was homogeneous and could be issued under one certificate of analysis.

All 30 individual representative samples were assayed

once and eight were repeated. Three of the values were outside

2 a and were not included in the average. (In this paper any

number omitted from the average in any Table is greater than

2 a and is not used to calculate the a given in the Table.)

After discarding these values, the standard deviation was 0.007 percent which indicates the arsenic trioxide was homogeneous within the limits specified. The results are given

in Table 1 and Figure 1. D . Impurities

Although arsenic trioxide is not hydroscopic, samples of

the material were dried at 105 to 110 °C to eliminate surface

moisture. After 1 hour of heating the loss was 0.001 percent

and after 24 hours 0.002 percent. All samples used in homogeneity testing and in the determination of effective purity

of SRM 83c were dried at 105 to 110 °C for 2 hours.

The arsenic trioxide was analyzed and found to meet the

ACS Specifications [5] (Table 2). Spark source mass spectrometric analysis also showed that sodium was present at the

50 ppm level but no other element was detected at a level

greater than 1 ppm.

E. Effective Purity of SRM 83c, Arsenic Trioxide

The effective purity of SRM 83c was determined by com- parison with SRM 83b, Arsenic Trioxide; by comparison with

SRM 136b and SRM 136c, Potassium Dichromate; by coulometry;

and by comparison with purified iodine. The effective purity

of SRM 83c for the provisional certification was determined

by the comparison with SRM 136b, Potassium Dichromate.

4
Table 1. Homogeneity and effective purity of SRM 83c, Arsenic

Trioxide, compared with SRM 136b, Potassium Dichromate.

SRM 83b SRM 83c

g As 2 0 3 a Effective Jar g As 2 0 3 Effective

g K 2 Cr 2 07 purity (56) No. per g K 2 Cr 2 0 7 purity(%)

1

"i . nnuu coo qyy'yy q q q 7

I

1 1 . 00867 99 .986

1 . UU Oyy qqy y qq8 y y u

0 1.00860 99.993

1 n n 9, h q inn nn q _L VJ VJ • VJ VJ 3 1° 1.00 865 99 .988

qyy>yyy q qqq J I.OO857 99y y .• y996y w

1 nn 8I18 J_1 00U VJ . ooh U VJ ^ 6 1 0086^ 99.990 y y • y y w 1 . UU04 ( 1UU 1 00 • nns U VJ ^) qy 1 008^8 qq 0015

y y • y y y

1 nn 8R7 qqy y • qq6 y y v 12 1 .00847 100 . 005

J. . UUUJ ( Q Q.7 • QQ6 y yv IS 1 . 00859 99.994

q.7 q^ • yyv qq6 18 I.OO857 99 . 996

1 nn 7^^; 100 . 117 21 1.00864 QQy y • yQ8Qw y

J.1 • nnSss UUUjJ q q qq 8 7. ' yy u 24 1 00866 yQQy • Q87 y u / 1 . 00852 100 . 000 27 1 .00857 99 . 996

qqy y • qq7 y y 1 ^0 1-L . 008^7 VJ VJ U _) / 100 . 0 15

1 . 00850 100 . 002 2b

1 .00511 100 . 340°

1.00862 99 .991 5 1. 00840 100 .012

1 . 00846 100 .006 11 1 . 00864 QQy y .• Q8g y y 1.00898 c 99 .955° 14 1 .00851 100 . 001

1 .00852 100 .000 17 1 . 00852 100 . 000

20 1. 01048° 99 . 807°

23 I.OO857 QQy y • yQQ6y u 26 1 . 00853 100 . 000

29 1. 00867 yQQy • Q86 y w

4 1. 00866 QQy y • yQ87<j i 7 1. 00863 QQy y • yyv QQ0

10 1 .00848 100.004

13 1 . OO857 QQy y • yQQ6y u 16 1. 00859 yQQy • yQQ4y

~ 19 1 .00861 QQy y • yQQ2y 22 1 .00857 qqy qq6

y

• yyv

25 1 .00860 QQy y « yQQ3y 0 28 1.00858 QQ-7 -? .• QQ5 yyy 0 K 1.00883 99 . 970

5 b 1.00852 100 .000

8^ 1.00850 100 .002

io5 1. 00861 QQ> y ' yQQ2y t- 12b 1.00854 99.999

1.00862 99.991

30

b 1.00852 100 .000

1.00853 99.999 1.00857 99.995

0.00004 0.0044 0 .000071 0 .0070

b Effective purity of SRM 136b, Potassium Dichromate = 99 .979%

.

Repeat analysis. Omitted from average.

Note: SRM 83c would have an effective purity of 99.992% when

compared to SRM 83b (99-996%).
100.020

100.010

« 100.000

99.990

99.980

100.337 T

• o r

-0-O- r* o

o o

o - JU _ _ _ _ L

o o

o

1

99.967

J L

o

i

99.804

J L

4 8 12 16 20 24 Drum Number o initial measurements • additional measurements

28

Figure 1. Homogeneity of SRM 83c, Arsenic Trioxide,

compared with SRM 136b s Potassium Dichromate.

6
Table 2. SRM 83c, Arsenic Trioxide, ACS Specifications.

Test

Requirements

not more than {%) Pound {%)

Residue after ignition 0 .020 0.006

Insoluble in HC1 0.010 0.001

Chloride (CI)

Sulfide (S)

Antimony (Sb)

Iron (Fe)

0 .005 To pass test ( . 001$)

0.0005

0 .001 Pass test <0.005

0 .0001

0.0005

1 . Comparison with SRM 83b, Arsenic Trioxide

a. Comparison through SRM 136b, Potassium Dichromate

- SRM 83c was compared to SRM 83b at the same time the effective

purity for certification and the homogeneity of the material

were determined through SRM 136b, Potassium Dichromate.

In the published methods for the determination of dichromate ion with an arsenite solution, an excess of arsenite

solution is added and this excess is determined iodiome trically

[6] or with a standard potassium bromate solution [7]. A direct potentiometric titration of the dichromate ion with

arsenic trioxide has been described in which manganous sulfate

and a catalyst (1 drop 0.025 M KI or KI0 3 solutions) are added

to the solution at the start of the titration [8].

Prom the inter comparison of the potassium dichromate

standards with ferrous ammonium sulfate it was known that the

approach of the endpoint was rapid and easily detected. Using

this information a method involving addition of excess potassium

dichromate to the arsenic trioxide solution and back-titrating

with a ferrous ammonium sulfate solution was developed.

Method : The arsenic trioxide was dissolved in a sodium carbonate

solution and acidified with sulfuric acid. The potassium

dichromate, dissolved in a little water, was added and the

7
excess potassium dichromate was titrated potentiometrically

with a dilute ferrous ammounium sulfate solution.

Reagents

(a) Water - All the water used was freshly boiled and

cooled distilled water.

(b) Standard ferrous ammonium sulfate solution (8 g/1)

- Eight grams of Fe (NfU ) 2 (SO n ) 2

* 6H 2 0 were transferred to a liter volumetric flask and 200 ml of H 2 SCW (1+19) were added.

When the salts were dissolved, the solution was diluted to the

mark with R^SOi* (1+19) and thoroughly mixed. Standardization—

0.050 g of potassium dichromate was transferred to a 600 ml

beaker and 300 ml of H2SO4 (1+19), freshly boiled and cooled, were added. After the salt was dissolved, the solution was titrated potentiometrically with the ferrous ammonium sulfate

solution. Grams of potassium dichromate equivalent to 1 ml

of ferrous ammonium sulfate solution is calculated.

Procedure

One gram of arsenic trioxide accurately weighed to the

nearest 0.00001 g was transferred to a 150 ml beaker and 10 g of solid sodium carbonate were added. Twenty ml of water were added in 5 ml portions and the solution was warmed to dissolve

the sample. The solution was transferred to a 750 ml Erlenrneyer flask and 30 ml of sulfuric acid (1+1) were cautiously

added in small portions through a short stem funnel inserted

in the flask. When the reaction ceased, 1.000 g of potassium

dichromate, accurately weighed to the nearest 0.00001 g, was dissolved in 10 ml of water, and was added. The solution was allowed to stand for 10 minutes, then transferred to a 600 ml

beaker and diluted to 300 ml with water.

The solution was cooled in ice water and the excess potassium dichromate was titrated potentiometrically with the

standard solution of ferrous ammonium sulfate. All weights

were corrected to vacuum.

8
Effective purity of As 2 0 3> in percent =

c

x —

where

A = g of potassium dichromate added,

B = ml of standard ferrous ammonium sulfate solution,

C = g of potassium dichromate equivalent to 1 ml ferrous

ammonium sulfate solution,

D = g of arsenic trioxide taken,

E = effective purity of potassium dichromate in percent,

P = equivalent weight AS2O3, and

G = equivalent weight K 2 Cr 2 07.

The effective purity of SRM 83c is 0.004 percent lower

than the effective purity of SRM 83b when compared through

SRM 136b (Table 1). Since SRM 83b was certified for 100.00

percent (99-996) the effective purity of SRM 83c would be

99.99 percent (99.992) . b. Comparison through Purified Iodine - The arsenic

trioxide standards were also compared during the determination

of the effective purity of SRM 83c by comparison with purified

iodine. A procedure involved the addition of solid iodine and

the excess iodine beyond the end point of the titration was determined photometrically.

The method for titration of arsenic trioxide with iodine

was developed by Mohr [3] in 1859 and today it is used essentially in its original form. The arsenic trioxide is dissolved

in a sodium hydroxide solution and the starch indicator solution

is added directly to the arsenic trioxide solution followed

by the iodine titrant . In order to improve the precision and

accuracy of the method by reducing possible errors such as impurities in iodine, end point detection, loss of material,

use of solid iodine, etc., the method was investigated.

(1) Impurities in iodine: The impurities in the iodine

were removed by purifying the iodine according to Treadwell

and Hall [9]. Commercial resublimed iodine was again resublimed from a potassium iodide-ignited calcium oxide mixture.

9
The iodine was then resublimed two more times without the use

of potassium iodide and the calcium oxide. The iodine was crushed in an agate mortar, transferred to a glass stoppered

bottle and stored over magnesium perchlorate in an ungreased

desiccator

(2) Detection of end point: A slight excess of iodine

is needed to detect the end point of the titration regardless

of whether the solution is titrated potentiometrically or an indicator is used. The excess iodine is usually determined

by adding the iodine solution to a solution containing all

reagents with the exception of arsenic trioxide either to an indicator or the potentiometric end point. All titrations

are corrected for the amount of iodine used to titrate the

blank solutions. Anhorn and Hunt [10] have shown that the

iodine color is sensitive enough to detect the end point in

titrations with 0.02 N or stronger iodine solutions if the

solution being titrated is clear. Using this information the

iodine solution itself is the indicator and the excess iodine

is determined by photometry. In this procedure, an accurate

correction can be applied to each determination because the excess iodine is determined for each titration.

(3) Use of solid iodine: In the preparation of the

standard iodine solution for the assay of arsenic trioxide,

the iodine was added to a potassium iodide solution in a storage flask and after the iodine dissolved, the solution

was diluted to the desired volume [11,12,13,1^,15]. Weight

burets are used to increase the precision and accuracy of the

delivery of the standard iodine solution by weight instead of

volume. During this investigation it was observed that iodine,

after standing in a glass stoppered bottle, did not lose

weight. Using this information a weighed amount of solid

iodine, less than the calculated equivalent of the arsenic

trioxide, was added to the reaction flask containing the

arsenic trioxide solution and the titration completed with a few ml of 0.05 N iodine solution. The use of solid iodine

10
allows one to determine the iodine equivalent of arsenic

trioxide more accurately.

(4) Loss of material: The above procedure gave good

results but when the sodium carbonate solution of arsenic

trioxide was neutralized, the escaping carbon dioxide caused

a fine mist above the solution and a small amount of arsenic

' trioxide could be carried along with the carbon dioxide and

! might be lost. The iodine-arsenic trioxide reaction proceeds

i quantitatively up to a pH of 9.2 [16, p. 218], and when the

neutralization step was omitted the pH of the solution, even after the titration, was below 9.2, therefore the neutralization of the sodium carbonate was deleted.

Method:

After the arsenic trioxide was dissolved in a sodium

carbonate solution, a sodium bicarbonate solution and potassium

iodide salt were added. After the potassium iodide was dissolved, a weighed amount of iodine, less than the calculated

equivalent of the arsenic trioxide, in a glass stoppered

weighing bottle was added. When the iodine had dissolved and

reacted with the arsenic trioxide the remaining arsenic trioxide

was titrated with a 0.05 N iodine solution previously standardized against arsenic trioxide. The titration was stopped when

the yellow color of iodine appeared and the excess iodine

determined by photometry. For the preparation of the calibration curve see Appendix II. The weight of arsenic trioxide

was corrected for the arsenic trioxide equivalent of the iodine

solution used in the titration and the weight of iodine was corrected for the photometrically determined excess iodine.

Reagents

:

(a) Iodine - Resublimed iodine (ACS grade) was additionally resublimed thrice [9].

(b) Water - All the water used was freshly boiled and

cooled distilled water.

(c) Sodium bicarbonate solution (60 g/1) - Sixty grams

11
of NaHC0 3 were dissolved in 900 ml of water and diluted to one liter.

(d) Standard iodine solution (0.05 N) - To a liter volumetric flask 200 ml of water and 100 g of potassium iodide

were added. When the potassium iodide was dissolved 6.3 g of

iodine were added and after the iodine dissolved the solution

was diluted to the mark with water. Standardization of iodine

solution—10 g of sodium carbonate were transferred to a 600 ml

beaker and 25 ml of water were added. When the salt was dissolved, 100 ml of sodium bicarbonate solution were added and

the solution diluted to 200 ml with water. The solution was cooled in ice water and 25 ml of the standard arsenic trioxide

solution (e) and 10 g of potassium iodide were added. When

the potassium iodide was dissolved the solution was titrated

with the iodine solution to the yellow color of iodine. The

solution was diluted to 300 ml with water and the excess iodine determined photometrically.

