DEPARTMENT

OF COMMERCE

National Bureau

of Standards

Standard Reference Materials:

Boric Acid; Isotopic and Assay

Standard Reference Materials
NATIONAL BUREAU OF STANDARDS

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

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

Standard Reference Materials:

Boric Acid; Isotopic, and Assay

Standard Reference Materials

£. J. Gatanzaro, G. £. Ghampion, £. L. Garner, G. Marinenko, K. M. Sappenfield, and W. R. Shields

tjv S (National Bureau of Standards Special Publication 260-17

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-17, 70 pages (Feb. 1970) CODEN: XNBSA

Issued February 1970

nstitute for Materials Research

National Bureau of Standards Washington, D. C. 20234

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 (Order by SD CaUlog No. C 13.10:260-17), Price 65 cents
PREFACE

Within the framework of the NBS Institute for Materials

Research the area of standard reference materials Is a broad

and Important one, including the preparation, characterization and distribution of a wide variety of materials in

such diverse fields as metallurgy, polymers and inorganic

materials. In carrying out such a program there is much

interaction with representatives of industry and science,

beginning with discussions as to which primary standard

materials will do most to advance technology, the furnishing

of materials and fabrication of samples, and the characterization and certification of the materials by cooperative

efforts. The many groups participating in a standards

program are very Interested in detailed information on specific aspects of the program — but to date there has

been no publication outlet for such written discussions.

To meet this need, NBS Special Publication 260 has been

reserved for a series of papers in the general area of

"standard reference materials". This series will present

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the Institute for Materials Research with emphasis on the

preparation and characterization of standard reference

materials. This subject-oriented series will provide a means for rapid dissemination of this detailed information

and we hope will stimulate the use of standard reference

materials in science and industry.

W. Wayne Melnke, Chief

Office of Standard Reference Materials

11
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iv
TABLE OP CONTENTS

PAGE

INTRODUCTION 1 A. Development of a Coulometric Method for

Assay of Boric Acid 1 1. Introduction 1

2. Investigation of Analytical Parameters 2 a. Effect of Mannltol on the Apparent

Ionization Constant of Boric Acid 2 b. Effect of Mannltol on the Electrochemical

Reduction of Hydrogen-Ion 3 c. Effect of Variation of Mannltol

Concentration 5 d. Evaluation of Analytical Bias 8 3. Coulometric Procedure for Assay of Boric Acid. . 8 a. Introduction 8 b. Coulometric Cell 9 c. Constants and Standards 11

d. Procedure. 12

B. Chemical Characterization of SRM 951, Boric Acid . . 13

1. Acldlmetrlc Assay 13

2. Effect of Relative Humidity on the

Stolchlometry of SRM 951, Boric Acid 19

3. Determination of Strong Acid In Crystalline

Boric Acid 22

C. Assay of SRM 952, Enriched Boric Acid 23

D. Preparation and Allquotlng of Separated Boron

Isotope Boric Acid Solutions 24

E. Containers and Conditions for Stable Storage of

Boric Acid Solutions 27

F. Development and Evaluation of a Mass Spectrometrlc

Technique for Boron-Isotope Ratio Measurements

and Evaluation of Possible Alternative

Techniques 28

1. Introduction 28

2. Sample Mounting Procedure 29

3. Filament Material 30

4. Sample Size 31

5. Total Sample Composition 31

6. Sodium Concentration 32

7. Filament Temperature 37

8. Mass Spectrometrlc Procedure 38 V
PAGE

G. Absolute Isotopic Abundance Ratios of Two

Boron-Isotope Standards, SRM 951 and SRM 952 ... 39

1. Introduction 39

2. Experimental Procedure hO

a. Mass Spectrometry 40 b. Purification of the Separated Isotopes ... 41

c. Preparation and Boron Concentration of

the Separated Isotope Solutions 43

d. Isotopic Analysis of the Separated

Isotope Solutions 44

e. Preparation of the Calibration Mixes .... 44

f. Isotopic Analysis of the Calibration

Mixes and SRM Samples 48

3. Results and Discussion 48 REFERENCES 52 APPENDIX 1 55 APPENDIX 2 57

APPENDIX 3 58

LIST OF TABLES

PAGE

Table No.

1. Effect of mannitol on electrochemical

reduction of 11+ at platinum cathode 6 2. Homogeneity of SRM 951, Boric Acid 15

3. Assay of SRM 951, Boric Acid (equilibrated at 34^ RH for at least 30 min) 17

4. Assay of SRM 951, Boric Acid composites I8

5. Absorption of water by boric acid 19

6. Assay of SRM 952, Boric Acid 23

7. Precision of aliquoting and analysis of boric

acid solutions 25

vi
PAGE

8. Effect of the size of boric acid solution

aliquot on the coulometric determination of

boric acid concentration 26

9. Concentration changes of solution (A) stored

in polyethylene bottle in room atmosphere ... 27

10. Stability of H3BO3 solution stored in glass

bottles in 90-100^ RH atmosphere 28

11. ^1b/^°B ratios (relative) of ANL-Il boric acid

solutions with known amounts of NaaCOs 3^ 12. iib/^°B ratios (relative) of ANL-II boric acid

solutions with known amounts of NaOH 35

13. Estimation of impurities {%) in separated

isotopes 41

14. Concentration of boron isotope solutions. .... 45

15. Determination of strong acid in boron isotope

solutions 46 16. Isotopic composition of separated boron

isotopes used in calibration samples 46

17. Composition of boron calibration samples 47

18. Determination of mass spec trometric bias 49 19. Observed and corrected ^^B/^°B ratios of the

SRM samples 50 20. Absolute isotopic ratios for the SRM samples. . . 50 21. Atom percents and atomic weights for the SRM

samples 51

1. Determination of the end-point in coulometric

titration of boric acid (Appendix 3) 59

vii
LIST OP FIGURES

PAGE

Figure No.

1. pKa of Mannltoborlc Acid as a Function of the

Concentration of Mannitol 4 2. Effect of Mannitol Concentration and Sample

Size on the Coulometrlc Titrations of

Boric Acid 7 3. Coulometric Titration Cell for Precise

Acidlmetry 10

4. Homogeneity of SRM 951, Boric Acid l4

5. Effect of Titration Sequence and Composite

Number on the Assay of SRM 951, Boric Acid. . . 1?

6. Effect of Relative Humidity in the Balance

Case on the Approach of Boric Acid to H3BO3

Stoichiometry (15 min exposure time) 20

7. Effect of the Humidity of the Environment on

the Assay of Boric Acid 21

1. Determination of the End-Point in Coulometric

Titration of Boric Acid (Appendix 3) 60

viii
Boric Acid

Isotopic and Assay

Standard Reference Materials

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

G. Marinenko, K. M. Sappenfield, and W. R, Shields

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

A precise coulometric titration method has been developed

for the assay of boric acid. The method is capable of providing boric acid assays precise to within 0.0025 percent

(1 Sigma) which is an order of magnitude better than existing

techniques. Using this method, a lot of high purity boric

acid has been assayed and found to be virtually stoichiometric

H3BO3. Its acidimetric assay value of 100.00 ± 0.01 percent

characterizes this material as a primary chemical standard for

boron determinations.

Multiple samples of boric acid were studied and characterized with respect to homogeneity and the effect of

relative humidity on stoichiometry . Procedures for precise

preparation, aliquoting and storage of boric acid solutions

were devised.

A mass spectrometric technique for the precise measurement of boron isotope ratios was developed. Single-filament

tantalum-ribbon sources are used, and ^^B/^°B ratios are determined by measuring the relative abundances of Naa^^BOa"*" and Naa^^BOa"^ ions, at masses 89 and 88, respectively. The

effects of various parameters such as sample mounting

procedure, filament material, sample size, total sample

composition and sodium concentration were studied and

alternative procedures were evaluated.

Ix
Absolute values were obtained for the Isotopic abundance

ratios of two boron isotopic standards, using surface emission

mass spectrometry. Samples of known isotopic composition,

prepared from nearly pure separated boron isotopes, were used

to calibrate the mass spectrometers. The resulting absolute

values are: SRM 951, Boric Acid, i^B/^°B = 4.04362 ± 0.00137;

and SRM 952, Boric Acid, ^^B/^°B = 0.053199 ± 0.000032. The

indicated uncertainties are overall limits of error based on

95 percent confidence limits for the means and allowances for

effects of known sources of possible systematic error.

Key words: Absolute ratios; assay standards; boron;

coulometry; isotopic standards; mass spectrometry.
INTRODUCTION

This publication describes in its entirety work by the

Microchemical Analysis Section and the Analytical Mass

Spectrometry Section of the National Bureau of Standards on the development and characterization of reference materials

for the chemical assay and isotopic composition for boron.

The work culminated in the certification of two Standard

Reference Materials. SRM 951, Boric Acid is a highly purified

boric acid of normal boron isotopic abundances, for which

both the acidimetric assay and the ^°B/^^B absolute abundance

ratio are certified. SRM 952, Boric Acid is an enriched

boron-10 boric acid which is certified in the same manner. Pinal certificates distributed with the samples are shown as Appendixes 1 and 2. The abundance ratio determination depends upon the now well established method of mass spec trometric comparison of the

material with standards prepared from high-purity separated

isotopic solutions. This procedure required the development

of a highly precise and accurate coulometric method which was also used for acidimetric assay of SRM 951, Boric Acid.

A . Development of a Coulometric Method for Assay of Boric

Acid

1 . Introduction

In aqueous solutions, boric acid is a very weak acid

(pK = 9.2). The hydrolysis of borate ions, in the course of

a titration with a standard solution of base (or with electrogenerated base, in the case of coulometry), precludes accurate

evaluation of the inflection point [ ( ApH/Aequiv . ) ] on the max potentiometric curve. Fortunately, borate ions can be com- plexed and effectively removed from solution; and, with the

decrease in borate concentration, the apparent dissociation

constant of boric acid increases.