AB Grams of As 2 0 3 equivalent to 1 ml of I 2 solution = ^—

where

A = ml of arsenic trioxide solution,

B = g of arsenic trioxide per ml arsenic trioxide solution,

C = ml iodine solution used, and

D = excess ml iodine solution determined photometrically.

(e) Standard arsenic trioxide solution (1 mg/ml) - One

gram of the arsenic trioxide SRM weighed to the nearest 0.00001

g was transferred to a 500 ml Erlenmeyer flask and 10 g of

sodium carbonate were added and the salts mixed. Twenty-five

ml of water were added in 5 ml portions and the solution was warmed to dissolve the salts. The solution was made just acid

by adding sulfuric acid (1+1) through a short stem funnel

inserted in the flask. The solution was transferred to a liter volumetric flask and diluted to the mark with water.

12
Procedure

:

A 1 g sample of arsenic trioxide accurately weighed to the nearest 0.00001 g was transferred to a 500 ml glass stoppered Erlenmeyer flask, 10 g of sodium carbonate were added

and the contents of the flask was thoroughly mixed. A short

stem funnel was inserted into the flask and 25 ml of water

were added in 5 ml portions. The flask was warmed to dissolve

the arsenic trioxide and sodium carbonate. When the salts were dissolved the solution was cooled in ice water, 100 ml of

the sodium bicarbonate solution were added and the solution

was thoroughly mixed. Ten grams of potassium iodide were added and the flask was swirled to dissolve the salt. The

funnel was rinsed and removed from the ice bath.

A small weighing bottle containing 2.5 g solid iodine

weighed to the nearest 0.00001 g was added to the solution

and the flask stoppered. When the iodine was dissolved, the excess arsenic trioxide was titrated with the standard iodine

solution to the yellow color of iodine.

The solution was diluted to 300 ml with water and the excess iodine was determined photometrically at 375 nm using

water as reference for zero absorbance.

Effective purity of As 2 0 3 j in percent = x 100 |

where

A = g of solid iodine added,

B = g of excess iodine used to detect the end point and

determined photometrically,

C = g of arsenic trioxide,

D = g of excess arsenic trioxide found by the iodine

titration,

E = equivalent weight AS2O3, and

P = equivalent weight of I 2 .

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The effective purity of SRM 83c is 0.005 percent lower

than the effective purity of SRM 33b when compared through

purified iodine (Table 3). Since SRM 83b was certified as 100.00 percent (99.996), the effective purity of SRM 83c

would be 99.99 percent (99-991).

2 . Comparison with SRM ' s 136b and 136c, Potassium

Dichromate

The provisional certification of the effective purity of

SRM 83c, Arsenic Trioxide, was based on a direct comparison

of the composite samples with SRM 136b, Potassium Dichromate.

Since the homogeneity of the arsenic trioxide was determined

by comparison with SRM 136b, a supplementary determination

was available for consideration in the certification of the

effective purity.

The arsenic trioxide was dissolved in a sodium carbonate

solution, acidified with sulfuric acid, and an excess of

potassium dichromate added. After the oxidation of the

arsenic trioxide, the excess potassium dichromate was titrated

potentiometrically with a standard ferrous ammonium sulfate

solution. The details of the method followed are given in

Section II.E.l.a.

The effective purity of SRM 83c was 99-995 percent with

a standard deviation of 0.007 percent when compared to SRM

136b (99-979$)- This value is the combined results of

Table 1, page 5 and the comparison to the composite sample of

SRM 83c, Arsenic Trioxide, Table 4.

15
Table 4. Effective purity of SRM 83c, Arsenic Trioxide,

based on composites compared with SRM 136b,

Potassium Dichromate.

g As 2 0 3 Effective purity

site No. per g K 2 Cr 2 0 7 {%)

I 1 .00853 100 . 000

II 1. 00863 99 -990 II 1 . 00802b 100 . 050

b

I

"] r\ p. Q n 1

99 .979

I 1 .00848 100 004

II 1.00860 99.993

II 1.00847 100 . 005

I 1.00849 100 .003 ge 1.00856 99.996

a 0. 00010 0 .010

Effective purity of SRM

= 99.979%.

Omitted from average

136b, Potassium Dichromate

SRM 83c was also compared to the composites of SRM 136c

for the certification of SRM 136c. Using the effective purity

value of 99.983 percent for SRM 136c (obtained by coulometry)

SRM 83c would have an effective purity of 99-986 percent with

a standard deviation of 0.006 percent. The results are given

in Table 5.

16
Table 5. Effective purity of SRM 83c, Arsenic Trioxide,

compared to SRM 136c, Potassium Dichromate

.

SRM 136c g K 2 Cr 2 0 7 a Effective purity

Composite No. per g As 2 0 3 {%)

I o .991294 99.979

II 0.991358 99.985

III 0.991259 99.975

IV 0 .991278 99-977

IV 0.991427 99.992

III 0 .991^12 99.990

II 0.991375 99.987

I 0 .991^^7 99.994

I 0 .991384 99.988

II 0.991397 99.989

III 0.991340 99.983

IV 0 .991424 99.992

Average 0.991366 99.986

a 0.000062 0 .006

a Effective purity of SRM 136c, Potassium Dichromate

= qq q

3. Coulometry

The basis for precise iodimetry was established by the

work of Edward Washburn who treated the subject in detail [12]

in 1908.

For coulometry it has been shown that for current densities

between 1.25 and 3 mA/cm 2

, iodine can be generated with 100

percent current efficiency to better than 1 part in 10 6 [17].

On the basis of these measurements the coulometric titration of SRM 83c with electrogenerated iodine was preformed

utilizing 0.1 M KI solutions and a current of 100 mA with an anode area of 40 cm 2

, corresponding to a current density of

2.5 mA/cm 2 - i.e., conditions under which the efficiency of

the generation of iodine should be 99.9999+ percent.

17
The supporting electrolyte, selected for these titrations,

was 0.05 M with respect to KH^POi*, 0.05 M with respect to Na 2 HP0if 3 and 0.1 M in KI

The end point indicator system was similar to the one used by Ramsey, Parrington and Swift [18]. It consisted of two 1 cm 2 platinum electrodes, arranged in horizontal planes,

facing each other and approximately 0.5 cm apart, with 150 mV

impressed between them. A recording polarograph was used as the source for applying the potential across the two indicator

electrodes as well as for recording the indicator current. Although at the end point, the generating current was passed increment-wise, recording of the indicator current

helped in deciding when equilibrium was reached.

The high accuracy timing and coulometric current measuring

circuit is shown in Figure 2. After an initial warm up period

of approximately 1 hour, its output seldom changed by more than

0.001 percent in the course of any one day. The accurate value

of the current was determined by comparison of the IR drop

60 VOLT

SOLID STATE REGULATED

POWER SUPPLY n

TO TIMER

L-..J SATURATED WESTON CELL Figure 2. High accuracy constant-current and timing circuit.

18
across a standard resistor with the voltage of a saturated

Weston cell. The error signal was displayed as the deflection

of a galvanometer. The comparison of these quantities was made

continually throughout the whole titration and the output of

the power supply was adjusted whenever the difference between

the voltage of the Weston cell and the IR drop across the

standard resistor differed by more than 0.0003 percent.

It was thought desirable to investigate experimentally

the changes which one might anticipate in arsenite-iodine

reaction stoichiometry as a function of pH. This point was treated by Washburn in detail [12]. In contrast to his work,

we felt that, for the purpose of titration, 5 minutes time

for equilibration of the system after addition of each increment of reagent was ample and any longer period of waiting

would be impractical.

The dependence of the assay of arsenic trioxide on pH is

shown in Figure 3. For the purpose of this comparison, the

+ 0.03r

+ 0.02 -

£ +0.01

Figure 3. Effect of pH on the stoichiometry of arseniteiodine reaction.

19
assay at pH 7 was defined as 100 percent. It can be seen that

between pH 6 and 8 the difference in the amount of required

Iodine to bring the system to the end point is on the order

of 0.004 percent. The error rapidly increases on the alkaline

side, and at pH 9 it is of the order of 0.01 percent. On the

acid side, on the other hand, below pH 6 the rate of the

reaction becomes extremely slow and with the 5-minute waiting

period no meaningful data could be obtained. Therefore it

must be concluded that for titrations where accuracy higher

than 0.01 percent is desired, pH must be maintained below a value of 9 and above a value of 6. An assessment of all known sources of error accounts for

approximately half of the experimental titration uncertainty

or approximately 15 ppm. It must be assumed that the other

half of the uncertainty results from indeterminate sources of error such as oxidation of trivalent arsenic by the residual

oxygen, loss in transfer, sample inhomogeneity , etc. Procedure

The arsenic trioxide samples were weighed into 125 ml

Erlenmeyer flasks, dissolved in 10 ml of 10% NaOH solution,

deaerated by purging with N 2 , delivered into the cell containing 100 ml of deaerated 0 . 1M KH 2 PO^-0.1M KI solution, and

titrated with iodine, elec trogenerated in situ at 101 mA. When the precalculated amount of iodine was generated (equal

to 99.9 percent of the theoretically required amount), the

passage of the coulometric current was stopped. The intermediate compartments of the coulometric cell were emptied and

rinsed back and forth by applying vacuum or nitrogen pressure

as required. Finally, only a minute amount of solution was

left in the intermediate compartments to enable the passage

of a small current. The pH of the solution was measured and

adjusted to 7.00 by the addition of deaerated NaOH or H 3 P0 4 as required. Then generation of iodine was resumed increment20
wise, using a current of 6.43 mA and indicator current readings

were taken after the passage of each increment of charge.

The indicator current remained constant and close to zero before the end point. After the end point, the indicator

cathode was depolarized by the excess iodine and the indicator

current was a linear function of the excess iodine with a slope of approximately 150 uA/yeq of iodine. The intersection

of this function with the residual current, recorded prior to delivery of arsenious acid, was taken as the equivalence point.

A set of results for titration of 0.5 gram samples of

SRM 83c, Arsenic Trioxide, is shown in Table 6. It can be seen that in these titrations, a precision of 32 ppm has been

obtained. On the basis of these titrations and after making

adjustments for changes in NBS volt and the value of the

faraday [19] 9 an assay value of 99.9873 percent is assigned

to SRM 83c, Arsenic Trioxide, with a standard deviation of

0.0032 percent.

Table 6. Effective purity of SRM 83c, Arsenic Trioxide,

by coulometry.

Titration No. Assay, %

3

4

5

6

12

Average

99.9859

99.9834

99 .9844

99.9896

99.9830

99-9904

99.986la

a 0.0032

a After applying the corrections for the changes

values of the ampere and the faraday [19]> the

assay becomes 99.9873 percent.

in the

average

21
4 . Comparison with Purified Iodine

SRM 83c, Arsenic Trioxide, was assayed by a direct titration with purified iodine. After the arsenic trioxide was dissolved in a sodium carbonate solution, a sodium bicarbonate

solution and potassium iodide were added. When the potassium

iodide dissolved, a weighed amount of iodine, less than the

calculated equivalent of the arsenic trioxide, in a glass

stoppered weighing bottle was added. After the iodine had

dissolved and reacted with the arsenic trioxide, the excess arsenic trioxide was titrated with a 0.05 N iodine solution

previously standardized with arsenic trioxide to the appearance

of the yellow color of iodine. The excess iodine was then

determined by photometry. The weight of arsenic trioxide

was corrected for the arsenic trioxide equivalent of the iodine

solution used in the titration and the weight of iodine was corrected for the photometrically determined excess iodine.

The details of the method are given in Section II.E.l.b.

The effective purity of SRM 83c based on titrations with

purified iodine is 99-9879 percent with a standard deviation

of 0.0085 percent. The results are given in Table 3 (page 14).

5 . Summary

In chemical characterization of materials, as in any

other area of metrology, one is compelled at some point to either make an absolute determination of the measured parameter

or define a "standard state", to which all other measurements

are compared.

To date two absolute methods are available for the

determination of trivalent arsenic content in arsenic trioxide:

titration with high purity iodine and coulometric titration

of arsenic (+3) with elec trogenerated iodine. These two methods can serve conveniently as the starting point for

correlation of the measurements of a chemical property to the

fundamental system of units. In the case of the former method

the measurements involve only the mass of iodine required to

22
react with a given mass of arsenic trioxide . The accurate

knowledge of the molecular weight of arsenic trioxide and

the atomic weight of iodine is mandatory in order to arrive

at an accurate value of the assay (where the assay represents

the molar ratio of moles of arsenic trioxide found to the

moles of arsenic trioxide calculated and expressed in

percent). Thus this method involves only accurate mass measurements and as such it is absolute.

Coulometry is also an absolute method. It entails the

measurement of charge needed to convert the constituent in

question from some initial state to a final state (i.e.

As +3 -> As +5

+ 2e ) . Three measurement parameters are involved

here; mass, electrical current and time.

Both methods were employed independently for the determination of the assay of SRM 83c, Arsenic Trioxide, and these

determinations consistitute the basis for the final certification of this material. The coulometric assay is 99.9873

percent (a=0.0032%) and the assay by comparison with iodine

is 99.9879 percent (a=0.0085$). The best value for the

effective purity of SRM 83c is 99.9876 percent. The results

are given in Table 7. The assay values obtained by other comparison which can

not be considered absolute are given in the lower part of the

table. Since SRM 83b was certified, the method for comparing

arsenic trioxide to iodine has been improved to give more precise and accurate results. SRM 136b, Potassium Dichromate,

was certified on the basis of comparisons to SRM 136 and SRM

136a. The comparison of SRM 136c, Potassium Dichromate, to SRM 83c was used for the certification of SRM 136c, therefore

this comparison was not used for the certification of SRM 83c.