1
The most widely used methods for the determination of

milligram quantities of boron involve titration of boric acid

complexed with some polyhydric alcohol (polyol) . These

methods are somewhat empirical, particularly with respect to the amount of polyol required, and are generally precise to only a few tenths of one percent.

For the determination of the absolute isotopic composition of standard boron samples, it was necessary to prepare

synthetic isotopic mixtures with an uncertainty less than

0.02^. A coulometric analytical method, developed by LeDuigou

and Lauer [1], approached this requirement in that a standard

deviation of 0.02% was obtained. However, a discrepancy of

0.1% between this method and conventional acidimetric

titrimetry was noted which they were unable to resolve.

Extensive experience in the NBS Microchemical Analysis

Section has established coulometry as perhaps the most

accurate and precise chemical assay technique providing

analyses of a variety of materials with an accuracy within a few parts in a hundred thousand. This provided the incentive

to investigate pertinent parameters to develop a coulometric

method suitable for the present analytical problem. This was successful and the research and method that was developed are described below and in a forthcoming publication [2].

2 . Investigation of Analytical Parameters

a . Effect of Mannitol on the Apparent Ionization

Constant of Boric Acid - For many years mannitol has been

widely used as a complexing agent for boron, and various

proposals have been made for the optimum mannitol/boron ratio.

In this work, the question of optimum ratio was studied in a precise methodical manner by the measurement of the pK of

a boric acid as a function of mannitol concentration. The

measurements were conducted in IM KCl because it was anticipated that further coulometric work would be conducted in

that electrolyte.

2
The results of the measurements are shown in Figure 1. The limiting value of the slope is:

dpK

= 2 dpC^ ^ mannitol

The highest concentration of mannitol investigated was l.OM,

which corresponds to a saturated solution at 25 °C.

The results show that varying the concentration of

mannitol can produce a whole spectrum of "mannitoboric acids"

with varying smoothly over five orders of magnitude (pK

a a from 4 to 9)- This fact was successfully utilized to determine

the relationship between the concentration of weak acid, K , and the difference between the inflection point and equivalence

point in a titrimetric determination. The results show that

Roller's equation [3j^] is equally applicable to the complex

mannitol-KCl-boric acid system as it is to the monoprotic and

polyprotic organic acids titrated in aqueous media.

Using Roller's equation, the error due to the difference

between the inflection point and the equivalence point is

given as

:

-300 K Error (%) = ^^a

where K is the dissociation constant of water, c is the

w

' concentration of borate at the end of the titration, and K ' a is the dissociation constant of boric acid in the given

medium. From a practical standpoint, above cK^ = 3xio~^,

a the error due to ApH between the inflection and equivalence

points is less than 0.0001^ and therefore negligible. All

titrations reported here were carried out under conditions

exceeding this minimum value.

b . Effect of Mannitol on the Electrochemical

Reduction of Hydrogen-ion - Previous work in this laboratory

has verified the accuracy of coulometric acidimetry conducted

in IM KCl medium. For example, measurements of the faraday

in this laboratory by reduction of hydrogen-ion in benzoic

3
4
acid and in oxalic acid dihydrate coulometers [5], under

conditions similar to those of the present boric acid experiments, yielded values of 96487.9 ± 2.5 and 96486.6 ± 3-4 abs

C (g-equiv)~^ , respectively. These results are in excellent

agreement with the value for the faraday of 96487.0 ± 1.6 abs

C(g-equiv)~^ , recommended by NAS-NRC [6]. In titrations of boric acid, however, the supporting

electrolyte is IM KCl plus mannitol , and it is necessary to determine if the addition of the mannitol has any effect on the electrochemical reaction at the platinum cathode.

Specifically, is the current efficiency 100^ with respect to the reaction H"*" + e = 1/2 H2 , or any equivalent reaction?

To answer this question, HCl solution, single crystal

oxalic acid dihydrate, and acid potassium phthalate were each

analyzed coulometrically in IM KCl, both with and without

mannitol. This choice of test materials was made to cover

a large range of pK values including the pK value for

a a mannitoboric acid (in 0.75M mannitol, the apparent K of

a boric acid is 4.17 x 10"^).

The results (Table 1) for each pair of analyses, with

and without mannitol, were identical to within 0.003^. It was therefore concluded that the accuracy of coulometric

acidimetry is unaffected by the addition of mannitol to the

IM KCl solution. Moreover, it was noted that the pH of the

inflection points on the potentiometric titration curves, obtained by use of the glass electrode - saturated calomel

I electrode system, is virtually unaffected by the presence of

mannitol.

c . Effect of Variation of Mannitol Concentration

- A number of samples of boric acid were assayed in the

following electrolytes: IM KCl - 0.25M mannitol, IM KCl - 0.75M mannitol, and IM KCl - 1.05M mannitol (saturated).

The results showed no significant difference (<0.005^)

between the three media for sample sizes of boric acid

5
Table 1. Effect of mannitol on electrochemical reduction of H"*" at platinum cathode. Acid

Hydrochloric

acid

Oxalic acid^

dihydrate^

Electrolyte Ratio IM KCI

2.18055 meq/g

2.18070

2.18032 Average 2.18052 meq/g

99.985 percent 99.985

99.983

99.987 99.987

99.983 Average 99.985 percent

IM KCI + 0.75M

mannitol 2.18105 meq/g

2.17992

2.18060 Average 2.18052 meq/g

99.972 percent 99.987 99.988 Average 99.982 percent

1.00000

1.00003

Potassium acid phthalate, SRM 84h

Single crystal chips

99.995

99.996 Average 99.992 percent*^ Average 99.995 percent 0.99997

The assay is the mean value obtained coulometrically during analysis of the material for NBS certification ranging from 1 to 12 meq (Figure 2). However, In the case of

the 0.25M mannitol solution, the slope of the potentlometrlc

curve near the equivalence point was too small to permit

precise evaluation of the end-point.

On the basis of the magnitude of the Inflection point

alone, an electrolyte containing 1.05M mannitol would be

preferable. However, this solution has a rather high

viscosity which Is troublesome when It Is forced through the

fritted glass disks of the coulometrlc cell and also presents

-some difficulty of mixing during the course of the analyses.

6
1 11

111 UJ • — RO

-J _j O TOI o o

LxJ h- h- M 7 EL INN INN NNI

RTo If) CL q c OL d1 3

C/) 1 1 —

OF o o < 2 ~~

TIO o • oa

OdIAlO ~ 0 o

-— ccp o1 1 odo oodo

CM

CD

(N 0E -s

00 liJ 1 N <5

(0 «

liJ =o

"o-«—'

"c:c=rao

7o*AVSSV
Hence, as a matter of convenience, with no significant loss

in the sensitivity of the end-point determination, a concen- tration of 0.75M mannitol was used in the subsequent work.

During the above-described tests, the boric acid sample

size was varied from 0.02 to 1.0 g. Over this range of

sample sizes, assay values were identical.

d. Evaluation of Analytical Bias - Boric acid of

known purity was not available for evaluating the accuracy

of the method described in this work. However, the absence

of analytical bias due to composition of electrolyte or sample

size supports the conclusion that the method is free of bias,

with an accuracy limited only by the random measurement errors. The various measurements related to this work were made

with a high degree of reliability (see Section A.3.C.,

Constants and Standards), and numerous determinations of boric

acid have shown that a relative standard deviation of at most

0.003 percent may be expected. This precision is consistent

with that obtained for a wide variety of coulometric procedures

developed in this laboratory.

3 . Coulometric Procedure for Assay of Boric Acid

a. Introduction - The coulometric acidimetric

titration method has been described previously [5]. It consists of the reduction (at the platinum cathode) of

hydrogen-ions derived from the weighed acid in a suitable

supporting electrolyte according to the reaction:

h"^ + e~ = 1/2 H2 . A glass electrode - saturated calomel electrode system

is used for measuring the change in hydrogen-ion concentration

as a function of the passed charge, to identify a differential

potentiometric inflection point. The supporting electrolyte

is initially acidified to facilitate the removal of COz,

swept with scrubbed nitrogen, and then pretitrated to a pH of

6.98 by passing small increments of charge. The charge

consumed in going from the end point of pretitration to the
differential pH inflection point is then taken to be equivalent to the amount of hydrogen-ion introduced with the

weighed acid.

b. Coulometric Cell - The coulometric cell used

in this work is shown in Figure 3. The anode and cathode

compartments are l80-ml electrolytic beakers connected

horizontally by a 25-mm o.d. glass tube approximately 6 cm

in length. Three sintered glass disks, one each of coarse, medium and fine porosity, are sealed in the connecting tube.

Each of the two intermediate compartments between the disks

has a tube with a stopcock which enables it to be filled or emptied by the application of a vacuum or nitrogen pressure.

The volume of the compartment between the coarse and the

medium porosity disks is about 10 ml while that between the

medium and fine porosity disks is about 20 ml.

There are two advantages in using a cell of relatively

small volume. Conventional end-point detectors, including

the glass electrode system, are sensitive to concentration

changes. With a decreased volume, a desired change in con- centration can be achieved with a smaller amount of reagent.

That is, for the same amount of reagent, the sensitivity of

the detector system varies inversely with the volume. The

second advantage of a smaller volume arises from the fact

that, concentrations being the same, the total amount of

impurities in the supporting electrolyte varies inversely

with the volume.

The molded polyethylene cathode compartment cover can be

snugly locked onto the beaker by means of a polyethylene lock

ring. Both the covers and the lock rings are commercially

available

The following accessories are mounted in the cover of

the cathode compartment: a combination glass electrode - saturated calomel electrode, an inlet tube to allow nitrogen

to be passed over the solution, an outlet tube for the

9
Figure 3. Coulometric Titration Cell for Precise Acidimetry.

1. polyethylene top; 2. polyethylene lock ring; 3. combination glasscalomel electrode; 4. nitrogen inlet tube; 5. nitrogen outlet tube;

6. platinum cathode; 7. liquid sample delivery and cell rinse assembly;

8. polyethylene spray shield; 9. 3% agar gel in 0. IM KCI;

10. silver anode. 10
nitrogen, a platinum coil cathode and a polyethylene tube with

polyethylene bottle. The polyethylene tube-bottle system can

be moved up and down to permit withdrawal of the solution from

the cell. The withdrawn liquid may then be squirted back into

the cell to rinse any material deposited on the cell walls and

top as a result of the liberation of gas during the electrolyses. The system is also used for introduction of samples in

the form of solutions.