A summary of the effective purity of SRM 83c, Arsenic Trioxide,

is given in Table 7.

23
Table 7- Effective purity of SRM 83c, Arsenic Trioxide.

Effective purity Standard deviation

(a)

For Certification

Coulometry

Purified Iodine

99.9873

99.9879

0.0032

0 .0085

Average 99.9876

Comparison with other SRM's

SRM 83b (thru K 2 Cr 2 0 7 ) 99-992 0.007

SRM 83b (thru I 2 ) 99.991 0.009

SRM 136b 99.995 0.007

SRM 136c 99.986 0.006

24
III. SRM 136c , POTASSIUM DICHROMATE

A . Introduction

The use of potassium dichromate as a titrant was first

proposed by Penny in 1850 [20] for the determination of iron

in ores. He standardized the potassium dichromate against

harpsichord wire and used potassium ferricyanide as an external

indicator. Sarver and Kolthoff [21] proposed the use of

dipheny laminesulfonic acid as an internal indicator. Since

the presence of tungstates does not interfere, this indicator

is generally used today. An advantage is that the barium and

sodium salt of the indicator are soluble in water.

The Bureau issued the first SRM 136, Potassium Dichromate,

in 1944 and has had three renewals since then. The present

potassium dichromate standard, SRM 136c, was issued in

March 1970.

The material for the potassium dichromate standards was purchased under special specifications to insure high homogeneity and purity so that the entire lot could be issued

under one certificate of analysis.

B. Sampling

Five hundred pounds of potassium dichromate in 20 fiberboard drums, lined with polyethylene bags, of 25 lbs each were received as the renewal for SRM 136b. After thorough mixing

by rolling, a 30 g sample was removed from each drum by taking

equal portions of material from the top, middle and bottom

while the drum was lying on its side. After the material was tested for homogeneity, four composite samples were prepared

by combining 10 g of material from the five representative

samples with the highest assay values, the five representative

samples with the lowest assay values, the five representative

samples with the high medium assay values and the five with

the low medium assay values. Composite I was composed of

drums 4, 5, 8, 9 and 18 (high); Composite II - drums 1, 2,

10, 11 and 17 (high medium); Composite III - drums 3, 6, 12, 25
14 and 15 (low medium); and Composite IV - drums 7, 13, 16,

19 and 20 (low)

C o Homogeneity

The representative samples of each jar were compared

through ferrous ammonium sulfate to determine if these lots were homogeneous and could be issued under one certificate

of analysis.

In the testing of SRM 136c each of the representative

samples was run. Since those run on the first day of testing

were higher than on the other days, they were repeated along

with two outliers. The standard deviation was 0.0046 percent

which indicates the potassium dichromate was homogeneous

within the limits specified. The results are given in Figure

4 and Table 8. In the determination of the effective purity

by coulometry each of the representative samples was run and

the homogeneity of the material was confirmed. The standard

deviation of the coulometric results was 0.0028 percent. For

the results see Figure 5 and Table 12 (page 47).

D . Impurities

The material for SRM 136c conforms with the ACS specifications for analytical reagent grade material [5]. Emission

spectrometry reported only traces of impurities. Spark source mass spectrometry reported vanadium present at 8 ppm in SRM

136c and detected no other impurity at a level greater than

1 ppm.Potassium dichromate is not hygroscopic but water can

be occluded in the crystals. SRM 136c was heated at 260 °C and the water occluded in the crystals was found to be 0.018

percent

E. Effective Purity of SRM 136c, Potassium Dichromate

The effective purity of SRM 136c was determined by com- paring it with SRM 136b, Potassium Dichromate, by comparing

it with SRM 83c, Arsenic Trioxide, and by coulometry.

26
100.000

99.970

1 1 I 1 I

o

o o

• ~

i 0 1

'

• ° » o

'

° •

o —i

° o o

-

o o —

1 1 99.917 99.962

1

1

1

1

1 4 8 12 16 20 Drum Numbei o initial measurements # repeat measurements additional measurements

Figure 4. Homogeneity of SRM 136c, Potassium Dichromate,

compared with SRM 136b, Potassium Dichromate.

27
Table 8. Homogeneity and effective purity of SRM 136c, Potassium

Dichromate, compared with SRM 136b, Potassium

Dichromate

SRM 136b d

g Fe(NH lt ) 2 (S0 It ) 2 '6H 2 0 Date per g K 2 Cr 2 0 7 9/15/69 8.00960

8. 00969

8.00928

9/17/69

9/19/69

10/13/69

10/16/69

8.00896

8.00906

8.00844

8.00858

8.00892

8.00877

8.OO878

8.00883

8.00895

8.00868

Drum

No.

1

35

7

9 11

13

1517

19

2

4

6

8 101214

1618

20

c

4

c 5^

8

c 17°

Average 8.OO896

a 0.00037(0.0046$)

17

l

11

SRM 136c

per g K 2 Cr 2 0 7 purity (%}

P.0 . nU _LU 1 n c.or\) 99. 995

8.01003 99. 992

8.01056 99 999

8.01002 99 992

8.01048 99 998

8.01013 99 994

8 .00991 99 991

8.01004 99 992

ftO . nrwRii^ U U / O H 99

^ b .965°

8 .00935 99 .984

8 .00962 99 .987

8 .00992 99 .991

8.00951h 99 .986b 8.00423 99 .920

8.00964 99 .987

8.00948 99 n Or .905

R DDQ1

U • U u y _L (J 99 .982

8.00909 99 .981

8.00944 99 .985

8.00910 99 . 9 ol

8.00956 99 .986

8.00924 99 .982

8 .00962 99 .987

8 . 00941 99 .985

8.00951 99 .986

8.00965 99 .988

Q nnnli7 0 . U U y h I Q Q

y y . y CO 8.00961 99 .987

8.00983 99 .990 8.00998 99 .992

8 .00985 99 .990

8.00983 99 .990

8.00970 99 .988

8.00971 99 .988

0 .00037 0 .0046

Effective purity of SRM 136b, Potassium Dichromate 99.979$

Omitted from average

Repeat analysis 0 28
99.

99.986

^ 99.984

99.982 -

99.980 -

99.978

12

Drum Number

16 20

Figure 5. Coulometric determination of the homogeneity of

SRM 136c, Potassium Dichromate.

29
1 . Comparison with SRM 136b, Potassium Dichromate

SRM 136c was compared with SRM 136b through ferrous ammo- nium sulfate hexahydrate and through SRM 83c, Arsenic Trioxide

a. Comparison through Ferrous Ammonium Sulfate - The end point in the titration of chromate ion with ferrous

ion was determined potentiometrically with a direct current

voltmeter of the Garman and Droz design [22] using a platinum

electrode and a calomel electrode.

Several lots of ferrous ammonium sulfate hexahydrate had

to be tested to obtain satisfactory material. In fact, with

all lots tested the variation of the results was larger than

the acceptable precision for the assay of the potassium dichro

mate standards. One lot of the material was crushed in an agate mortar to an approximate fineness of 100 mesh and, when

tested, the variation was reduced so that the material met

the desired homogeneity. Enough ferrous ammonium sulfate

hexahydrate was crushed and thoroughly mixed at one time to complete all anticipated titrations.

Method:

In the intercomparison of the potassium dichromate standards, the potassium dichromate was dissolved in sulfuric

acid (1+19) and solid ferrous ammonium sulfate hexahydrate

(less than the calculated amount required) was added. The excess potassium dichromate was titrated potentiometrically

with a dilute ferrous ammonium sulfate solution.

Reagents

(a) Sulfuric acid solution (1+19) - One hundred twentyfive ml of sulfuric acid were added to 2375 ml of distilled

water in a 3 liter Florence flask. The solution was boiled

for 1/2 hour and cooled to room temperature . The flask was covered with a plastic cover.

(b) Ferrous ammonium sulfate solution (8 g/1) - Eight

grams of ferrous ammonium sulfate hexahydrate were transferred

to a 1 liter volumetric flask and 200 ml of sulfuric acid

30
(1+19) were added. When the salt was dissolved, the solution

was diluted to the mark with sulfuric acid (1+19) and mixed

thoroughly. Standardization—0.050 g of potassium dichromate

was transferred to a 600 ml beaker and 300 ml of sulfuric

acid (1+19) were added. After the salt was dissolved, the

solution was titrated potentiometrically with the ferrous

ammonium sulfate solution. Grams of potassium dichromate

equivalent to 1 ml of the ferrous ammonium sulfate solution

was calculated.

Procedure

:

A 0.98 g sample of potassium dichromate weighed to the

nearest 0.00001 g was transferred to a 600 ml beaker and 300

ml of sulfuric acid (1+19) were added. The solution was cooled in ice water and when the potassium dichromate was dissolved, 7.8 g of solid ferrous ammonium sulfate hexahydrate

were added. After the salt was dissolved the excess potassium

dichromate was titrated potentiometrically with the ferrous

ammonium sulfate solution.

All weights were corrected to vacuum. Grams Fe (NH^

)

2 (SCU

)

2

• 6H 2 0 equivalent to 1 g K 2 Cr 2 0 7 = ^"4^ D— OD where

A = g of ferrous ammonium sulfate hexahydrate added,

B = g of potassium dichromate taken,

C = ml ferrous ammonium sulfate solution used, and

D = g potassium dichromate equivalent to 1 ml of ferrous

ammonium sulfate solution.

SRM 136c was found to have an effective purity of 99.990

percent with a standard deviation of 0.0034 percent when the

composite samples were compared to SRM 136b through ferrous

ammonium sulfate hexahydrate and 99-988 percent with a standard deviation of 0.0046 when the homogeneity testing was compared to SRM 136b. The results of the comparisons are given in Table 8 page 28 and Table 9. 31
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32
b . Comparison through SRM 83c, Arsenic Trioxide - SRM 136c was also compared to SRM 136b through SRM 83c,

Arsenic Trioxide. The arsenic trioxide was dissolved in a sodium carbonate solution and acidified with sulfuric acid. An excess of the potassium dichromate was added and after the

chromium equivalent to arsenic trioxide was reduced the excess potassium dichromate was titrated with a dilute ferrous ammo- nium sulfate solution. Details of the method are given in

Section II.E.l.a.

A EF Effective purity of K 2 Cr 2 0 7j in percent = B_ CD

x —

where

A = g of arsenic trioxide taken,

B = g of potassium dichromate added,

C = ml of standard ferrous ammonium sulfate solution

used,

D = g of potassium dichromate equivalent to 1 ml of

ferrous ammonium sulfate solution,

E = effective purity of arsenic trioxide in percent,

F = equivalent weight of pocassium dichromate, and

G = equivalent weight of arsenic trioxide

.

The effective purity of SRM 136c is 0.012 percent more

than the effective purity of SRM 136b when compared through

SRM 83c, Arsenic Trioxide, therefore SRM 136c has an effective

purity of 99-991 percent when compared with SRM 136b. The

results are given in Table 10.

2 . Comparison with SRM 83c, Arsenic Trioxide

The effective purity of SRM 136c was determined by a com- parison with SRM 83c, Arsenic Trioxide. The arsenic trioxide

was dissolved in a sodium carbonate solution and acidified with

sulfuric acid. Potassium dichromate (more than the calculated

amount required) was added and after the reaction with the

arsenic trioxide was complete the excess potassium dichromate

was titrated with a dilute ferrous ammonium sulfate solution.

Details of the method are given in Section II.E.l.a.

33
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34
Only the composite samples of SRM 136c were compared to SRM 83c. The effective purity of SRM 136c was found to be

J, 99.985 percent with a standard deviation of O.OO63 percent.

SRM 136b was also compared to SRM 83c at the same time as SRM

136c. The effective purity of SRM 136b was found to be 99-973

j percent with a standard deviation of O.OO69 percent. These

I results are also given in Table 10.

3 • Coulometry

The effective purities of SRM 136b and SRM 136c were determined coulometrically . SRM 136b was assayed in connection

with the evaluation and development of a highly precise and

accurate analytical method for a typical oxidation-reduction

titration by coulometry [23]. SRM 136c was assayed coulometrically for certification of effective purity.

Coulometric methods are absolute methods in that the

results depend directly upon the reliability of measured

physical quantities. This is in contrast to many analytical

methods which are comparative in principle, in that an unknown

is compared with a material of known composition.

The application of constant-current coulometric titrations

to the analysis of dichromate is not novel [24,25,26], but

highly precise and accurate results have not been reported.

The method developed consists of the reduction of dichromate

with electrogenerated ferrous ion. The technique and procedure were refined to permit results reliable to a few parts

in 100,000, which are about two orders of magnitude better

than those reported by previous investigators.

The conditions for 100-percent efficient generation of

ferrous ion at a platinum cathode are readily obtained from

current density versus potential studies of the supporting

electrolyte alone and with several additions of ferric ion.

This technique has been employed by Lingane and co-workers for

the evaluation of the current efficiency of generation of

eerie ion [27,28].

35
Prom this study, solutions more concentrated than 0.01 M with respect to ferric ion in a supporting electrolyte of 2 M sulfuric acid can be reduced with a current efficiency of 100

percent, provided the current density is less than 1.2 mA/cm 2

. It is also indicated that the presence of phosphoric acid

is not essential for theoretically efficient reduction of

ferric ion to ferrous ion, as has been reported by previous

investigation [24]. In fact, it is evident that the presence

of phosphoric acid lowers the limiting current-density for

efficient reduction of ferric ion. In this work ferrous ion was pregenerated from a solution

0.4M with respect to ferric ion in 2 M sulfuric acid supporting

electrolyte at current densities never exceeding 0.6 mA/cm 2

. The pregeneration technique, described later, reduced the

ferric ion concentration to 0.3 M but this was well above the

limit for efficient generation of ferrous ion.