A polyethylene cap is mounted about 1 cm above the top

edge of the cathode. The purpose of this cap is to catch the

spray resulting from the liberation of gaseous hydrogen at the platinum cathode.

The constant-current and timing circuits have beeK

described elsewhere [5]. The charge passed through the

coulometric cell (in excess of 100 coulombs, corresponding

to about 1 meq of sample) can be measured accurately to a few tenths of a millicoulomb

.

c. Constants and Standards - A number of constants

and standards were used in this study. These are described

or defined below.

1) Resistance: A ten-ohm resistor calibrated

by the NBS Resistance and Reactance Section, was used. The

value at 25 °C was certified to the nearest 10 \iQ. and the

equation

T^ = R25 [1 + a (t-25) + 3 (t-25)']

was used to correct for the small temperature deviations

encountered in the work.

2) Voltage: A set of saturated Weston cells,

which were calibrated by the NBS Electrochemistry Section and

kept in an air thermostat, was used as the emf reference

source. The value of the most reproducible cell in the set

is known to within ±0.8 yV and this cell was used as the

primary emf reference.

11
3) Current: The ampere used in this work

was based on the volt and ohm as maintained by the NBS

4) Time: The time measurements were made by means of a 100-kHz quartz crystal oscillator time-Interval

meter which was calibrated against the WWV frequency and

found to be accurate to better than within 0.001^.

5) Mass: The weights used In this work were calibrated by the NBS Mass and Volume Section. The mass of

each Individual weight was known to the nearest microgram.

6) Air Density: The density of air was com- puted on the basis of measurements of atmospheric pressure,

temperature, and relative humidity.

7) pH: The NBS phosphate buffer, SRM l86b,

(pH = 6.865 at 25 °C) was used in the standardization of the

glass electrode - saturated calomel electrode system.

d. Procedure - The samples of acid were weighed

by substitution on a 20-gram capacity, unequal-arm, constantload mlcrobalance . The standard deviation of the weighing

process was 0.003 mg. The weighings were free of systematic

error except, possibly for the uncertainties in the standard

weights, which were less than 0.003 mg. The weight of each

sample was corrected for air buoyancy, based on the density

of the air at the time of the weighing.

In general, the titration procedure is the same as that

described by Marinenko and Taylor [5]. The stirrer was operated only during the dissolution of the samples . It was

not used during the titration for the following reasons:

(1) to eliminate swirling of gas bubbles liberated at the

cathode into the bulk of the solution, thereby minimizing

spray-loss of the sample; (2) to minimize the transfer of

impurities in the supporting electrolyte to the electrode

surface; and (3) to minimize transfer of the sample into

the intermediate compartments.

12
After electrolysis at about 102 mA, for a predetermined

period of time sufficient to allow reaction with all of the

sam.ple if it were 100.00% stoichiometric H3BO3, the coulometric current was interrupted. The intermediate compartments

were flushed and the cell wall and cover were rinsed in the manner previously described. The pH of the solution was measured to the nearest 0.001 pH unit and recorded. Then a small amount of HCl was added to the cell to return the system

to a state well in advance of the equivalence point and, by

successive passage of charge increments and measurements of

pH, the equivalence point, identified by (ApH/AC) is max established. This procedure may be repeated as often as desired to increase the precision of locating this point.

A correction is then applied for the difference between the

pH of the system at the end of the titration and that of

(ApH/AC)^^^.

A step-by-step description of a complete typical analysis

is given in Appendix 3- B. Chemical Characterization of SRM 951, Boric Acid

1 . Acidimetric Assay

This lot of boric acid was prepared to ensure material

of high purity and homogeneity. It consisted of 550 pounds

of acid prepared at one time and was received in 22 containers

of approximately 25 pounds each. A preliminary spectrochemical

survey analysis indicated a high degree of freedom from

contaminants in that no metallic impurities were present at a greater concentration than a few parts-per-million

.

The coulometric assay of this material was done using

samples which had been equilibrated at 50% RH as described in

Section B.2. Initially, 600-mg samples were used but this was later reduced to 200 mg to conserve analytical time, since

earlier experiments had shown no significant differences in

assay due to sample size.

13
Representative samples of approximately 50 g were taken

from each of the containers and these were screened by a preliminary coulometrlc analysis. The results of these

determinations are given In Figure 4 and Table 2. The Initial

100.010 1 1 1 r tf

T

1 r

100.000

Oi

99.990

OI

' I ' ' I I I I I 1 1 \ 1 0 4 8 12 16 20 24

BOTTLE NO.

Figure 4. Homogeneity of SRM 951, Boric Acid. NOTE: •200 mg Samples O 600 mg Samples

Numbers adjacent to the points in the figure represent the sequence

of repeat measurements. 1^
Table 2. Homogeneity of SRM 951, Boric Acid.

10-31-67 17 99.9889 11- 2-67 12 99.9999 11- 3-67 1

1 AA AA A A 100.0022

4

1 AA AA A A 100.0024

9 An AATA

99.9978

11 n ri 11- 6-67 13 AA AAAT

99.9997

2

1 A A AA 1 A 100.0010 11- 7-67 3 AA AAAA

99.9983 11- 8-67 22

A A A A il 99.9964

18 100.0032 11- 9-67 16 99.9910

19

A A A A T n 99.9975 11-13-67 7

1 A A AAA il 100.0004

10 AA AAAA99.9896

11 in ri 11-14-67 20 AA AA/*n 99.9960

1

1

11 AA AAAO

99.9993 11-15-67 21

A A AAA A 99.9990

5

1 A A AA OA 100.0028

15

1 AA AA T

A

100.0078

14 inn ftnnr 100.0005 n11—10—0/ ifi fi7 00 inn nn/19 iuu.uuh^

c0 inn nn^7 lUU.UUo/ ii_?n_fi7 1 1 — iU—0/ lU inn nn9n

15a 100.0089 16a 99.9955 11-21-67 17a 99.9975 15a 100.0055 Average 99.9989^

<r = 0.0044k>

Repeat runs. Based on initial runs, excluding Sample No. 15.

15
results for all containers, except numbers 10, 15, 16 and 17,

fell within 0.005^ of the mean which was considered to be a limit of acceptability. These four samples were reanalyzed.

The duplicate analysis for the three samples which produced

low assay values on the first runs (nos. 10, 16 and 17) fell

within 0.005% of the mean. However, triplicate analysis of

samples from container No. 15 remained outside the 0.005%

limit. The material in this container appears to have

slightly different composition from the others, perhaps due

to the conversion of a minute amount of boric acid to boric

oxide. Accordingly, this container has been set aside, and

the material will not be used as SRM 951.

The overall acidimetric assay value may be obtained from

a consideration of the analyses described above. From the

Initial analyses of the 22 containers (excluding container

number 15), an average assay value of 99.9988 ± 0.0020% is

obtained where the uncertainty represents the half width of

the 95% confidence interval. The standard deviation of these

measurements about their mean was 0.0044 which includes

components of variation due to both measurement and compositional variations

.

In a further evaluation of the assay of SRM 951, Boric

Acid, the samples from the 22 original containers were combined into five composites, containing equal amounts of

material from each bottle. The make-up was as follows:

Composite I - 5, 6, 8, 15, 18 (the five containers that gave

the highest results) i Composite 11 - 10, 16, 17, 20, 22 (the five containers that gave the lowest results);

Composite 111 - 1, 2, 4, 7, 12, l4; Composite IV - 3, 9, 11,

13, 19, 21; Composite V - equal amounts of I, II, 111 and IV. Figure 5 and Table 3 show the results of replicate analyses

of the composites. Each symbol represents an individual

titration plotted with respect to the sequence in which

titrations were performed.

16
100.01

5 100.00 <

99.99

^

COMPOSITES

O COMPOSITE I

COMPOSITE m. 0 COMPOSITE IE Q COMPOSITE HE

o nD (a

O

6 9 12 TITRATION SEQUENCE

15 18

Figure 5. Effect of Titration Sequence and Composite Number on the Assay of SRM 951, Boric Acid.

Table 3. Assay of SRM 951, Boric Acid (equilibrated at 34% RH for at least 30 min). Run No. Composite No. Assay,

%

Run No. Composite No. Assay, %

1 V 100.0008 10 IV 100.0009

2 V 99.9978 11 IV 100.0002

3 I 99.9990 12 IV 100.0012

4 I 99.9941 13 IV 99.9961

5 II 99.9983 14 I 100.0042

6 II 99.9992 15 II 100.0007

7 III 99.9998 16 III 99.9973

8 III 100.0024 17 III 100.0025

9 IV 99.9987 18 II 100.0006 Average 99.9997 <^ = 0.0025 0.0013 17
The average assay of each composite and the grand average

of all measurements of the composites is shown in Table 4. The standard deviation of the individual measurements of the

composites is from their respective average values and is

based upon a total of 18 measurements providing a total of 13

degrees of freedom for the estimation of a. This represents

precision estimate for a single coulometric assay of boric

acid

Table 4. Assay of SRM 951, Boric Acid composites.