The end points of these titrations were determined

amperometrically using a polarized platinum indicator anode

and a saturated calomel electrode as the nonpolarizab le reference cathode. Using this electrode system the current-voltage

curves shown in Figure 6 were obtained. An applied-potential

of +0.85 v is midway on the diffusion-current plateau for the

reaction Fe

++

-> Fe

+++ +e~ . In actuality, this plateau is not completely flat; however, this voltage corresponds to a region

where the indicator current is the least dependent on the

applied voltage.

From studies of Anson [29], it is evident that the oxide

film which is formed on platinum affects the behavior of the

ferrous-ferric couple at the electrode. According to him, the

reduction of such a film is easily accomplished by ferrous ion

in a sulfuric acid medium. If such a film were not removed

prior to generation of ferrous ion, it would cause positive

errors in the final analysis of oxidant, dichromate in the

present case. To avoid such interference, precautions were

36
+0.6 + 0.7 + 1.1 E VS SCE, V Figure 6. Current, potential curves for oxidation of ferrous

ion at a polarized platinum electrode.

+++

1.

2.3.

4.

2 M H 2 SCU

2 M H 2 SCH

5 M Pe

5 M Fe +++

2 M H 2 SCH; 0.5 M Pe

2 M H 2 S0i 0.5 M Pe

40 yeq K 2 Cr 2 0 7 40 yeq Cr+++

; 40 yeq Cr

40 yeq Cr

0.5 yeq Fe

10

20 yeq Fe yeq Fe

++

taken. The electrodes (both the indicator anode and the

generator cathode) were pretreated by immersion for 10 minutes

in a chromic acid solution with subsequent rinsing, followed

by a 10 minute immersion in 0.1 M ferrous ammonium sulfate

solution in 2 M H2SCU. Thus the electrode surfaces were

freshly "reactivated" prior to each titration.

37
All calculations were based on the 1967 12 C atomic weight

scale [30]. Prom it the molecular weight of potassium

dichromate was calculated to be 294.1918.

The value of the faraday constant which was used in the

calculations was 96,487.2 coul/g-equiv (based on the determination of D. N. Craig et al . [31] and corrected to the 1 2 C atomic weight scale).

Apparatus

The titration cell used in this work was similar to the one described by Marinenko and Taylor [32]. The anode and

cathode compartments are separated by a connecting tube

containing a series of porous glass frits to provide two intermediate sections. Tubes sealed to the tops of these

sections permit application of suction or pressure to fill or empty the compartments. A silicic acid gel plug, prepared by

gelation of sodium silicate solution with sulfuric acid, was formed on the fine-porosity disk of the anode compartment to eliminate flow of electrolyte. The plug remained for long

periods of time without deterioration provided it was kept wet with supporting electrolyte.

A piece of corrugated platinum foil (5 x l6 cm) served as the generator cathode. The anode was constructed from sheet

lead (12x10x0.2 cm) which was contoured to fit the anode

compartment of the cell.

The electric circuit was the same as the one used by

Marinenko and Taylor [32]. The current was adjusted manually

to maintain the IR drop across a standard resistor constant

and equal to the voltage of a saturated standard cell. Because

the dummy resistance only approximated that of the electrolytic

cell, a momentary initial imbalance was invariably observed.

Such imbalances were small and corrected quickly so that they

could not affect the titrations by more than 0.00001 percent.

The saturated standard cell and the standard resistor

used in this work were calibrated by the appropriate sections

38
of the NBS. Although the resistor was placed in a thermostated

oil-bath, there was always a slight temperature rise of the

resistor due to heat dissipation. The correction for this

temperature change was applied using the mean temperature

value of the resistor for a given titration and the appropriate

temperature coefficients.

The time was measured by means of a 100 kHz-quartz crystal

controlled time interval meter (TIM) whose calibration was checked with respect to standard frequency signals of WWV:

the calibration was found to be accurate to at least 1 ppm.

Near the equivalence point of the titration, it was more convenient to pass current increment-wise using a commerciallyavailable constant-current coulometric source. The current

and timing accuracy of this apparatus were calibrated and the

current times time (Ixt) product was found to be accurate to ±0.005 percent. Since the fraction of the total titration

performed with this instrument was of the order of 0.0005, the errors encountered as a result of the use of this equipment

were negligible.

The amperometric indicator system consisted of a platinum foil electrode (1 cm 2 in area) and a saturated calomel

electrode with a 3-percent agar-agar gel in 0.1 N potassium

chloride in a salt bridge. A recording polarograph was used

as the source of applied emf (0.85 V) between these two electrodes and also as the microammeter for measuring or recording the indicator current.

All weighings were done on a 20 g capacity microbalance

of the single-arm, constant-load type and were accurate to ±0.003 mg. All weights were corrected for air buoyancy.

Procedure

:

The potassium dichromate standards were dried at 110 °C

for 6 hours and stored in a desiccator over P 2 0 5 . One-half gram of SRM 136b ( 1 g of SRM 136c) contained in

a platinum boat was weighed by the method of substitution

39
using a calibrated tantalum weight. Sample weights agreed

with the nominal value and with each other within 3 mg. The

difference between the mass of the sample and the calibrated

mass was determined on the optical scale of the balance.

The weighed samples were transferred from the boat into 30 -ml

weighing bottles and stored in a desiccator until used.

The weighings were done in sets, each representing a number of samples weighed on the same day. Measurements were

made of air temperature, barometric pressure, and relative

humidity during the weighings and the appropriate corrections

for air bouyancy were applied.

The supporting electrolyte was prepared in advance and

was 0.4M with respect to ferric ion and 2 M with respect to sulfuric acid. Ferric ammonium sulfate, reagent grade salt,

or electrolytically oxidized ferrous ammonium sulfate served

as the source of the ferric ion.

The anode and cathode compartments were filled with 100

ml of 2 M sulfuric acid and 120 ml of supporting electrolyte,

respectively. At this point about 1 ml of 0.005 N potassium

dichromate was delivered into the cathode compartment to remove traces of ferrous ion and to facilitate the pretitration step described later.

Dissolved air was removed from the catholyte by purging

with nitrogen gas that was introduced through the coarse frit

of the side compartment adjacent to the cathode compartment.

The nitrogen used for this purpose was pretreated by passing

it through a heated copper-filled tube to remove oxygen and

through towers containing acidified potassium permanganate, ferrous ammonium sulfate and supporting electrolyte respectively.

After purging, the catholyte was permitted to flow into

the intermediate compartments to the extent that it would just cover the bottom of each compartment, thus establishing electrolytic contact between the cathode and anode compartments.

This was accomplished by application of suction to each

intermediate compartment.

40
The catholyte was then pretitrated by passage of increments

of current equivalent to 0.200 yeq, using the 0.643 ma current

range of the coulometric power supply. At the conclusion of

each increment, the indicator current was observed. This was

small and essentially constant up to the equivalence point,

exhibited a curvature in the vicinity of the equivalence point,

and became a linear function of the ferrous ion concentration

with slope of about 5-0 ya/yeq beyond the equivalence point.

This linear portion was extrapolated graphically and its

intersection with the zero-current line was taken as the end

point

.

After completion of the pretitration , the intermediate

compartments were rinsed by repeated emptying and filling with

catholyte by applying suction or nitrogen pressure as required.

The walls of the cathode compartment and its cover were rinsed

by withdrawing catholyte using a syringe in which a glass tip

replaced the hypodermic needle. The tip was bent so that the

expelled catholyte would wash down all parts of the cathode

chamber. The final reading of the indicator current was then

taken from which the amount of over-titration of the pretitration step was determined.

The intermediate compartments of the cell were then filled

with catholyte and ferrous ion was elec trogenerated at constant

current using the high precision circuit and the TIM. The

amount of ferrous ion generated was a predetermined quantity

sufficient to reduce about 99.95 percent of the dichromate in

a given sample.

After the generation of this major portion of ferrous

ion, a dry funnel was inserted into the cover of the cathode

compartment through an auxiliary opening which was stoppered

with a piece of glass rod during the time of generation of

ferrous ion. The stem of this funnel reached slightly below

the level of the liquid In the cell. The weighing bottle

containing the weighed sample was inverted into the funnel

41
and lightly tapped, so that all visible crystals of the dichromate were delivered into the cell. A complete dissolution of

sample took place in about 5 min after delivery into catholyte.

Following dissolution of sample the intermediate compartments

were successively emptied by applying nitrogen pressure until

only the bottoms of these compartments were wetted enough to make the electrical contact between the anolyte and the

catholyte. The compartment which is adjacent to the cathode

compartment was rinsed several more times, by permitting the

catholyte to flow into it, and by forcing the same out with

nitrogen pressure. This was done because a perceptible amount

of ferrous ion diffused into that compartment during the course

of its generation. Thus, it required more than one rinse to remove all ferrous ion which was entrapped in the glass frits.

The second compartment contained an imperceptible amount of

ferrous ion. At this point the titration was continued increment-wise,

using the coulometric current source as in the case of pretitration. When the first rise of the indicator current was observed, the cell walls, the cell cover, and the intermediate

compartments were rinsed as already described, and the weighing

bottle which originally contained the sample and the delivery

funnel were rinsed with catholyte withdrawn from the cell by

the syringe. This procedure was repeated after each succeeding

increment. The extrapolation of the indicator current line,

where it becomes a straight line function of the ferrous ion

concentration was again taken as the end point at the point

where it intersects the zero current line. A typical end-point

determination is shown in Figure 7.

42
Figure 7. Typical end-point determination.

Discussion

:

The factors affecting the precision and accuracy of

coulometric titrations have been discussed in previous papers

[32,33]. In the present work, much effort was expended to minimize all errors to produce results of high reliability.

An attempt has been made to evaluate the precision of all

measurements involved. These are summarized in the following

43
Weighings were precise to about 3 Ug 3 which amounts to 0.0006 percent in the case of 0.5 g samples and uncertainties

in the buoyancy corrections did not exceed 0.0003 percent.

Manual control of the current was maintained within a 0.5 cm galvanometer deflection which would amount to 0.00015 percent.

Timing errors are not believed to exceed 0.0001 percent for

the 10 ,000-second period of electrolysis. Uncertainties in

the current due to temperature fluctuation of the standard

resistor were no more than 0.00007 percent, while endpoint

uncertainties may have amounted to about 0.02 yeq which represents 0.0002 percent in a 10-meq sample. On the basis of

these estimates an overall precision of 0.0008 percent should

have been attained, which is about one-third of the standard

deviation of the results.

There are of course other sources of error, the magnitudes

of which are difficult to estimate. Such indeterminate errors include mechanical losses due to transfer of sample, spray

losses, and residual amounts of oxygen in the cell. Efforts

were made to minimize some of the indeterminate errors by

the washing procedure already described and by pretreatment

of the electrolyte.

The stability of the generating current was greatly improved by the pregeneration technique, and this was the reason

for its adoption. In preliminary work, it was found that the

current became quite unsteady when about 30 percent of a 10 meq sample of dichromate which had been added directly to the

supporting electrolyte was reduced. Potential measurements

showed that the cathode potential shifted abruptly at this

point from that of the dichromate-chromic couple to that of

the ferric- ferrous couple. Undoubtedly, due to concentration

polarization effects, the primary electrode reaction changed

from the reduction of dichromate to reduction of ferric ion,

well in advance of the stoichiometric end point. The transition was too rapid to compensate by manual means and conse- quently large uncertainties were introduced. The pregeneration

44
of the ferrous ion as already described eliminates this

uncertainty

.

Another improvement in precision of the endpoint determination resulted from the pretitration technique. Some

uncertainty was introduced by small but measurable variations

in the current prior to the complete reduction of dichromate

(residual current). However, by alternate reduction and

oxidation in the region of the end point, it became evident

that extrapolations of the linear residual-current curve to zero current gave reliable values for a sample added to one previously titrated, and the pretitration technique described

earlier was adopted. Also, it was hoped that this procedure

would eliminate blank corrections, but it was observed that

a pretreatment of the supporting electrolyte was required

as well.

It has also been shown conclusively that a pretreatment

technique Is required for the supporting electrolyte if the

results of highest obtainable degree of accuracy and precision

are desired.

The results for the determination of the effective purity

of SRM 136b are given In Table 11 and for SRM 136c in Table

12. The average value for the effective purity of SRM 136b

is 99.9772 percent with a standard deviation for all measurements of 0.0029 percent. After correction of the assay value

for changes in the value of NBS ampere and the faraday [19]

the assay of SRM 136b is 99-9786. The average value for SRM

136c is 99.9822 with a standard deviation of 0.0028 percent.

No correction is needed for SRM 136c because the assay was determined after the value of NBS ampere and the faraday had

been changed.

4 . Summary

SRM 136c, Potassium Dichromate, will be issued with a final "Certificate of Analysis" with an effective purity of

99.983 percent. This value is based on the direct coulometric

method and on the comparison with SRM 83c, Arsenic Trioxide.