Composite I Composite 11 Composite III Composite IV Composite V

99.9990 99.9983 99.9998 99.9987 100.0008 99.9941 99.9992 100.0024 100.0009 99.9978 100.0042 100.0007 99.9973 100.0002

100.0006 100.0025 100.0012 99.9961 Average 99.9991 99.9997 100.0005 99.9994 99.9993 Average of all determinations: 99.9997 Within-composite a (pooled): 0.0027 Half-width 95%-confidence interval for mean, based on within-composite variations only: 0.0013 The excellent agreement between the average assays of the

composites and the excellent agreement of this value with that

obtained from the determinations of the original containers

leads to considerable confidence in both the overall assay

and the high degree of homogeneity of this standard reference

material. The two series of assays allow a semiquantitative

estimate to be made of the latter. In the first series,

Oi = a.^ + 0^ where 0\ is the overall standard deviation,

a is the standard deviation of measurement , and a is the m ' c standard deviation of the component of between-container

varaibility. Taking a = 0.0025 from measurements of the

composites, and Oi = 0.0044, a value of = 0.0036 may be

calculated

18
For final certification purposes, an acidimetric assay

value of 100.00 ± 0.01 has been established. The limit of

uncertainty is believed to be a conservative estimate of any

deviations in composition that might be encountered due to errors of assay, inhomogeneity of material, or departures

from stoichiometry due to absorbtion of moisture under

adverse conditions.

2 . Effect of Relative Humidity on the Stoichiometry of

SRM 931, Boric Acid

The study of the behavior of this lot of boric acid

Indicates that the material is somewhat deficient in water.

Apparently it was prepared under conditions which resulted

in the formation of small amounts of boric oxide. This

conclusion is based on the fact that samples weighed directly

from the original containers and then left standing in a room

at 28^ relative humidity for 30 minutes showed significant

increases in weight (Table 5).

Table 5. Absorption of water by boric acid.

Sample

number

Initial wt

of H3BO3

Wt of H3BO3

after 30-min

exposure to 28% RH AW X 106g

Change in weight, percent

1 1.351348g 1.351484g + 136 + 0.010

2 1.340345 1.340394 + 49 + 0.004

3 0.934555 0.934655 + 100 + 0.011

4 1.685807 1.685968 + 161 + 0.010

5 2.249267 2.249379 + 112 + 0.005

6 1.343683 1.343823 + 140 + 0.010

Average + 0.008%

19
The data in Figure 6 show that at 28% RH the time

required to weigh out a sample ('^15 min) was not long enough

for the material to absorb sufficient water to attain stoichiometric composition. At 3^% RH this same weigh-out time

allowed sufficient water adsorption so that assays of the

material corresponded exactly to H3BO3 Once the stoichiometric amount of water had been absorbed, exposure of the

material to even a very dry atmosphere did not result in a loss of water.

100.01

100.00

99.99 -

27 29 31 33

RELATIVE HUMIDITY IN THE BALANCE CASE .%

35

Figure 6. Effect of Relative Humidity in the Balance Case on the Approach of Boric Acid to H3BO3 Stoichiometry (15 min exposure time). 20
The effect of humidity was further tested by placing

boric acid samples In various controlled humidity environments for periods of at least four days with occassional

stirring of the crystals. The results of these tests are given In Figure 7- Within the 0 to ^2% RH range, the assay

of boric acid remained constant and close to stoichiometric

H3BO3. At relative humidity greater than 52^, assay values

decreased, reaching 99.98% at 90% RH . The assay decrease

Is undoubtedly due to the adsorption of water on the surface

of the polycrystalllne boric acid.

100.01

100.00

y 99.99

99.98 -

99.97 -

20 40 60 80

RELATIVE HUMIDITY, 7o

Figure 7. Effect of the Humidity of the Environment on the Assay of Boric Acid. 21

100
3 . Determination of Strong Acid In Crystalline Boric

Acid

For accurate coulometric analysis of boric acid solutions

it is necessary to either prove that the titrated hydrogen ion

is contributed totally by the boric acid or to correct for any

hydrogen ion contributed by a contaminating acid. In other

words, it is necessary to test the unknown solution for the

presence of acids other than boric acid; and, pragmatically,

this reduces to a test for strong acid. A simple procedure

such as direct titration of strong acid in the presence of

boric acid in IM KCl solution is not sufficiently sensitive

because of the buffering effect of the boric acid. Tests

showed that under those conditions, it was impossible to resolve less than a few hundredths of a percent of strong

acid in the boric acid. Preconcentration of the strong acid

by extraction of boric acid into an organic solvent was unsatisfactory; tests showed that some of the HCl is also

lost in the process.

The procedure finally developed is based on the 20-fold

decrease in solubility of boric acid in water between 100

and 0 °C. A special sealed tube was devised to recrystallize

the boric acid from which the supernatant liquid could be

removed for determination of its strong acid content by direct

titration. The results of analyses of SRM 951, Boric Acid,

show that it contains 5-5 ppm of strong acid. Thus, after

the correction for the strong acid, the B/H ratio of this

material is 0.999982 on a molar basis.

The above procedure requires large amounts of material

(1 to 2 g) and, because of cost considerations and the availability of only small amounts of material, could not be

directly used in the evaluation of the st oichiometry of

H3^°B03 and Ha^^BOa isotopically "pure" samples. Consequently

a micro method had to be developed. The key to this problem

was to establish the amount of strong acid in the SRM 951,

Boric Acid, as described above. The solutions containing

22
known widely varying concentrations of boric acid were prepared and their pH's measured. A three-dimensional calibration

graph was constructed relating pH, concentration of boric

acid, and concentration of strong acid (HCl). Then, knowing

accurately the total acid concentration, from coulometric

analyses, and the pH of the boric acid solution, one can calculate the concentration of strong acid by the method of

successive approximations. The sensitivity of the threedimenstional calibration graph is such that 0.01^ to 0.001^

of strong acid in the original sample of boric acid can be

determined with an accuracy of a few relative percent.

C . Assay of SRM 952, Enriched Boric Acid

This standard reference material is an enriched boron-10

boric acid which was furnished by the Oak Ridge National

Laboratory. The material as received was not deemed to be

of sufficient purity for use as a standard reference material.

Accordingly, it was recry stallized twice from a water solution

to assure homogeneity and freedom from metallic impurities,

and then dried at room temperature.

The purified acid was assayed by coulometric titration

as described above. The results are shown in Table 6 in

Table 6. Assay of SRM 952, Boric Acid. Sample No. Assay, %

1 99.968

2 99.971

3 99.982

4 99.968

5 99.968

6 99.969

7 99.973

8 99.958

Average 99.970 a = 0.0066 ^= 0 0056
which an average assay of 99-971^ is indicated. An assay of

99.97/^ ± 0.02 is certified where the uncertainty limit is

believed to be a conservative estimate of variation of

composition of the material due to all causes. The deviation

from stoichiometry results from the fact that the standard

reference material is slightly contaminated by occluded mother

liquor. Accordingly, the assay value should be used in the case of direct weighing for a "spiking" material for boron

assays in mass spectrometry.

D . Preparation and Aliquoting of Separated Boron Isotope

Boric Acid Solutions

One of the basic aims of this study was to develop a boron isotopic standard to be used in the absolute calibration of mass spectrometers. Toward this aim, the following

subjects had to be investigated: the precision of aliquoting

relatively small amounts of boric acid solutions; suitable

conditions for accurate preparation and storage of boric acid

solutions without significant changes in the composition of

the solutions (storage for approximately one week); a reliable

procedure for delivery of boric acid solution aliquots into

the coulometric cell; evaluation of any possible bias which

might be associated with the size of the aliquots; and, as previously described, development of a method for the determination of small amounts of strong acids in small samples

of boric acid solutions. Composite II of the SRM 951, Boric

Acid, was used throughout the exploratory part of this work.

The procedure developed for the aliuqoting of solutions

[7] involves the use of 10-ml polyethylene syringes as weight

burets. After the syringe is filled, the needle is removed and

the syringe is closed with a special cap. The solutions were prepared of such concentrations that aliquots from 1 to 10 ml

in size would bracket the sample sizes of separated isotope

solutions required for the accurate preparation of the boron

isotopic ratio standard solutions (approximately 0.3 meq/g of

solution)

.
Prom such a syringe, solutions were weighed by difference

Into 4 oz polyethylene bottles. A closed syphon arrangement,

shown In the cell assembly (Figure 3) was constructed which

fitted tightly over the bottles and connected to the coulometric titration cell. With this arrangement, solution

samples could be quantitatively transferred into the cell

without the need for disassembling it. Such an arrangement

is highly desirable since, in this closed system, nitrogen

flowing over the electrolyte keeps atmospheric carbon dioxide

out of the cell.

Initially two sets of experiments were conducted to establish the precision of aliquoting, transfer and titration.

Equal-size samples of boric acid solutions were aliquoted on the same day and titrated soon thereafter (1 to 2 days later).

The results of these analyses are summarized in Table 7. Prom

these data the standard deviation of a single determination is

0.000052 meq/g of solution. On a percentile basis, this

amounts to approximately 0.017^.

Table 7. Precision of aliquoting and analysis of boric acid solutions. Aliquot

Solution No. Wt of aliquot, g meq found meq/g

1 1 10.95358 2.988735 0.272855

2 10.05553 2.742278 0.272713

3 10.45754 2.852758 0.272794

4 10.27975 2.804252 0.272794 Average 0.272789

II 1 10.73511 2.964446 0.276145

2 10.59936 2.926541 0.276105 Average 0.276125

Estimated wittiin-solution precision, based on

4 degrees of freedom: a — 0.000052 25
Subsequently the procedure was refined and a study was done on the effect of the aliquot size on the analytical

determination of boric acid. The samples were weighed on the same day and the results are given in Table 8. It can be seen from these data that there is no systematic difference

between analytical results for the larger and smaller aliquots

(within 0.01^)

.

Table 8. Effect of the size of boric acid solution aliquot on the coulometric determination of boric acid concentration.

Solution Aliquot No. Wt of aliquot, g meq found meq/g

b 3 10.20413 2.834423

4 2.02505 0.562549

5 2.20348

n r 1 ono

0.612081 0.277779 6^ 1.14274^ 0.324266^ 0.283762**

7 1.17222 0.325603 0.277766

U.ll H 10

1 1 6.828209 1.193722 0.174822

2 8.370246 1.463594 0.174857

3 8.755848 1.531033 0.174858

0.174846

II 1 8.493900 1.281755 0.150903

2 8.303518 1.253004 0.150900

3 21.30825 3.215547 0.150906

4 9.040497 1.364261 0.150906

0.150904

This sample is excluded from consideration. A gross error (2%) was apparently involved in handling it.

26
E . Containers and Conditions for Stable Storage of Boric

Acid Solutions

The storing of boric acid solutions in polyethylene

bottles stoppered with rubber septa and left at room atmosphere conditions was found to be unsuitable. This conclusion

is based on the analytical data given in Table 9. Solution A was prepared and aliquoted on December 7, 1967, then aliquoted

again on December 12 and on December 15. The data in Table 9 show the average concentration of the solution on the various

days of aliquoting. The increase is obviously due to evaporation. The nonlinearity of the increase is understandable

from the fact that the volume of the solution was constantly

being depleted by the removal of aliquots, so that a constant

rate of evaporation would yield an ever-increasing percentage

loss per unit time.

Table 9. Concentration changes of solution (A) stored

in polyethylene bottle in room atmosphere.

Date of aliquoting Concentration, meq/g

December 7 0.277666 December 12 0.277778 Decembers 0.279337

storage of boric acid solutions in glass or Teflon at 90 to 100^ RH appears to be totally satisfactory as illustrated

by the data in Table 10, there was no significant change in

concentration over the testing period of 20 days.

The last column in Table 10 shows percent recovery.

These values are based on the ratio of meq HaBOs/g of solution

calculated from the weight of boric acid originally used in

the preparation of the solution to the meq HsBOs/g of solution

found by coulometric analysis of each aliquot. The average is

100.0025, with a 95% confidence interval (df = 7) of 0.0021%.

27
Since the assay of the solid boric acid is 99-9997, with a 95^ confidence interval (df = 17) of 0.0013f«, the difference

is not significant.

Table 10. Stability of H3BO3 solution stored in glass bottles in 90-100% RH atmosphere. Date of aliquoting - 3-68 - 3-68 -23-68

-23-68

- 4-68 - 5-68 - 5-68

Solution No.

1

1

1

1

2

2

2

2

meq/g calc.

0.277604

0.277604

0.277604 0.277604

0.150901

0.150901

0.150901

0.150901

meq/g found

0.277609 0.277622

0.277617 0.277603

Recovery, %

100.0018 100.0067 100.0047 99.9996

F . Development and Evaluation of a

0.150903 100.0013 0.150900 99.9996 0.150906 100.0034 0.150906 100.0029 Average 100.0025 t95^

Mass Spectrometric= 0.0021%

Technique for Boron-Isotope Ratio Measurements and

Evaluation of Possible Alternative Techniques

1 . Introduction

Since the large percentage mass difference between ^ "b"*" and ^^B might cause prohibitive fractionation problems, the

boron isotope ratios were measured using the molecular ion

NaaBOa"*". The efficiency of production of this ion by surface

ionization techniques was readily affected by variations in

the chemical preparation, sample mounting procedure, filament

material, sample size, total sample composition, sodium

content, and filament temperature; and the development of a precise mass spectrometric technique necessitated a careful

study of numerous variations and permutations of these

parameters

.

28
Argonne - II (ANL-II) boric acid was used as a reference

standard in the testing and evaluating of all of the chemical

and mass spectrometric parameters. Unless otherwise stated,

all samples contained 1 mg of boron per ml of aqueous

solution, with sufficient NaaCOs added to give an atomic B:Na

ratio of 1:1. In preliminary tests NaOH was tried as a Na

source, but this proved inferior to NaaCOa in the mass spectrometric analyses.

Preliminary work showed very quickly that mass spectrometric results varied quite drastically when different chemical

preparation techniques were used. Studies showed that there

was no isotopic fractionation during the chemical preparation

but that differences in chemical preparation caused differences in the ionization efficiency of the mass spectrometric

technique and these, in turn, caused variations in the biases

involved in the surface ionization phenomenon. Consequently

it became obvious that it would be necessary to use a meticulously consistent preparation procedure for all samples.

2 . Sample Mounting Procedure

A sample was mounted on a tantalum-ribbon (1x30 mils)

filament in the following manner: one small drop of solution

(^5 yg B) was placed on the filament and dried with a heat

lamp and a 1.2 A current for 5 minutes; the lamp was turned

off and the current was increased until the sample color

changed to gray-white (1.5 - 1.8 A); the current was maintained for 2 minutes after the color change. This procedure

was selected over other variations because it led to good mass spectrometric performances and because the easily recognizable

fiduciary point (the color change) allowed for consistency in

the procedure.

Solutions which were dried only with a heat lamp, or with a heat lamp plus an electrical current of less than 1 A,

usually lead to either intense ion currents that decayed too rapidly for precise measurements or ion currents that were

not stable enough for precise measurements.

29
Solutions that were acid or neutral gave very low intensity ion currents (10~^^ A). It appeared that the boron was

lost through sublimation when these solutions were dried on the filaments. Basic solutions gave 10" -^^ A ion currents

even when the filament was heated to a dull red color during

the sample mounting. Limited experimentation with solutions

with pH greater than 8 did not reveal any significant prospect

for improving the analytical technique.

Sample mounting was performed in a hood and the variable

draft made it impossible to impose rigid temperature controls

on the drying procedure. Best results were obtained when the

hood door was adjusted so that the color chang'e in the sample

occurred between 1.5 and 1.8 A. It was found that gross

changes in the volume of air flowing across the filament

could cause the color change to occur outside of the normal

1.5 - 1.8 A range.

3 . Filament Material

Of the four materials tested (Ta, Re, W, Pt), tantalum

was selected as the filament material because its ionization

efficiency was most consistent over a wide range of sample

mounting conditions and it was least sensitive to variations

in total sample composition. In addition, tantalum filaments

showed no strontium background (<10~^^ A) even at a temperature of 950 °C. No pre-baking was necessary. Both rhenium

and tungsten had lower ionization efficiencies than tantalum

and both required pre-baking in order to reduce the strontium

background to <10~-'-^ A. Platinum filaments showed no strontium background at temperatures below 1000 °C. Efforts to develop a triple-filament technique using

any of the four filament materials were unsuccessful; . the

ion currents were usually unsuitable both in magnitude and

stability

.

Preliminary investigations with platinum filaments

indicated that it is more efficient than tantalum for surface

ionization of boron but is extremely sensitive to sample

30
mounting conditions, total sample composition, and "age" of

the solution. Efforts to demonstrate that the apparent gross

changes In Ionization efficiency are due to Impurities which

produced a poisoning or suppressive effect on the Ionization

process were unsuccessful. Investigations subsequent to the

completion of the measurements of the absolute Isotoplc

compositions of the two boron standards (Section G) have

revealed that the use of platinum filaments gives excellent

results when the B/Na ratio of the samples Is >1. Although

boric acid dissolved In water without any addition of sodium

and solutions with B/Na = 1 did not yield Ion currents of

sufficient Intensity or duration for good analyses, solutions

with B/Na ratios from 2 to 80 yielded excellent analyses. It appears that the use of platinum filaments for the surface

Ionization of boron may eventually produce the best results

but more work Is needed to define the optimum conditions.

4. Sample Size

Sample sizes ranging from 5 to 100 yg were tested to determine the optimum amount of boron necessary for a good

analysis. The experiments Indicated that sample sizes >20 yg

had a greater frequency of erratic emission and signal

Instability. The larger samples did not have long periods

of signal growth, as might have been expected, but yielded

decaying signals, as with smaller samples. In general It was more difficult to maintain standard Intensities with larger

samples than with smaller samples.

The final prepared sample size was 5 yg. This amount of

boron yielded the same magnitude of Ion currents as the larger

samples without any detectable Increase In fractionation.

Ion signals were generally very stable and rejection of an analysis because of erratic emission was very rare.

5 . Total Sample Composition

The presence of extraneous material In the boron samples

was detrimental to the analytical precision of the mass

31
spectrometry and was the most probable cause for some of the

observed variations in ionization efficiency. In a series of

tests, barium, calcium, and lead were added to ANL-II solutions

in sufficient concentrations to give 20 to 40 yg sample sizes

of known impurity per sample loading. The general effect of

these impurities was to require higher filament temperatures

to produce the standard (3 x io~^^ A) NazBOa ion currents.

This in turn resulted in observed ^^B/-^°B ratios significantly

higher than those observed in analyses of high purity ANL-II

solution, reflecting significant depletion of ^ °B under the

higher-temperature conditions.

In another series of tests approximately 2 yg aliquots of

strontium were added to several tantalum filaments along with

the normal 5 to 20 yg boron samples. There were no detectable

(<10~^^ A) strontium signals at temperatures <950 °C.

6 . Sodium Concentration

The effect of sodium concentration on the production of

NaaBOa"*" was evaluated by preparing samples of ANL-II boric

acid with known B/Na ratios. This experiment was conducted

twice, over an l8 month interval, with two completely different sets of solutions. The purpose of the second set of

experiments was to provide critical evaluation of previous

conclusions formulated during the fluid stages of development

when data was acquired under conditions where the criticalness

and control of all parameters were not firmly established.

In the first experiment sufficient sodium, as NaOH, was added to solutions of ANL-II to yield solutions with B:Na

ratios of 2:1, 1:1, 1:2 and 1:4. All of these solutions

were analyzed under the conditions of standard ion current

(1 - 2 X 10"^^ A) and restricted filament current (1.15 - 1.30 A); results from analyses which could not meet these

requirements were rejected. Under these conditions the

observed ^^B/^°B ratios of the solutions with B:Na ratios of

1:1 and 2:1 were nearly identical and did not change grossly

32
even after the filaments had been heated for several hours.