45
Table 11. Effective purity of SRM 136b, Potassium Dichromate,

by coulometry

Sample Set Set

No. I II

1 99-9800 99.9755

2 99.9788 99.9736

3 99.9751

4 99.9757 99.9739

5 99.9789 99.9787

6 99.9812 99.9783

Average 99.9783 99.9760

a 0.0024 0.0024

Set Set Set

III IV V

99.9742 99.9835 99-9785

99.9734 99.9737 99.9777

99.9735 99.9772

99.9747 99.9771

99.9778 99.9762

99.9747 99.9786 99.9773

0.0018 0.0049 0.0009

Sample Set Set

No. VI VII

1 99.9769 99.9846

2 99.9766 99.9820

3 99.9775 99.9685

4 99.9784 99.9752

5 99.9787

6 Average 99.9776 99.9776

a 0.0009 0.0060

Average all values

Set Set Set VIII IX X 99.9789

99.9771 99.9783 99.9760

99.9764 99.9769

99.9785 99.9787

99.9796

99.9769

99.9781 99.9780 99.9765

0.0013 0.0010 0.0005

99 . 9772 a

a 0.0029

a After applying the correction for the change in the values

of NBS ampere and the faraday [19] the average assay value

becomes 99.9786 percent.

46
Table 12. Effective purity of SRM 136c,

Dichromate , by coulometry.

Potassium

Titration

Sequence Drum No. Assay

1 3 99.9849

2 19 99.9821

3

8 99 .9840

4 10 99.9847

5 1 99.9797

6 11 99.9845

7 2 99.9790

8 17 99.9801

9 5 99.9825

10 20 99.9864

11 14 99.9874

12 15 99.9792

13 12 99.9786

14 6 99.9794

15 9 99.9794

16 18 99.9825

17 4 99 .9804

n P 10 ID O Q Q P £ "3 19 7 99 .9796

20 13 99.9833

Average 99 .9822

a 0 .0028

47
The coulometric assay is an absolute method and depends only

on the accuracy of the measurements of mass, electrical current

and time. SRM 83c was assayed by the absolute methods,

coulometry and comparison with iodine, therefore the effective

purity is accuratey known. The certified value is also compatible with the previous potassium dichromate standard, SRM 136b.

The summary is given in Table 13. The lower part of the table

gives the assay of SRM 136c when compared to SRM 136b.

Table 13. Effective purity of SRM 136c, Potassium Dichromate.

Effective

purity

(*)

For Certification

SRM 83c, As 2 0 3 (composite) 99-985

By coulometry 99.982

Average 99.983

Other Comparisons

SRM 136b (thru Fe

++ -composite ) 99-990

SRM 136b (thru Fe

++ -homogeneity) 99.988

SRM 136b (thru As 2 0 3 ) 99-991

In studies of SRM 136 [34] and SRM 136b [35], it was shown that the removal of occluded water by appropriate heat

treatment increases the assay of the standard to 100.000 ±0.005

percent. In one study [34], the occluded water was determined

and in the other the standard was assayed after the removal

of the occluded water. From our limited determination of

water and the work of other investigators [34,35], SRM 136c

would have an effective purity of 100.000 percent with a standard deviation of 0.006 percent if it were heated to remove occluded water.

Standard

deviation

(a)

0.006

0.0028

0.003

0.005

48
IV. SRM 40h. SODIUM OXALATE

A . Introduction

The use of sodium oxalate as a primary standard was suggested by Sorensen [36] who described its preparation, testing

and use in subsequent papers [37]. The National Bureau of

Standards issued the first Standard Reference Material, Sodium

Oxalate, as an oxidimetric standard in 1912. Since then, 8 renewals have been issued with the SRM 40h, Sodium Oxalate,

as the present Standard Rreference Material.

The sodium oxalate was purchased under special specifications to insure high homogeneity and purity so that the

entire lot could be issued under one certificate of analysis.

B. Sampling

Twenty-eight fiberboard drums containing 25 lbs each of

sodium oxalate were received to be used as the renewal for

SRM 40g. Each drum was lined with a polyethylene bag. After

thoroughly mixing by rolling each drum, a 30 g sample was removed by taking equal portions of material from the top,

middle and bottom while the drum was lying on its side. Four

composite samples were prepared from the above representative

samples as follows:

Composite I - drums 1, 5, 9, 13, 17, 21 and 25; Composite II - drums 2, 6, 10, 14, 18, 22 and 26; Composite III - drums 3, 7, 11, 15, 19, 23 and 27; Composite IV - drums 4, 8, 12, 16, 20, 24 and 28.

C . Homogeneity

The individual samples were titrated with a 0.1 N potassium permanganate solution to determine whether the material

was homogeneous and could be issued under one certificate of

analysis

.

The results of the titrations of the 28 representative

samples of SRM 40h are given in Table 14 and Figures 8 and 9. With the exceptions of Nos . 9 and 23 of the 0.1 N potassium

49
Table 14. Homogeneity of SRM 40h, Sodium Oxalate, and

comparison with SRM 40g } Sodium Oxalate.

SRM 40) SRM 4 Oh

I KMnOi, per

g Y\Q, 2 C z Q h 128.5644

128.533S)

127.9741

128.5897

128.5627

128.5339

128. 4785

128.5703

128.5740

128.5660

128.5372

KMnO i,. Solution No. 3 KMnO h Solution No. 4 Drum g KMnO 4. per

No. g NaaCaO^

1 153.9448

2 153.9608

3 153.9650

4 153.9485

5 153.9711

6 153.9425

7 153.9582

8 153.9568

9 153.9583

10 153.9718

11 153.9705

12 153.9748

13 154.0024

14 153.9513

15 153.9919

16 153.9498

17 154.0072

18 153.9909

19 154.0012

20 154.0056

21 154.0116

22 153.9811

23 153.9837

24 153.9893

25 153.9817

26 153.9745

27 153.9996

28, 153.9672

15" 153.9829

13 153.9788

Ave. 128.5511

o 0.0316(0.026$)

Drum g AJYinUit per

ViO . 1

1 O ft C ft Jl £ 1^ 0 0 0 40

d 12o .0202

00 120 . 5773 0 9 120 . ( 4o7

15 "l 0 Q r On/" 12o . 5o0o

lo 12o .5613

dd 12o.5435Q 2 3 127. 9985

3 128 .5619

12o .5917

1U 128 . 5567

11 12o . 5573

17 12o .5349

11 ft0 120.5201

dohH 120 . P^W 5 do 12 0 .5^91

c

D 12o . pl2o

u l 0 ft ii n 0 0 I*! 0 . d

±dl 0 I2O.5236

1 ft 1^0 1 0 ft £ n o c . oudo

1 Q 1^0 10ft £ n 1 Ji

. 01)^4 20 l£19ft0 . on 0 1 j y 26 128.5183

7 128.6384

14 1 p ft fil iiq 21 128.6034

27 128.5948

2 128.6351

% 128.5834

128.6179

128.5947

128.5736

0.0390(0.030$)

153.9758

0.0199(0.013$)

Omitted from average.

Repeat runs

50
128.650 -

= 128.600 -

128.550 -

128.500 -

I I I

I I I L

4 8 12 16 20 24 28 Drum Number o initial measurements • additional measurements

Figure 8. Homogeneity of SRM 40h, Sodium Oxalate, using

KMnOit solution no. 3.

51
154.025

154.000 -

153.975

153.950 -

153.925 -

_ JLoS- — — — — — ___°J.

I I I I I l_ L

4 8 12 16 20 24 28 Drum Number o initial measurements • additional measurements

Figure 9. Homogeneity of SRM 40h, Sodium Oxalate, using

KMnOit solution no. 4.

52
permanganate solution No. 3 all results fell within the limits

of acceptance for this lot of material. On repeat runs these

outliers fell within this limit.

D . Impurities

SRM 40h, Sodium Oxalate, conforms to the American Chemical

Society specifications for reagent-grade material [5] but small

amounts of water, sodium hydrogen oxalate and sodium bicarbonate are present.

E. Effective Purity of SRM 40h, Sodium Oxalate

The effective purity of SRM 40h for the provisional certification was determined by comparison with SRM 40g, Sodium

Oxalate, and by determining the impurities most likely to effect the neutrality of the sodium oxalate. SRM 40h has also

been compared to SRM 83c, Arsenic Trioxide, using a 0.1 N potassium permanganate solution to titrate both standards.

SRM 40h was also compared to SRM's 136b and 136c, Potassium

Dichromate. Since sodium oxalate and potassium dichromate do not react efficiently with one another, the sodium oxalate

and ferrous ammonium sulfate hexahydrate were titrated with

a 0.1 N potassium permanganate solution and then the ferrous

ammonium sulfate compared with SRM's 136b and 136c.

1 . Comparison with SRM 40g, Sodium Oxalate

The provisional Certificate of Analysis for SRM 40h,

Sodium Oxalate, was issued on the basis of the determination

of the impurities and also comparison with SRM 40g by titrating

with 0.1 N potassium permanganate solutions. The procedure

used in these titrations was generally that recommended by

R. M. Fowler and H. A. Bright [38].

Method

The sodium oxalate was dissolved in sulfuric acid (I+19)

and most of the potassium permanganate solution was added.

The solution was heated to 55 - 60 °C and the titration

completed. The excess potassium permanganate solution was determined with a sodium thiosulfate solution.

53
Reagents

(a) Potassium Permanganate Solution (0.1 N) - Twenty-five

and six tenths grams of recrystallized potassium permanganate

were transferred to a 10 liter glass bottle and 8 liters of

water were added. When the salt was dissolved and the solution

thoroughly mixed it was allowed to stand in the dark for 2 weeks. The solution was filtered through a fine porosity glass

filter to remove the precipitated manganese dioxide and was stored in the dark.

(b) Starch Solution - Two and one half grams of soluble

starch were triturated with 5 ml of water to a thin paste which

was then poured into 250 ml of boiling water and boiled for

10 minutes. The solution was cooled, diluted to 500 ml with

water and stored in the refrigerator in a polyethylene bottle.

(c) Sodium Thiosulfate Solution (0.02 N) - Five grams of

sodium thiosulfate pentahydrate was dissolved in 1 liter of

water, freshly boiled and cooled. Ratio of the sodium thiosulfate solution to the potassium permanganate solution—One

gram of the 0.1 N potassium permanganate solution was transferred to a 600 ml beaker containing 300 ml of sulfuric acid

(1+19) freshly boiled and cooled. A 0.5 g portion of potassium

iodide was added and the liberated iodine was titrated with

the sodium thiosulfate solution. Two ml of the starch solution

were added just before the yellow iodine color disappeared and

the titration was continued to the starch-iodide end point.

Procedure

A 0.3 g sample of sodium oxalate, dried at 110 °C and

weighed to the nearest 0.00001 g, was transferred to a 500 ml

glass stoppered Erlenmeyer flask and 250 ml of sulfuric acid

(1+19), freshly boiled and cooled, were added. When the salt was dissolved, approximately 40 ml of 0.1 N potassium permanganate were added. The solution was stirred slowly until the

color disappeared. The solution was heated to 55-60 °C and the

titration completed to the pink color of potassium permanganate.

54
To determine the excess potassium permanganate, the

solution was cooled to room temperature and 0.5 g of potassium

iodide was added. The liberated iodine was titrated with the

0.02 N sodium thiosulfate solution. Two ml of the starch

solution were added just before the yellow iodine color disappeared and the titration was continued to the starch-iodide

end point [39]

.

Weight burets were used for the potassium permanganate

solution and all weights were corrected to vacuum. Grams of KMnO i+ solution equivalent to 1 g Na 2 Cz0k = b-CD

where

A = g of sodium oxalate taken,

B = g of potassium permanganate solution used,

C = ml sodium thiosulfate solution used to determine

excess KMnO 4, and

D = g of potassium permanganate solution equivalent to

1 ml of sodium thiosulfate solution.

The effective purity of SRM 40h is 0.02 percent more than

SRM 40g or 99.97 percent when the homogeneity values are used

and 0.03 percent more than SRM 40g or 99-98 percent when the

composite values are used. The comparison from the homogeneity

is considered to be the better value because a larger number

of samples were run, spread over several days. The comparison

of the composites to SRM 40g was performed in a single day.

The results are given in Table Ik page 50 and Table 15.

2 . Determination of Impurities

The impurities most likely present to effect the neutrality of sodium oxalate are sodium carbonate, sodium bicarbonate,

sodium hydrogen oxalate and gaseous carbon dioxide. A spectrograph! c analysis of the sodium oxalate showed that metal

impurities were less than 0.001 percent.

55
Table 15. Comparison of SRM 40g, Sodium Oxalate, with

composites of SRM 40h, Sodium Oxalate.

SRM 40g SRM 40h

g KMnOit sol Composite g KMnOi, sol

per g Na2C20it No. per g Na 2 C 2 0it 128.6043 I 128 .6214

128.5635 II 128 .5700

128.5106 III 128 .6548 IV 128 .5137 IV 128 .4295 III 128 .5963

II 128 .6131

I 128 .6662

Average 128.5595 128 .6051

a 0.0470(0.037$) 0 .0520(0.

a Omitted from average.

Since sodium bicarbonate and sodium carbonate are inert

materials in the redox system, their effect is proportional

to the amount present [40]. Sodium hydrogen oxalate causes

a positive error of 16 percent of its amount. Sodium bicarbonate rather than sodium carbonate was present in SRM 40h

because a solution of this standard gave a pH of 7.3 which

is on the acid side of the neutral point [4l].

a. Determination of sodium hydrogen oxalate - The

sodium hydrogen oxalate present in the sodium oxalate would

be equivalent to the amount of sodium hydroxide needed to neutralize a sample of sodium oxalate [4l].

A two g sample of sodium oxalate was transferred to a 250 ml beaker containing 130 ml of carbon dioxide-free water

When the salts were dissolved the solution was titrated with

a 0.01 N sodium hydroxide solution to a pH of 8.1. A carbon

56
dioxide-free atmosphere was maintained above the solution

during the dissolution and titration. A glass electrode - saturated calomel electrode system was used for measuring the

pH of the solution. The percent sodium hydrogen oxalate in

the sample was calculated from the reaction:

NaHCzOi, + NaOH * Na 2 C 2 0 lt + H 2 0. The sodium hydrogen oxalate found in SRM 40h was 0.031

percent (Table 16). The correction for the sodium hydrogen

oxalate is positive since more potassium permanganate would

be needed to react with sodium hydrogen oxalate than with an equal weight of sodium oxalate . The correction for the sodium

hydrogen oxalate in SRM 40h was +0.006 percent.