The observed ^^B/^°B ratio of the solution with a B:Na ratio

of 1:2 was 1 percent higher than that of the B:Na ratio of

1:1 and 2:1 solutions and was observed to change with time.

However, under the standard conditions the rate of change

was sufficiently slow so that precise ratio measurements

could be made. For the solution with a B:Na ratio of 1:4

the NaaBOa"*" ion current was completely suppressed at normal

filament temperatures. Even at a filament current of 1.5 A,

the ion signal was only 1 x io~^^ A and decayed too rapidly

to make precise ratio measurements.

On the basis of these observations, it was postulated

that sodium ions, which are formed with a high degree of

efficiency on most filament materials, were so voluminous for

solutions with excess sodium that suppression of the NaaBOa

ion occurred. In order to obtain the standard signal intensity (1 - 2 X io~^^ A) it was therefore necessary to have a higher filament temperature. The higher filament temperature

caused an increase in the rate of isotopic fractionation and

the observed ^^B/^°B ratios reflected the consequent depletion

in ^ °B abundance. It was therefore concluded that excess sodium was to be avoided and that the absolute isotopic

abundance measurements would have to be made with solutions

that all had only one B:Na ratio - either 1:1 or 2:1.

The second experiment to evaluate the effects of sodium

on the production of NaaBOa"*" was performed after completion

of the measurements of the absolute isotopic compositions of

the two boron standards (Section G), when the total analytical

experience with this element was more than 1 year. Two groups

of ANL-II solutions were prepared under identical chemical

conditions. NaOH was added to one group and NaaCOa was added

to the other group. Within each group there were solutions

with B:Na ratios of 2:1, 1:1, 1:2 and 1:4. In the mass spectrometric measurements the filament currents were adjusted

33
to give specific temperatures as measured with an optical

pyrometer. The filament current was not restricted to a particular range as in the first experiment nor was the ion

current set to a specific magnitude as in the first experiment, although at the temperatures chosen the ion current was generally in the standard range (1 - 2 x lO"-^^ A).

Duplicate analyses were made on each sample and within each

analysis ratio measurements were made first at 750 °C and

then at 780 °C. The results are summarized in Tables 11

and 12. The ratios given are not absolute; they have been

corrected for the ^^0 effect but not for filament bias.

Table 11. ^^B/^^B ratios (relative) of ANL- 1 1 boric acid solutions with known amounts

of Na2C03.

Ion current (A)

at 780 °C

2.5-1.5 X lO-is

3.0-2.2 X lO-is

4.0-3.0 X lO-^s

5.0-3.0 X 10-12

B/Na 11B/10B Ion current (A) 11B/10B

Ratio at 750 °C at 750 °C at 780 X

2:1 4.0362 2.0-1.0 X lO-is 4.0372

4.0339 1.2-0.8 X lO-is 4.0367 4.0350 4.0370

1:1 4.0368 2.5-1.5 X 10-12 4.0416

4.0379 2.0-1.6 X lO-is 4.0417

4.0374 4.0416

1:2 4.0375 5.0-4.0 X lO-is 4.0635

4.0379 3.2-2.0 X lO-is 4.0574

4.0377 4.0604

1:4 4.0477 7.0-5.0 X 10-12 4.0686

4.0448 2.0-1.2 X 10-''2 4.0715

4.0462 4.0700

1.7-1.4 xlO-""!

1.3-1.1 xlO-ii

1.7-1.4 xlO-ii

7.4-6.1 x 10-''2

34
Table 12. '"'B/^°B ratios (relative) of ANL-I I boric acid solutions with known amounts

of NaOH. B/Na hb/^ob Ion Current (A) hb/i^B Ion Current (A) Ratio at 750 X at 750 X at 780 °C at 780 X

2:1 4.0301 1.2-0.8 X lO-is 4.0336 2.0-1.6 x 10-12

4.0318 1.2-0.6 X 10-12 4.0308 2.5-1.5 x lO-^s

4.0310 4.0322

1:1 4.0391 2.5-1.9 x 10-12 4.0443 6.4-4.5 x IO-12

4.0377 2.0-1.6 x IO-12 4.0407 5.0-2.5 x IO-12

4.0384 4.0425

1 : 2 4.0408 2.0-1.5 x 10-''2 4.0594 7.0-4.0 x 10-''2 4.0483 3.2-2.0 X 10-''2 4.0696 1.3-0.9 x 10-11

4.0446 4.0645

1:4 4.0645 1.2-1.0 x 10-12 4.0831 2.6-1.0 x 10-12

4.0559 1.3-1.0 x 10-12 4.O8O6 4.0-2.1x10-12

4.0602 4.0818 From the results of the first experiment It had been

expected that. In the second experiment, the observed ^^B/^°B

ratios of samples with B/Na <l/2 would show significant

depletion In ^ °B when compared to observed ^^B/^°B ratios of

samples with B/Na >1. The results (Tables 11 and 12) bear

out this expectation, with the average effect (B/Na = 2 vs B/Na = 1/4) being significantly greater at 78O °C (1.0^) than

at 750 °C (0.5%) . The complete suppression of the NaaBOa"*" Ion signals from

the sodium hydroxide solution with B/Na = 1/4, observed In the first experiment, was not observed In the second experiment. The first two analyses of this solution gave Ion currents of <5 x 10"^ A at 750 °C, but at 8OO °C currents of

35
1 to 2 X 10"'^^ A were obtained which were sufficiently stable

for ratio measurements. The two analyses were made within

one week after the completion of the chemical preparation of

the sample. The solution was allowed to remain on the shelf

for another two weeks without any additional processing.

Then another mass spectrometric analysis was performed and

ion currents of 2 x 10"^^ A were obtained at 750 °C. This

phenomenon was observed to occur in a small percentage of the

many boron solutions that were prepared and is referred to as "aging". These boron solutions apparently changed in some manner while enclosed in sample vials and sitting on a shelf.

For some samples this change improved the analytical performance of the sample on the mass spectrometer; for other

samples the change caused a deterioration in the analytical

performance of the sample with time. Most of the samples used

in this study, and particularly those used for the absolute

ratio determinations, did not show any aging effect, but this

phenomenon presents the only plausible explanation for the

discrepency between the results of the two experiments on the

NaOH solution with B/Na = 1/4.

The data for the samples with B/Na >1 confirmed the

general observations of the first experiment. The ^^B/^°B

ratios did not change significantly during an analysis and were relatively insensitive to small temperature increases.

Within the sodium carbonate group of samples (Table 11)

there is a strong indication that, at a given temperature, the

ionization efficiency increases as the sodium concentration

increases. The results from the sodium hydroxide group of

samples (Table 12) are neutral in this respect but still con- tradict the conclusion, based on the first experiment, that excess sodium inhibits the production of NaaBOa"*" ions. However,

in the final analysis, the samples with excess sodium do give

observed ratios which are depleted in ^ °B and changing with

time, indicating that excessive fractionation is occurring.

36
Although there are some discrepancies in the observations

of the two experiments, the conclusion that sodium concentration is a critical parameter which must be kept constant for

all solutions is strengthened by both experiments. Failure

to use a constant B/Na ratio for all samples will seriously

affect both the precision and accuracy of the ^^B/^°B ratio

measurements

.

7 . Filament Temperature

For precise isotoplc ratio determination using surface

ionization techniques it is Important to be able to reproduce

the same filament temperature for all analyses. Variations

in the filament temperature of analyses will cause variations

in the rate of Isotoplc fractionation and, consequently,

variations in the observed ratios.

In the determination of the absolute isotoplc compositions

of two boron standards (Section G) a consistent temperature

level was maintained through a combination of filament current

and signal intensity settings. Preliminary work had established an average optimum temperature to be equivalent to a filament current of from 1.20 to 1.30 A. In general this

filament temperature resulted in NaaBOa ion signals of from

1 to 3 X 10~^^ A. Any analysis in which the parameters were Inconsistent with these ranges was discarded.

Admittedly, the use of filament currents and signal

intensities as temperature indicators is Imperfect. Even minor

variations in the thickness of the filament materia and in

the chemical compositions of the samples will affect these

parameters. However, it was proven empirically that these

temperature guides gave results with an analytical limit of error on the order of 0.1^, which was quite acceptable.

Subsequent to the determination of the absolute isotoplc

compositions of two boron standards (Section G), it was found

that optical pyrometers available in the laboratory could be

used for filament temperature measurements even though their

37
minimum temperature measurement was 700 °C. At this low

temperature it first appeared to be impossible to match the

reference line of the pyrometer and the faint glow of the

filament with any reasonable precision. However it was subsequently discovered that by turning off ceiling lights

and/or shielding the pyrometer and source housing with a black cloth a precision of ±10 °C could be achieved in the

range between 750 °C and 800 °C. Use of the pyrometer quickly revealed that filament cur currents of 1.2 and 1.3 A corresponded approximately to temperatures of 710 °C and 830 °C, respectively. Taking data

indiscriminately over this filament current range could

produce ^^B/^°B results which differed by as much as 1% . By

also restricting the ion current intensity, the variations in

the ratio can be reduced to an acceptable level (0.10 - 0.15^)3

but use of the pyrometer to restrict filament temperatures

to within ±10 °C of the optimum temperature gives the best

precision

.

8 . Mass Spectrometric Procedure

The mass spectrometric procedure used in the determination of the absolute isotopic compositions of two boron

standards (Section G) is as follows.

The filament current is rapidly increased (1.2 - 1.25 A)

until the NazBOa ion current is barely detectable (1 x io~-^^ A) on the transmission grid, then slowly increased until the grid

ion current is 60 - 80 x 10~^^ A. The signal will decay

slowly. At 18 minutes the filament current is increased to give a grid ion current of 60 ± 2 x 10~^^ A. The signal will

continue to decay slowly. At 24 minutes the filament current

is increased to give a 50 ± 2 x lo"-^^ A grid ion current. The

signal is given a final focussing^ it will decay very slowly.

At 30 minutes the signal should be 40 ± 5 x lO"^^ A; if

necessary it is adjusted to this level. The signal will

continue to decay slowly for the remainder of the analyses.

Data-taking begins at 40 to 42 minutes into the run. Ten peak

38
sets (10 ratios) are taken and base line readings are taken

Immediately before and Immediately after the data. The peaktop data are taken by step-wise changes In the magnet current

and each peak-top Is monitored for 30 seconds.

The filament heating procedure subsequently devised for

use with an optical pyrometer Is as follows.

The filament current Is Increased until the NaaBOa"*" Ion

current Is barely detectable (1 x 10~^^ A) on the transmission

grid. At 6 minutes the filament temperature Is adjusted to

720 °C. The Ion current will usually grow to a maximum of

20 - 40 X 10"^ 3 A and then decay to ^10 x lo"^^ A. At l8

minutes the filament temperature Is Increased to 750 °C; the

ion current Is normally 30 - 50 x 10~^^ A and will decay

slowly for the remainder of the analyses. If the ion current

is much less than 20 x 10~^^ A at 30 minutes, the filament

temperature may be Increased slightly (750 - 760 °C). Datataking begins at 40 - 42 minutes into the run and is

accomplished as previously described.

G. Absolute Isotoplc Abundance Ratios of Two Boron-Isotope

Standards, SRM 951 and SRM 952

1 . Introduction

The NBS is conducting a long-term program of absolute

isotoplc abundance ratio and physical atomic weight determinations using solid-sample surface or thermal ionization mass spectrometry. Previous elements studied include silver [8],

chlorine [9], copper [10], bromine [11], chromium [12],

magnesium [13] and lead [l4]. The present work extends the

study to boron.

The isotoplc composition of natural boron appears to vary

significantly [15], [l6], [17], so there is little value in

attempting to establish an atomic weight. So, the purpose of

the present study was to establish the absolute isotoplc com- positions of a normal (^^B/-^°B '\^4) boron sample and an enriched

( ^ °B "^95%) boron sample. The former would be of great use as

39
an absolute standard for people studying natural boron

isotopic variations and the latter would be useful to people

concerned with enriched ^ °B samples used in nuclear reactors.

The mass spectrometers used in this study were calibrated

for bias by the use of samples of known ^^B/^°B ratio, prepared from nearly pure separated ^ °B and ^^B isotope solutions.

The collector and recorder systems used in this laboratory

have been designed so that any biases they might contribute

are constant. The only "random" bias associated with the

isotopic ratio measurements is due to the mass-dependent

fractionation of the isotopes during ionization, and this bias

is independent of the isotopic composition of the sample.

Thus, for each instrumental system used in this study, a single

experimentally determined correction factor is valid over the

entire range of isotopic compositions measured. The validity

of this single-correction factor assumption based on multiple

previous studies was re-tested in this study by comparison of

correction factors obtained with calibration mixes with ^^B/^°B

ratios of approximately ^/l, 1/4, 1/12 and 1/18.

2 . Experimental Procedure

a. Mass Spectrometry - Isotopic measurements of

the calibration mixes and SRM samples were made with three

different single-stage solid-sample mass spectrometers which

are identical except for minor differences in the vacuum

systems. Each instrument has a 6-inch radius of curvature 60°

analyzer tube, 48° sector magnet and a Z lens in the source assembly [l8]. Single filament tantalum-ribbon (1 x 30 mils) sources were used.

The samples were prepared to a concentration of 1 mg of

boron per milliliter of boric acid solution containing enough

sodium to yield B/Na of 1/1. One drop of solution was deposited on the filament and dried with a heat lamp and a 1.2 A current for 5 minutes; then the heat lamp was turned off and

the current was slowly increased until the sample turned

whitish-grey (1.6 - 1.8 A) and held there for two minutes.

40
An accelerating voltage of 6 kV was used. The ^^B/^°B

ratios were determined by measuring the relative abundances

of the Naz^^BOa"*" and Na2^°B02"^ ions, at masses 89 and 88 respectively. Corrections for Na2 ^ ^0 ^ ^o"*" were made by

measuring the Na2 ^ ^B^ ^Oa'^/Naz ^ ^B^ ^0 ^ ^0^ ratio of the ^^B separated isotope solution (masses : 89/90 )

.

Preliminary studies showed that isotopic fractionation

was significant and varied with time and filament temperature.

To minimize the effect of this fractionation, all analyses

were made in an identical manner; data were always taken

during the same time interval, and the total ion signal was always kept within strict limits.

b . Purification of the Separated Isotopes - Approximately O.65 g each of elec tromagnetically separated

^ °B and ^ ^B isotopes, in the form of boric acid (H3BO3), were obtained from the Isotopes Division, Oak Ridge National

Laboratory, Union Carbide Nuclear Company. The H3^°B03 was designated sample No. 98726, and the Hs^^BOs was designated

sample No. 98802.

The chemical purity of the samples (Table 13) was not

Table 13. Estimation of impurities (%) in separated isotopes. H3IOBO3 H3IIBO3

Lithium 0.01 <0.01

Magnesium 0.02 <0.05

Silicon 0.05 <0.05

Calcium <0.01 0.02 Copper <0.01 <0.05

Sodium 0.05 <0.01

Semi-quantitative determination at Oak Ridge National Laboratory by emission spectroscopy. sufficient for their use as received and they were dissolved

and eluted through hydrogen-form cation exchange columns to

41
remove the cationlc Impurities. Unfortunately, the abundances

of sodium and magnesium were sufficiently high so that, in the

exchange, enough strong acid was produced so that a strongacid correction to the assay could not be made with sufficient

accuracy (refer to page 22). It was therefore decided to recrystallize the isotopic boric acids from water as a preliminary purification. This procedure was shown to be effective

in removing NaCl (at the 0.1% level) from natural boric acid.

Each sample of isotopic boric acid was treated as follows:

About 2.5 g of material was dissolved in 20 ml of water and

the solution was filtered through a polyethylene filter. The

filtrate was concentrated to about 10 ml at '80 °C and then

cooled to about 5 °C. The recrystallized boric acid was caught

on a polyethylene filter, washed with cold distilled water, and

dried by passing clean dry air through the filter. In both

cases the weight of the recrystallized separated isotope boric

acid was about 1.6 g, or about 6^% of the starting material. A second sample of H3^°B03 was recrystallized in the same manner.

Two solutions of the "BIO" separated isotope and one solution of the "Bll" separated isotope were then prepare from the

recrystallized materials and further purified in the following

manner. About 1.2 g (B-lO-2) and 1.08 (B-lO-3) of Hs^^BOg and

1.6 g (B-11-2) of Ha^^BOa were transferred to platinum boats,

weighed on a microanalytical balance and transferred to 125 ml

teflon bottles. Each sample was then dissolved in 30 ml of

water and eluted through a cation exchange column to remove

any remaining cationic impurities. The elute was caught in

a weighed 125 ml teflon bottle and the column was washed with

15 ml of water which was added to the original elute.

The cation exchange column consisted of a 0.5><8.8 cm polyethylene dropping pipet filled to a height of 3 cm with

100-200 mesh AG 50x8 cation exchange resin. The resin which

was supplied in the hydrogen form, was pretreated by passing

10 ml of HCl (1+3) through the column and then washing it

with 30 ml of water.

42
c . Preparation and Boron Concentration of the

Separated Isotope Solutions - The three purified separated

Isotope solutions were diluted with water so that the concen- tration of boron was approximate! 3 mg per g of solution. A diluted solution of Ha^^BOa (B-11-4), with a concentration of

approximately 0.45 mg of boron per g of solution, was then

prepared by transferring l6 g of the first Ha^^BOa (B-11-2)

solution to a weighed 125 ml teflon bottle and diluting It to 100 g with water. This diluted Ha^^BOa (B-11-4) solution was used to prepare calibration mixes with ^^B/^°B ratios <l/4.

Four samples containing from 1.2 to 1.5 meq of boron

were withdrawn from each of the two Ha^°B03 (B-10-2 and

B-12-3) and the Ha^^BOa (B-11-2) solutions, and seven samples,

containing 0.45 meq of boron, were withdrawn from the diluted

Ha^^BOa (B-11-4) solution. In the following manner.

The cap of the Teflon bottle containing the solution was replaced with a rubber serum stopper. A platinum hypodermic

needle and another smaller hollow needle to serve as a vent were Inserted through the stopper. The appropriate syringe

was attached to the platinum needle and the aliquot was withdrawn. The tip of the needle was raised above the solution

and a small amount of air was drawn Into the syringe. The

syringe was detached from the needle, the tip was wiped with

a piece of tissue paper and capped with a Kel-P cap. The two needles which were still In the stopper were capped to prevent

evaporation of the solution from the bottle.

The syringe was wiped with a damp towel to dissipate any

static charge and It was weighed. The cap was removed from

the syringe and the aliquot was discharged Into a bottle

(Polyethylene bottles were generally used, but Teflon bottles

were used In the preparation of the synthetic mixtures). A little air was drawn Into the syringe which was then recapped

with the Kel-P cap. The syringe was wiped with a damp towel

and weighed. The difference between the filled and empty

syringe was taken as the weight of the aliquot.

43
The solutions were transferred to 4 oz polyethylene

bottles and the boric acid concentrations were determined as described in Section C. Results are given in Table l4 . Two samples, each containing 0.3 meq H3BO3, were withdrawn from

each of the boric acid solutions for the determinations of

strong acid concentrations, (see Section B.3) Results are given in Table 15

d . Isotopic Analysis of the Separated Isotope

Solutions - The separated isotope solutions were analyzed

twelve times each; four times each on the standard 6-inch

instrument described in Section G (MS-4), and 8 times each

(four times each by two operators) on a 12-inch instrument

[19] which was equipped with an electron multiplier.