Table 16. Sodium hydrogen oxalate and sodium bicarbonate

in SRM 40h, Sodium Oxalate.

Percent NaHC 2 CU Percent NaHC0 3 0 029 0.022

0 032 0 .032

0 032 0.035

0 032 0.026

0 032 0.028

0 030 0.022

0 030 0.022

0 030 0 .028

0 030 0 .026

0 031 0 .027

0 0012 0 .0046

57
b. Determination of sodium bicarbonate - Sodium

bicarbonate in a sodium oxalate solution was determined by

making the solution acid with oxalic acid and then removing

the carbon dioxide that was formed from the solution:

2 NaHC0 3 + H 2 C 2 0^ + Na 2 C 2 0^ + 2 H 2 C0 3 H 2 C0 3 -> H 2 0 + C0 2 + . After the carbon dioxide has been removed the oxalic acid was titrated with a sodium hydroxide solution. The difference

between the amount of sodium hydroxide solution used in the

titration and the amount of sodium hydroxide solution needed

for the neutralization of the added oxalic acid is equivalent

to the sodium bicarbonate present in the sodium oxalate.

After each 2 g sample of sodium oxalate was titrated

with sodium hydroxide to a pH of 8.1, 10 ml of 0.01 N oxalic

acid solution was added. A stream of carbon dioxide-free air was bubbled through the solution for 30 minutes to remove the

carbon dioxide. While maintaining the carbon dioxide-free

air above the solution, it was titrated with 0.01 N sodium

hydroxide solution to the neutral point of a pH of 8.1.

The sodium bicarbonate found in SRM 40h was 0.027 percent. The results are given in Table 16

c . Determination of gaseous carbon dioxide - The

gaseous carbon dioxide was determined with the apparatus and

method given in Hillebrand, Lundell, Bright and Hoffman [42].

The carbon dioxide was evolved by heating a hydrochloric acid

solution, cleansed of impurities, absorbed by ascarite and

weighed. Erratic results were obtained when carbon dioxide

was first determined in SRM 40g. In an attempt to discover

the cause of the erratic results, carbon dioxide was collected

for additional 30 minute periods. The weight of the carbon

dioxide increased from 0.001 to 0.002 g for each 30 minute

period indicating the sodium oxalate was slowly decomposing

(Table 17)

58
! Table 17 Evolution of Carbon Dioxide on Boiling in

Sodium Oxalate Solution

C0 2 evolved

Sample No. 1st 30 min 2nd 30 min 3rd 30 min 4th 30 min

(10 g) (g) (g) (g) (g)

1 0.0024 0.0021 0.0020

2 0.0013 0.0012 0.0011

3 0.0023 0.0013 0.0012 0.0013

Evolution of Carbon Dioxide Without Heating in

Sodium Oxalate Solution

CO2 evolved

Sample No. 1st 1 1/2 hours 2nd 1 1/2 hours 3rd 1 1/2 hours

1 0.0007 0.0000 0.0000

2 0.0010 0.0002 0.0000

Carbon dioxide can be removed from a solution at room

temperature by a stream of carbon dioxide-free air [41]. By

keeping the solution of sodium oxalate at room temperature,

the probable sources of error of thermal decomposition of the

sodium oxalate, steam distillation of oxalic acid and attack

of chemicals on the glass apparatus were eliminated. Any

carbon dioxide evolved would come from the entrapped carbon

dioxide in the sodium oxalate or from sodium bicarbonate.

When the heating of the solution was eliminated the

carbon dioxide evolved did not increase after the initial

time period (Table 17). The procedure was modified to eliminate the heating of the solution.

59
Using the absorption train given in Hillebrand, Lundell,

Bright and Hoffman [42], 125 ml of water and 30 ml hydrochloric

acid (1+2) was delivered to the flask. Carbon dioxide-free

air was bubbled through the solution for 30 min, the air flow

stopped, and the carbon dioxide absorption tube attached. A

10 g sample of sodium oxalate was added and the air flow was started and continued for 30 minutes after the salt was dissolved. The absorption tube was detached and weighed.

The carbon dioxide found includes the gaseous carbon

dioxide in the voids of the crystals and the carbon dioxide

from the sodium bicarbonate present in the sodium oxalate.

In order -to eliminate any gaseous carbon dioxide, crushed

samples of sodium oxalate were run. The sodium bicarbonate

equivalent to the evolved carbon dioxide agreed with that

found by titration indicating no gaseous carbon dioxide in

SRM 4 Oh.

d . Determination of water in sodium oxalate - Sodium oxalate is not hygroscopic and the small amount of

surface water is removed by heating for 2 hours at 105 °C

Three samples of SRM 40h, Sodium Oxalate, showed a loss

of 0.0004, 0.0005 and 0.0001 percent in 2 hours and after 94

hours an additional loss of 0.0004, 0.0003 and 0.0004 percent

(Table 18). Two hours of heating at 105 °C was considered to be sufficient for the removal of all surface water.

As with most materials crystallized from aqueous solutions,

water could be entrapped In the sodium oxalate crystals. The

removal of this water was accomplished by heating the sodium

oxalate at 240 °C, which is below the 250-270 °C decomposition

temperature. After the standards had been dried at 105 °C,

SRM 40h showed a loss of 0.0088 and 0.0090 percent on heating

at 240 °C (Table 18)

60
Table 18. Loss on heating of SRM 40h, Sodium- Oxalate

Percent loss

10 II heated for (1) (2) (3)

2 hours at 105 °C 0 .0001 0.0004 0 .0005

+ 20 hours at 105 O r\C 0 .0000 0.0003 r\ r\ o r\ r\ u . uu uu

+ 74 hours at 105 °C 0 .0004 0 .0001 0.0003

Total at 105 °C 0 .0005 0 .0008 0 .0008

20 hours at 240 °c 0.0088 0. 0090

e . Calculation of effective purity of SRM 40h , Sodium Oxalate - The effective purity of the sodium oxalate

was calculated by deducting (or adding) the effective percentage of impurities determined from 100. The total effect of

the impurities in SRM 40h was minus 0.030 percent, therefore

the effective purity o'f SRM 40h, Sodium Oxalate, is 99.97

percent (Table 19) . Table 19. Impurities in SRM 40h, Sodium Oxalate.

Impurity

Water

NaHCaO^

NaHC0 3 CO 3

SRM 40h

(%)

-0.009

+0 .006

-0 .027

Total 0.030

3 . Comparison with SRM 83c, Arsenic Trioxide

The sodium oxalate standard SRM 40h was compared to SRM

83c by using potassium permanganate as an intermediate standard

The sodium oxalate was titrated with a potassium permanganate

61
solution as in Section IV.E.l. The direct titration of arsenic

trioxide with potassium permanganate is unsatisfactory because

the reaction is not stoichiometric, but the addition of a small

amount (0.0025 M) of potassium iodide or potassium iodate acts

as a catalyst for the complete reduction of potassium permanganate with arsenic trioxide in a slightly acidic solution [39]

a . Dissolution using sodium hydroxide and hydrochloric acid - In the methods given in the literature, the

sodium carbonate solution containing the arsenic trioxide is

acidified using hydrochloric acid. Also potassium iodide is

generally used as the catalyst

Method:

The arsenic trioxide was dissolved in sodium carbonate,

acidified with hydrochloric acid and titrated either visually

or potentiometrically [43]. When the visual end point was used the excess potassium permanganate was determined

iodiometrically [39].

Reagents

(a) Potassium iodide solution (0.0025 M) - 0.0415 g of

potassium iodide was transferred to a 100 ml volumetric flask,

diluted to the mark with water, and mixed.

(b) Potassium permanganate solution (0.1 N) - see Section IV.E.l.

(c) Starch solution - see Section IV.E.l.

(d) Sodium thiosulfate solution (0.02 N) - see Section IV.E.l.

Procedure

A 0.25 g sample of arsenic trioxide was transferred to a H00 ml beaker and 10 ml of sodium hydroxide (20%) were added.

When the arsenic trioxide was dissolved 100 ml of water, 10 ml

of hydrochloric acid and one drop of the potassium iodide

solution were added. The solution was titrated with the 0.1 N potassium permanganate solution to the visual pink color of

potassium permanganate.

62
To determine the excess potassium permanganate, 0.5 g of

potassium iodide was added and the liberated iodine was titrated with the 0.02 N sodium thiosulfate solution. Two ml of

starch solution were added just before the yellow iodine color

disappeared and the titration was continued to the starchiodide end point [39].

Weight burets were used for the potassium permanganate

solution and all weights were corrected to vacuum.

Grams of KMnOi+ solution equivalent to 1 g As 2 0 3 = B_ cp where

A = g of arsenic trioxide taken,

B = g of potassium permanganate solution used,

C - ml sodium thiosulfate solution used to determine

excess KMnCU , and

D = g potassium permanganate solution equivalent to

1 ml of sodium thiosulfate solution.

Effective purity of Na 2 C 2 04, in percent = ~ x ~

where

A = g of potassium permanganate solution equivalent to

1 g sodium oxalate,

B = effective purity of arsenic trioxide in percent,

C = g of potassium permanganate solution equivalent to

1 g arsenic trioxide,

D = equivalent weight of sodium oxalate, and

F = equivalent weight of arsenic trioxide.

Results

:

In the first set of titrations a platinum electrode - saturated calomel electrode system was in the solution so that

the titration could be followed potentiometrically as well as visually. In the second set, the titrations were performed

visually with no electrodes present. The platinum electrode

- saturated calomel electrode system appeared to have a system

63
bias on the arsenic trioxide - potassium permanganate titration. For each set the potassium permanganate solution was standardized with SRM 83c and the effective purity of SRM 40h,

Sodium Oxalate was calculated.

The effective purity of SRM 40h, Sodium Oxalate, when

compared to SRM 83c, Arsenic Trioxide, is 99.862 with a stan- dard deviation of 0.018 percent when the platinum electrode

and saturated calomel electrode were present and 99.892 with

a standard deviation of 0.018 percent when no electrodes were

present. The results are given in Table 20.

b . Investigation of the Conditions of the Arsenic

Trioxide - Potassium Permanganate Reaction - When hydrochloric

acid was used to acidify the arsenic trioxide solutions the

effective purity of SRM 40h appeared to be somewhat low. In order to determine the accuracy of this value the following

conditions of the titration were studied: use of sulfuric

acid and acidity, catalysts, heating, and end point detection.

(1) Use of sulfuric acid and acidity: Lowenthal and

Lenssen [44] as early as 1862 showed that hydrochloric acid

reacted to a small extent with potassium permanganate during

the titration of sodium oxalate. Although later investigators

[^3,45,46] state that in the titration of arsenic trioxide

with potassium permanganate the side reaction

2 KMnOi, + 16 HC1 -* 2 MnCl 2 + 2 KC1 + 5 Cl 2 + 8 H 2 0 does not occur, sulfuric acid was substituted for hydrochloric

acid to eliminate the possibility of this side reaction.

Since a 1.8 N sulfuric acid solution is used in most

titrations using sulfuric acid, this concentration was tried.

The spread of the results was too large ('0.4$) so 3.6 N, 0.7 N and 0.35 N sulfuric acid were tested. The 3.6 N sulfuric acid

gave results similar to the 1.8 N sulfuric acid. In 0.35 N sulfuric acid the violet color of the potassium permanganate

solution was slow to disappear and the end point was reached

slowly and was too indistinct to be used. In 0.7 N sulfuric

64
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65
acid the spread of results was within the desirable limits

(0.05$), therefore, in the procedure for the titration of

arsenic trioxide with a potassium permanganate solution, a 0.7 N sulfuric acid solution was used.

(2) Catalysts: A catalyst is needed to accelerate the

reaction between arsenic trioxide and potassium permanganate

solution. One drop of potassium iodide solution (0.0025 M)

has been used as the catalyst [45] but after the addition of

45 ml of the potassium permanganate solution, the decolorization of the solution was slow and the solution went through a greenish yellow color.

Since an osmium tetroxide solution (0.01 M) has been used

as the catalyst in the standardizations with eerie ammonium

sulfate solutions [47], the use of this catalyst was tried to determine whether the speed of the initial reaction could be

increased. An improvement was noted in the reaction because

the greenish-yellow color did not persist but, from 3 to 5 minutes was needed for the reaction with arsenic trioxide to go to completion and decolorize the solution. Also it was determined that 1 ml of osmium tetroxide solution (0.25 mg/ml)

was sufficient to catalyze the reaction.

In order to reduce the time to decolorize the solution

after the addition of the first 45 ml of potassium permanganate

solution a manganous sulfate solution was added. It had been

noted that in titration of sodium oxalate with potassium permanganate the potassium permanganate reduction was rapid after a little manganous ion was formed. Accordingly a manganous

sulfate solution (0.3 mg/ml) was added to the arsenic trioxide

solution and the potassium permanganate color disappeared in 30 seconds. The addition of a manganous sulfate solution and a osmium tetroxide solution was incorporated into the procedure.

(3) Heating: A series of experiments was performed to determine the effects of heating the solution during the

titration. Arsenic trioxide solutions were titrated without

heating, then arsenic trioxide solutions were heated to 55 - 66
60 °C after the addition of 45 ml of the potassium permanganate

solution. No apparent change was observed on heating, therefore the heating of solutions was discontinued.

(4) End point detection: An attempt was made to determine the excess potassium permanganate needed for the detection

of the end point iodimetrically but with the osmium tetroxide

present no end point was reached in the sodium thiosulfate

titration. When using the pink color of potassium permanganate

as the end point of this titration the spread of the results

was more than was desired. Although the values obtained for

the potentiometric titration in hydrochloric acid appeared to

be low, this end point detection was tried in a sulfuric acid

solution and the results were more precise and appeared to be more accurate. Therefore, the arsenic trioxide in the sulfuric

acid solution was titrated potentiometrically

.

c . Dissolution using Sodium Carbonate and Sulfuric

Acid - Method:

In this method, the arsenic trioxide was dissolved in a sodium carbonate solution and then was acidified with sulfuric

acid. The solution was diluted to a 300 ml volume and osmium

tetroxide solution and the manganous sulfate solution were

added. Approximately 45 ml of the potassium permanganate

were added and the solution stirred until it was decolorized.

The titration was completed potentiometrically by adding the

potassium permanganate solution dropwise.

Reagents

:

(a) Water - All water used was freshly boiled and

cooled distilled water.

(b) Manganese sulfate solution (0.3 mg/ml) - Forty-six

mg of manganous sulfate monohydrate was transferred to a 50 ml volumetric flask, diluted to the mark with sulfuric acid

(1+19) and mixed.

67
(c) Osmium tetroxide solution (0.25 mg/ml) - Two-hundredfifty mg of osmium tetroxide was transferred to a 1 liter

volumetric flask and 200 ml of 0.1 N sulfuric acid were added.

When the oxide dissolved the solution was diluted to the mark

with 0.1 N sulfuric acid and mixed.

(d) Potassium permanganate solution (0.1 N) - see Section IV.E.l.

Procedure

A 0.25 g sample of arsenic trioxide was transferred to a 150 ml beaker, 5.00 g of sodium carbonate were added and thoroughly blended. Twenty ml of water were added in 5 ml portions

and the solution was warmed to dissolve the sample. The solution was transferred to a 750 ml Erlenmeyer flask and 17.5 ml

of sulfuric acid (1+1) were added cautiously through a short

stem funnel. When the reaction ceased, the solution was transferred to a 600 ml beaker, 1 ml of the manganese sulfate

solution and 1 ml of the osmium tetroxide solution were added,

and the solution was diluted to 300 ml with water. Approximately 45 ml of the 0.1 N potassium permanganate

solution were added and the solution was stirred slowly until it was decolorized. The titration with the potassium permanganate

solution was followed potentiometrically to the end point. The

last ml was added dropwise and the last few drops were split.

Weight burets were used for the potassium permanganate

solution and all weights were corrected to vacuum. Effective purity of Na 2 C 2 0 2 = ~ x §

where

A = g of potassium permanganate solution equivalent to

1 g sodium oxalate,

B = effective purity of arsenic trioxide in percent,

C = g of potassium permanganate solution equivalent to

1 g arsenic trioxide,

D = equivalent weight of sodium oxalate, and

E = equivalent weight of arsenic trioxide.

68
Results

:

In calculating the effective purity of SRM 40h, the

average of the ratios of grams of potassium permanganate

solution equivalent to 1 g of SRM 83c was used. The effective

purity of SRM 40h is 99.941 percent with a standard deviation

of 0.007 percent when compared to SRM 83c, Arsenic Trioxide.

The results are given in Table 21.

4 . Comparison with SRM's 136b and 136c, Potassium

Dichromate

Sodium oxalate is not a strong enough reducing agent to reduce potassium dichromate therefore SRM 40h cannot be com- pared directly with SRM's 136b and 136c.

Since ferrous ion can also be titrated with a potassium

dichromate solution, sodium oxalate was compared indirectly

to potassium dichromate by titrating the sodium oxalate and

ferrous ammonium sulfate hexahydrate with a potassium permanganate solution. The ferrous ammonium sulfate hexahydrate

was then compared to potassium dichromate.

The method for the titration of sodium oxalate with a potassium permanganate solution is given in Section IV.E.l

above, and the method for the comparison of potassium dichromate to ferrous ammonium sulfate hexahydrate is given in

Section IV.E.l. a. The method for the titration of ferrous

ammonium sulfate hexahydrate with a potassium permanganate

solution is given below.

Method:

The ferrous ammonium sulfate hexahydrate was dissolved

in sulfuric acid and titrated potent iometrically with a potassium permanganate solution.

Reagents

:

Potassium 'permanganate solution (0.1 N) - see Section

IV.E.l.

69
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on

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•H -H m X

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a-H

M

CD LH >

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-P -H O C

CU CD Cm CO Cm m W <

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j- cd CO tr— LTA VO -=r vo vo LfA tr— tr— tr- VO VO rH •H O S 1—

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CD M

-p <

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C— tr— [— VO C^- tr— tr— tr— CO c— O

C hO a

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W M OA OA OA OA OA OA OA OA OA OA OA OA OA CD

CD rH H rH rH H rH rH rH H rH H rH rH >

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70
Procedure

:

An accurately weighed 2.00 g sample of ferrous ammonium

sulfate hexahydrate was transferred to a 600 ml beaker and

250 ml of sulfuric acid (1+19), freshly boiled and cooled, were added. When the salt was dissolved, 45 ml of the 0.1 N potassium permanganate solution were added and the solution

was stirred until all the potassium permanganate had reacted.

The titration with the potassium permanganate solution was continued potentiometrically to the end point. The last ml

of the titration was added dropwise, splitting the last few

drops

.

Weight burets were used for the potassium permanganate

solution and all weights were corrected to vacuum.

AD E The effective purity of sodium oxalate = BC

x p where

A = g of potassium permanganate solution equivalent to

1 g sodium oxalate,

B = g of potassium permanganate solution equivalent to

1 g ferrous ammonium sulfate hexahydrate,

C = g of ferrous ammonium sulfate hexahydrate equivalent

to 1 g potassium dichromate,

D = effective purity of potassium dichromate in percent,

E = equivalent weight of sodium oxalate, and

F = equivalent weight of potassium dichromate.

SRM 40h, Sodium Oxalate, has an effective purity of

99.973 percent when compared with SRM 136b, Potassium Dichromate. In the comparison of SRM 136b with ferrous ammonium

sulfate hexahydrate, only four runs were made. One of these

runs was discarded since the titration did not follow the

expected pattern and the end point of the titration was doubtful. The average equivalent of SRM 136b to ferrous

ammonium sulfate hexahydrate was used for the calculations of

the grams of the potassium permanganate solution equivalent

to 1 g potassium dichromate.

71
The average of these values was used to compute the effective

purity. The results are given in Table 22. SRM 40h has an effective purity of 99-967 percent when

compared with SRM 136c, Potassium Dichromate . In the comparison of SRM 136c with ferrous ammonium sulfate hexahydrate,

twelve runs were made because this standard is the present

potassium dichromate Standard Reference Material. The

effective purity of SRM 40h was calculated in the same manner

as with SRM 136b. The results are given in Table 23.

5 . Summary

SRM 40h, Sodium Oxalate, will be certified with an effective purity of 99. 97 percent. This value is based on the

comparisons of SRM 40h to SRM 40g and to SRMs 136b and 136c,

Potassium Dichromate. The' effective purity of SRM 40h is

99.97 percent when compared to SRM 40g through a potassium

permanganate solution; 99.973 percent when compared to SRM 136b

through a potassium permanganate solution and ferrous ammonium

sulfate hexahydrate; 99.967 percent when compared to SRM 136c

in the same manner as SRM 136b. The results are given in

Table 24. The lower part of the table gives the other determinations of the effective purity of SRM 40h. When the effective

percentage of impurities were determined and subtracted from

100 percent the effective purity of SRM 40h is 99-97.

The effective purity of SRM 40h is 99-9^1 percent when

compared with SRM 83c, Arsenic Trioxide. This value could

be 0.03 percent low since the platinum electrode - saturated

calomel electrode system was used for the detection of the

end point. In the study for the detection of the end point

(acidification with hydrochloric acid), an 0.03 percent lower

value was found when electrodes were present in the solution

than when the titration was performed visually.

72
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ctf faO WOW

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LPv LT( LPi LT\

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74
24. Effective purity of SRM 40h, Sodium Oxalate.

Effective purity

(%)

For certification

SRM 136c (K 2 Cr 2 0 7 ) 99.967

SRM 136b (K 2 Cr 2 0 7 ) 99.973

SRM 40g (Na 2 C 2 0 lf ) 99.97

Average 99.97

Other comparisons

Determination of Impurities 99.97

SRM 83c (As 2 0 3 ) 99.9^1

75
V. SUMMARY OF RESULTS

The oxidlmetric standard reference materials have been

intercompared and a final certificate of analysis will be

issued for each standard. The interrelationship of the SRM's

is given in Figure 10 and the effective purity in Table 25- Table 25. Effective purity of oxidlmetric standard

reference materials.

SRM Effective puritya

(,%) 83c, Arsenic Trioxide 99-988

136c, Potassium Dichromate 99-983

40h, Sodium Oxalate 99-97

a Error terms discussed in text.

In Figure 10 the basis of the intercomparison is the

International System of Units (SI) with measurements through

the coulomb and the mass of the iodine used in the titrations.

The differences are recorded in the direction of the arrows. With the coulomb and mass of iodine taken as basic standards

and following the arrows in any manner the value obtained for

any SRM will agree within 0.007 percent except when the relationship of SRM 83c to SRM 40h is involved. However, in

Section IV. E. 5 it has been shown that the value for this relationship probably Is in error and should be disregarded D The coulomb and the mass of high purity iodine are taken

as standards because their measurement is based on SI ampere,

seconds and grams . Iodine is monomulcidic thus Its atomic

weight is known accurately to 1 part in 1,000,000. Using a microbalance the mass of iodine can be determined within one part in 100,000. The coulometoric measurement involves the

measurement of time, current and mass. The current was measured

by the use of a NBS calibrated secondary standard. A micro76

balance was used to obtain all weights and time was based on

the standard 100 k Hz WWV frequency. Therefore, the charge

involved in coulometric measurement is known within a few

parts in 1,000,000.

The effective purity of SRM 83c, Arsenic Trioxide, is 99.988 percent and is the average of the values determined by

comparison with purified iodine and by coulometry. The value

for the comparison with purified iodine is 99.9878 percent

with a standard deviation of O.OO85 percent for 16 degrees of

freedom and the value for coulometry is 99.9873 percent with

a standard deviation of 0.0032 percent for five degrees of

freedom.

The effective purity of SRM 136c, Potassium Dichromate,

is 99.9 83 percent and is the average value determined by

comparison with SRM 83c, Arsenic Trioxide, and by coulometry.

The value for the comparison with SRM 83c is 99.985 percent

with a standard deviation of O.OO63 percent for 11 degrees of

freedom and the value for coulometry is 99.982 percent with a standard deviation of 0.0028 percent for 19 degrees of freedom

The effective purity of SRM 40h, Sodium Oxalate, is 99.97 percent and is the average of the values determined by

comparisons with SRM 40g, Sodium Oxalate; SRM 136b, Potassium

Dichromate; and SRM 136c, Potassium Dichromate. The value

for the comparison with SRM 40g is 99.97 percent with a standard deviation of 0.030 percent for 29 degrees of freedom

(KMnOt, Solution No. 3) and 0.013 percent for 30 degrees of

freedom (KMnO^ Solution No. 4). The value for the comparison

with SRM 136b is 99.973 percent with a standard deviation of

0.011 percent for 15 degrees of freedom. The value for the

comparison with SRM 136c is 99-967 percent with a standard

deviation of 0.011 percent for 15 degrees of freedom.

The final Certificates of Analysis for SRM 83c, Arsenic

Trioxide; SRM 136c, Potassium Dichromate; and SRM 40g, Sodium

Oxalate; will be issued in the near future.

78
ACKNOWLEDGMENTS

The authors are indebted to Mrs. Virginia C. Stewart for

the spectrographic analyses, to Paul J. Paulsen and Charles

W. Mueller for the spark source mass spectrographic analyses,

and to Rolf A. Paulson and William P. Schmidt for ACS Specification testing of these standards. Special appreciation is

expressed for the assistance of Mrs. Joy J. Shoemaker in the

preparation of this report.

79
VI. REFERENCES

Gay Lussac, J. L. , Ann. lQ_, (1815).

Penot, B., J. Proct. Chem. 54, 59 (I85D; Dingel. Pol.

J. 127, 134 (1853).

Mohr, P., Lehrbuch der Chemisch - Analytischen

Titriermethode , Braunschweig, Edition 1, (1859).

Chapin, R. M., J. Ind. Eng. Chem. 10, 522 (1918).

Reagent Chemicals } American Chemical Society

Specifications, 196 8, Allied Publications, American

Chemical Society, Washington, D. C. (1961).

Spitalsky, E., Z. Anorg. Chem. 69, 179 (1911).

Bacho, P. de, Ann. Chim. Appl . 12, 153 (1919).

Zintl, E. and Zaimis, P., Z. Angew. Chem. 40_, 1288

(1927).

Treadwell, F. P. and Hall, W. T . , Analytical Chemistry

Vol II, 9th Edition, p. 587, Wiley, New York (19^2).

Anhorn, V. J. and Hunt, H, Ind. Eng. Chem,, Anal. Ed.

9, 591 (1937).

Milstead, K. L., J , Assoc. Office. Agr. Chemist 22,

567 (1939).

Washburn, E. W. , J. Am. Chem. Soc . 30_, 31 (1908).

Chapin, R. M . , J. Am. Chem. Soc. 4l, 351 (1919).

Rice, F. 0., Kilpatrick M. and Lemkin, W., J. Am. Chem. Soc. 45, 1361 (1923).

Popoff, S. and Whitman, J. L. , J. Am. Chem. Soc.,

47, 2259 (1925).

Kolthoff, I. M. and Belcher, B., Volumetric Analysis,

Vol. Ill, Interscience Publishers, New York (1957).

Marinenko, G. and Taylor, J. K., Anal. Chem. 39,

1568 (1967).

Ramsey, W. J., Farrington, P. S. and Swift, E. H.

,

Anal. Chem. 22, 232 (1950).

80
[19] Taylor, B. N., Parker, W. H. and Longenberg, D. N.

,

Rev. Modern Phys. 4l, 375 (1969).

[20] Penny, P., Brit. Assoc. Advancement Sci. Rept. 18 , (2) 58 (1850).

[21] Sarver, L. A. and Kolthoff, I. M . , J. Am. Chem. Soc.

53, 2902 (1931).

[22] Garman, R. H. and Droz. M. E., J. Ind. Eng. Chem.

Anal. Ed. 11, 398 (1939).

[23] Marinenko, G. and Taylor, J. K., J. Res. NBS 67

A

, (Phys and Chem) No. 5, ^53 (1963).

[24] Cooke, W. D. and Furman, N. H. } Anal. Chem. 22_, No. 7, 896 (1950).

[25] Shults, W. E., II, Thomason, P. F. and Kelley, M. T.,

Anal. Chem. 27, No. 11, 1750 (1955).

[26] Rawley, K. and Swift, E. H., Anal. Chem. 26, 373

(195^).

[27] Lingane, J. J. and Anson, F. C, Anal. Chim. Acta 16 , 165 (1957).

[28] Lingane, J. J. and Kennady, J. H., Anal. Chim. Acta

15, 465 (1956).

[29] Anson, F. C, Anal. Chem. 33, No. 7, 934 and 939 (1961).

[30] International Commission on Atomic Weights, Final

Version of Report 28-9-1967, pp. 130-141, Comptes Rendus

XXIV IUPAC Conference, Prague, Sept. 4-10, 1967.

[31] Craig, D. N., Hoffman, J. I., Law, C. A. and Hamer,

W. J., J. Res. NBS 64A, (Phys and Chem) No. 5, 38l

(I960).

[32] Marinenko, G. and Taylor, J. K., J. Res. NBS 67

A

, (Phys

and Chem) No. 1, 31 (1963).

[33] Taylor, J. K. and Smith, S. W. , J. Res. NBS 6_3A, (Phys and Chem) No. 2, 153 (1959).

[34] Schwab, F. W. and Wickers, E., J. Res. NBS 33, 121

(1944). —

[35] Knoeck, J. and Diehl, H. , Talanta 1_6, l8l (1969).

81
[36] Sorensen, S. P. L. s Z. Anal. Chem. 3§_, 639 (1897).

[37] Sorensen, S. P. L., Z. Anal. Chem. 4_2, 333 and 542

(1903).

[38] Fowler, R . M. and Bright, H. A., J. Res. NBS 15, 493

(1935).

[39] Bray, W. C, J. Am. Chem. Soo. 32, 1205 (1910).

[40] Blum, W., J. Am. Chem. Soo. _34, 123 (1912).

[4l] Clabaugh, W. S. and Knowles, H. B., unpublished paper

in authors possesion.

[42] Hillebrand, W. F . , Lundell, G. E. F., Bright, H. A. and Hoffman, J. I., Applied Inorganic Analysis, 2nd

Edition, page 768, John Wiley and Sons, Inc., New York,

(1953).

[43] Bright, H. A., J. Res. NBS 19, 691 (1937).

[44] Lowenthal, J. and Lenssen, E., Zeit. Anal. Chem. 1,

329 (1862).

[45] Lang, R . , Z. Anal. Chem. 152., 195 (1926).

[46] Kolthoff, I. M., Laitinen, H. A. and Lingane, J. J.,

J. Am. Chem. Soo. 59, 429 (1927).

[47] Gleu, K., Z. Anal. Chem. 95, 305 (1933).

82
S. Department* of Commerce APPENDIX I

Maurice,-fl.Stans Secretary

|.asfefcj National Bureali of Standards

L. M. Branscemb; Director

(Eerttfttate of JVnalyatg

STANDARD REFERENCE MATERIAL 136c

Potassium Dichromate

(Oxidimetric Standard)

Purity on basis of effective oxidizing power 99.98 ± 0.02 percent This lot of material was prepared to ensure material of high purity and uniformity. It conforms

to the American Chemical Society specification for analytical reagent grade material, but is not to be considered as entirely free from traces of impurities. It is certified only for its effective oxidizing power. -A Standardizations have been made by the coulometric methyl bf^sMapSrenko (see NBS J. Res. 67A, 453 (1963)), by direct comparison with arsenic triiraidi.' ^R^83ct and by comparison with

the previous SRM 136b through ferrous ammonium sujf^ iSyTt>Sbppenfield. Impurity tests to the ACS specifications were made by W. P. Schmi d L< errassio ii\sppC

t

r°

g

r aphy by V. Stewart, and spark- source mass spectrography by C. W. Mueller. Drying tests indicate that losses of fne c^derw 0.005 percent are obtained on a few hours drying

at 105 °C; long time drying indidH^s inat rpsses approach 0.01 percent. The problem of occluded and surface moisture will be/t?Tuaie OTaqfore a final certificate is issued. The effective assay is based on the sample as issue^Tfce tolerance indicated is at least as large as the 95 percent confidence

limits for a single^db^n^ajKm^nd include terms for inhomogeneities in the material as well as analytical error, "v The overall direction and technical measurements leading to certification were performed under

the chairmanship of R. A. Durst. The technical and support aspects in the preparation, certification, and issuance of this standard

reference material were coordinated through the Office of Standard Reference Materials by J. L. Hague.

Washington, D. C. 20234 J. Paul Cali, Acting Chief March 24, 1970 Office of Standard Reference Materials 83
U. S. Department of Commerce

Maurice, H. Stans

Secretary

irn trM potassium dichromate

icTrtOxide standard 83b, with the

orfoal uranium metal, using the 1961

ted experimental error is about 0.02

Certificate

STANDARD REFERENCE MATERIAL 83c

Arsenic Trioxide

Purity on Basis of Effective Reducing Power . .99.99 percent

This lot of arsenic trioxide was prepared to ensure material of high purity and uniformity. It conforms to the American Chemical Society specification for reagent grade ikaterial. The assay, for the present, is based on a direct comj

standard 136b. The value is compatible with the prevks potassium dichromate standard 136b, and with hi^-purity

table of Atomic Weights based on Carbon- l^T^liev esfefi percent, and work is in progress on additi^na^tanMrofzation procedures The sample as issued does 103 °C. The assay value<fo?th\

lot^TbseNjJ/er 6.002 percent in weight on drying up to 24 hours at )resent, is based on the material as issued.

Washington, D. C. 202* February 6, 1962

(Reprinted April 16, 1970)

Harry C. Allen, Jr., Chief

Analytical and Inorganic Chemistry Division

84
U. S. Department' of Commerce

Maurice H. Stans

Secretary

Certificate af JkxmlQzx*

Standard Reference Material 40h

Sodium Oxalate

Na2 C2 04 , effective purity 99.95 ± 0.05 percent

This lot of sodium oxalate was prepared to ensure material of purity and

uniformity compatible with its use as a working standard for oxidation-reduction

reactions. The material conforms to the American Chemical SocieUf specification for reagent-grade material, but should not be considered as entirely freWrom impurities such as moisture, sodium hydrogen oxalate (0.03%!, aW|Nso($h-tm hydrogen

carbonate (0.03%). The material is not appreciably hy^r^ac^ipia The effective purity of the sodium d&laie cam) berealized in the standardization of KMn04 solutions by use of the direct ion? on this certificate. Homogeneity

testing, effective purity, sodiumuhyoroge^/oxalate and sodium hydrogen carbonate

determinations were rnadgx^yll K. p-. Happenfield; American Chemical Society

specification tests were lwdeNby W. P. Schmidt of the Analytical Chemistry

Division.

Fection and coordination of the technical measurements leading

to certification were performed under the chairmanship of W. R. Shields. 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 J. L. Hague.

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

April 24, 1969 Office of Standard Reference Materials

(over) 85
DIRECTIONS FOR USE IN OXIDIMETRY

Standardization of 0.1N permanganate. Transfer 0.3 g of sodium oxalate (dried

at 105 °C) to a 600-ml beaker. Add 250 ml of diluted sulfuric acid (5+95)

previously boiled for 10 to 15 minutes and then cooled to 27 ± 3 °C. Stir until the

oxalate has dissolved. Add 39 to 40 ml (0.3 g of sodium oxalate requires approximately 44.8 ml of O.liV KMn04 ) of O.liV potassium permanganate at a rate of 25 to 35 ml per minute while stirring slowly. Let stand until the pink color disappears (about 45 seconds; if the pink color should persist because the permanganate is too strong, discard, and begin again, adding less of the KMn04 solution). Heat to 55 to 60 °C and complete the titration by adding permanganate

until a faint pink color persists for 30 seconds. Add the last 0.5 to 1 ml dropwise, with particular care to allow the solution to become decolorized before the next drop is introduced.

Determine the excess of permanganate required to impart the permanent pink

color to the solution. This can be done by matching the color by adding permanganate to the same volume of the specially treated dilute sulfuric acid at 55

to 60 °C. This correction usually amounts to 0.03 to 0.05 ml.

In potentiometric titrations the correction is negligible if the end point is reached slowly. For the standardization of more dilute solutions, the same conditions are recommended except that the initial volume and size of sample are proportionally reduced.

In very accurate work the correction is best obtained iodometrically [cf. W. C.Bray, J. Am. Chem. Soc. 32, 1205 (1910)] as follows: Cool the titrated solution to 25 °C, add 0.5 g of KI, 2 ml of starch solution, and titrate the liberated iodine with 0.02iV thiosulfate. To obtain the ratio of the thiosulfate to the permanganate solution, add 1 ml of the O.liV permanganate to 350 ml of the diluted

sulfuric acid (5+95), stir, add 0.5 g KI, and titrate with the thiosulfate solution, adding starch just before the end point is reached.

USCOMM-NBS-DC

86
APPENDIX II Preparation of Calibration Curve for the Determination of

Excess Iodine

In the preparation of the calibration curve, the concen- tration of iodine in a 300 ml volume of solution was used

since in all titrations the solution could be diluted to this

volume. The calibration solutions were prepared by adding

the iodine to 60 ml of a 300 ml solution which was prepared

in the same manner as a titration solution except arsenic

trioxide was omitted. After reading the absorbance of the

calibration solutions the calibration curve was prepared

by plotting iodine concentration in 300 ml volume against

absorbance

.

An absorption curve of iodine was prepared so that the

appropriate wave length could be selected. A spectrophotometer was used and the absorption measured at intervals between

325 nm and 500 nm. A wave length of 375 nm was selected, for

convenience, even though 365 nm gave the maximum absorbance.

Reagents

:

(a) Water - All the water used was freshly boiled and

cooled distilled water.

(b) Sodium bicarbonate solution (60 g/1) - Sixty grams

of NaHC0 3 were dissolved in 900 ml water and diluted to one liter

.

(c) Standard iodine solution (0.00025 g I/ml) - Ten ml

of 0.1 N iodine solution were transferred to a 500 ml volumetric flask and diluted to the mark with water.

Procedure

:

A 10 g sample of sodium carbonate was transferred to a 500 ml glass-stoppered Erlenmeyer flask and 25 ml of water

were added. The solution was warmed to dissolve the salt and

then was cooled in ice water. One hundred ml of the sodium

bicarbonate solution and ten grams of potassium iodide were

87
added. When the potassium iodide was dissolved, the solution

was diluted to 300.0 ml with water.

To 60.0 ml portions of the solution, 0.0, 0.5, 1.0, 1.5

and 2.0 ml of the standard iodine solution were added respectively, and mixed. A suitable portion of each solution was transferred to a 2 cm photometric cell and the absorbance of

the solution was measured using a 375 nm filter and using the

absorbance of water as zero. The curve was drawn by plotting

mg of iodine in a 300 ml of solution against absorbance. The

data for the curve is given in Table 26 and the plot in

Figure 11.

Table 26. Calibration curve. Equivalent I 2 ml I 2 mg I 2 in 60 ml in 300 ml Absorbance

(%)

0.00 0.000 0.0000 0.000

0.50 0.1254 0.6267 0.181

1.00 0.2507 1.2535 0.357

1.50 0.3761 1.8802 0.516

2.00 0.5014 2.5070 0.638

88
.7 -

0 0.5 1.0 1.5 2.0 2.5 Mg l 2/300 ml solution

Figure 11. Photometric calibration curve for excess iodine.

89
form NBS-114A (1-711 U.S. dept. of comm. 1 . PU B LIC AT ION OR RE PORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No- sheet ims-sp- 260-24

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Standard Reference Materials: Comparison of Redox Standards

5. Publication Date January 1972

7R^uF.

0R^appenfield, G . Marinenko and J. L. Hague 8. Performing Organization

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 Address Same13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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 methods described in this publication are those that have been used at the National Bureau of Standards for the comparison

of the current oxidimetric standard reference materials with each other

and for the determination of the effective purity of these standard

reference materials. The methods, which are modifications of well- established and previously published methods, were selected or designed

for their accuracy and dependability. An accurate method was devised

for the determination of the effective purity of the arsenic trioxide

standard SRM 83c by adding solid iodine and determining the excess iodine photometrically.

17. KEY WORDS (Alphabetical order, separated by semicolons) Analytical procedures; arsenic-iodine method; comparison; effective

purity; oxidimetric standards.

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