Results are given in Table 16. All results were corrected for fractionation during ionization, as determined

by analyses of the synthetic mixtures (see below), and the

results from MS-1 were also corrected for the theoretical

square-root-of-the-mass-ratio fractionation of the electron

multiplier. Tests made with and without the multiplier

showed that this correction was valid and necessary.

The isotopic compositions reported by ORNL are: "BIO",

99.77 ± 0.03 atom percent ^ °B and 0.23 ± O.O3 atom percent

^^B; "Bll", 0.23 ± 0.05 atom percent ^ °B and 99-77 ± O.O5 atom

percent ^^B. The ORNL limits quoted express the precision of

the measurements. From known sources of systematic error, the

absolute error is estimated by ORNL to be less than one percent.

e. Preparation of the Calibration Mixes - Twelve

calibration mixes were prepared by mixing weighed portions

of the "BIO" and "Bll" solutions to produce ratios from

iiB/^°B 0.05 to ^iB/^°B ^ 4 (Table 17). The B-11-4 solution

(see Section G.2.c) was used to prepare calibration mixes

whose ^^B/^°B ratios were 0.05, O.O8 and 0.25. In addition

another calibration mix with ^^B/^°B 0.25 was prepared from

the B-11-2 solution, thus providing a crosscheck of the two

"Bll" solutions.

44
Table 14. Concentration of boron isotope solutions.

Solution B-10-2

B-10-3

B-11-2

B-11-4

Weight of Concentration Sample solution Boron fneq boron/gram number (grams) (meq) solution

1 5.18162 1.551955 0.299512

2 5.08398 1.523070 0.299582

3 D.UUo25

1 cnnoon n OQQCCO

4 5.07867 1.521378 0.299562

nVcl dgc

Correction for strong acid

n nnnnoo

0.299532

1 4.12724 1.243347 0.301254

2 4.07599 1.227977

A OA 1 OT

1

0.301271

00/I nocQi 4.uy3yi 1 oooTon n oni ooc

4 4.14870 1.249785 0.301247 Mverage

Correction for strong acid

n nnnnoi - 0.00002/ 0.301222

1 5.22914 1.421235 0.271791

2 4.98995

1 1 rr on r 1.356285 0.271803

Q0C 10/170 1 oQoocn n 07 1 7Q0

4 5.14304 1.397974 0.271818

A w Of ^3 n Q nVcrdgc n ?7iRni

Correction for strong acid

n nnnn i

o

- 0.000013

0.221788

1 10.11226 0 447349 0 0442383

2 10 36477 0 458675 0 0442532

3 10.17779 0.450218 0.0442353

4 10.13191 0.448150 0.0442315

5 10.04164 0.444233 0.0442391

b

in oc yi Qo 1U.Zd49o n /I i:/ino 1 U.4o4Uol n n^ /looco

U. 044/039

7 in iRnnt; lU.loUUO n yicnoQi n n/i/10^ 1 1; Average 0.0442393

Correction for strong acid - 0.0000020

0.0442373

^5
Table 15. Determination of strong acid in boron isotope solutions.

Solution number

B-10-2

B-10-3 B-11-2

B-11-4

Strong acid in boric acid

(percent) 0.0074 0.0090

0.0047 0.0046

Table 16. Isotopic composition of separated boron isotopes used

in calibration samples.

Separated isotope Isotopic composition

(atom percent)

'BIO" "IOB 99.83871 0. 11B 0.161310.

Bll iob 0.1874 1 0.0020 11B 99.8126 10.

The uncertainties are based on a minimum error of 0.00002 for

tlie ratio determinations. The calculated 95 percent confidence

limits are below this level.
Table 17. Composition of boron calibration samples.

Weight of Concentration Sample solution meq boron/gram Boron number Isotope (grams) solution (meq)

9 4.41671 0.271788 1.200409

1 n10 1.01056 ft OOQCOO 0.Z99532 ft OftOCQC 0.30Zb95

10 4.68554 0.271788 1.273474

in10 1 ni 1 1

n

1.01110 ft OQOCOO ft OftOQCT U.iUZob/

11 1.94136 0.271788 0.527638

in 1 Dom/i 1.00/U4 n 00000 n RCROOQ

12 2.19255 0.271788 0.595909

1 n 1 Qcnyi7 n ooocoo n KQ700/1 y.!)o/ZZ4 13 1.11925 0.271788 0.304199

in 4.0!Jlo3 ft OQQCOO 1 OOCCO/I l.ZZt)bj4 14 1.07284 0.271788 0.291585

10 4.09134 ft OftOCOO 0.Z9953Z l.ZZ54o7

17 3.73430 0.0442373 0.165195

1 n

10 Z.0Z290 ft Oftl ooo 0.301ZZZ ft CftOO/l o 0.b0934Z

18 3.37683 0.0442373 0.149382

in Z.Uj/44 ft oni 000 O.oOlZZZ ft Cl OTOO 0.bl37ZZ

19 2.27225 0.0442373 0.100518

in ft 'iMOOO 1 OOCQOO l.ZZbooo 20 2.28611 0.0442373 0.101131

in /I 1 con/i ft Ofti 000 0.301ZZZ 1.Z5Z493

21 1.89970 0.0442373 0.0840376

10 5.12416 0.301222 1.543510 22 1.95068 0.0442373 0.0862928

10 5.23751 0.301222 1.577653

^7
All aliquots were withdrawn from the primary solutions

and weighed in the manner previously described (Section

G.2.c). To eliminate the effect of any possible changes in

the concentrations of the separated isotope solutions with

time, the aliquots for the calibration mixes were taken

interspersed with those used for the determinations of the

boron concentrations of the separated isotope solutions. All

aliquots were taken within a single 5-hour time period.

f . Isotopic Analysis of the Calibration Mixes and

SRM Samples - One complete and one partial set of analyses of

the calibration mixes and SRM samples were made, by different

operators on different mass spectrometers. The complete set

of analyses consisted of 32 analyses of SRM 951 made alternately with 32 analyses (4 each) of calibration samples 9,

10, 11, 12, 13, 14, 17 and l8, plus 8 analyses of SRM 952

made alternately with 8 analyses (4 each) of calibration

mixes 21 and 22. The partial set of analyses consisted of 12 analyses of SRM 951 made alternately with 4 analyses each of

calibration mixes 9 and 10 and 2 analyses each of calibration

mixes 11 and 12; plus 8 analyses of SRM 952 made alternately

with 8 analyses (4 each) of calibration mixes 21 and 22. In addition, calibration mixes 19 and 20 were analyzed H times

each by operator I and 2 times each by operator II. Ten peak sets (10 ratios) were taken in each analysis

and baseline readings were taken immediately before and after

the data. The peak-top data were taken by step-wise changes

in the magnet current and each peak-top was monitored for

30 seconds

3 . Results and Discussion

Table l8 summarizes the results for the calibration

mixes. Although there appears to be a real difference

between operators, due to slight differences in their procedures, there is no systematic variation of correction factor

with isotopic composition. This reaffirms the conclusion,

48
stated in Section G.l, that the biases of the mass spectrometers used in this laboratory are independent of isotopic

composition for a particular element.

Table 18. Determination of mass spectrometric bias. Isotopic rational Correction factor

H/ilihr^itinn \ja 1 lui aiiuii UpcidlUl uperaior Operator Operator mix no. 1 1 1 1 1 1

9 3.93700 3.93417 3.93258 1.00072 1.00112

10 4.17247 4.17100 4.16652 1.00035 1.00143

11 0.93323 0.93268 0.93254^ 1.00059 1.00074

12 1.01421 1.01306 1.01283^ 1.00114 1.00136

13 0.24963 0.24943 1.00080

14 0.23938 0.23920 1.00075

17 0.27251 0.27226 1.00092

18 0.24484 l.UUlZo

19 0.083514 0.083491 0.083414^ 1.00028 20 0.082325 0.082268 0.082231^ 1.00069 1.00114

21 0.056042 0.055992 0.055973 1.00089 1.00123

22 0.056292 0.056219 0.056229 1.00130 1.00112 Mean values of correction factors 1.00080 1.00119b

^Average of 2 determinations ail otiiers are average of 4 determinations. Weighted for average number of determinations. Table 19 gives the observed and corrected ratios for the

four sets of data. Table 20 gives the final absolute values

for the ratios along with the overall limits of error and

uncertainty components. Although more data was obtained by

operator I, the data of operator II show a better internal

precision. As a result, the standard errors for both sets of

data are almost identical, and the final values are a straight

49
Table 19. Observed and corrected ''B/''°B ratios of the SRM samples.

Sample SRM 951

Operator Observed i^B/iob

4.04074

4.03847

Corrected ^^B/iob

4.04397

4.04328

SRM 952 0.053138

0.053155

0.053180

0.053218

Table 20. Absolute isotopic ratios for the SRM samples.

SRM 951 Boric Acid

Overall limit of error^ Uncertainty components: 95% confidence limits on ratio determination Bounds due to possible systematic error in correction

factor (including errors in separated isotopes) Bounds due to possible systematic error in chemical analyses

SRM 952 Boric Acid

Overall limit of error^ Uncertainty components: 95% confidence limits on ratio determination (minimum Bounds due to possible systematic error in correction

factor (including errors in separated isotopes) Bounds due to possible systematic error in chemical analyses

0.00002)

11B/10B

4.04362 ± 0.00137 i 0.00054 ± 0.00060 ± 0.00023

0.053199 ± 0.000032 ± 0.000020 ± 0.000009 ± 0.000003 ^The overall limit of error is the sum of the 95% confidence limits for the ratio determinations and the terms covering effects of known sources of possible

systematic error. 50
average of both sets of data. Because of instrumental uncertainties, such as dead zone and recorder nonlinearity , it is

assumed that no ratio can be measured to better than 0.00002,

so this figure is used as an error value whenever the statistically determined 95 percent confidence limit is smaller

than 0.00002.

Table 21 gives the atom percents for each isotope in each

standard, and the atomic weights (^^C = 12) of each standard.

Nuclidic masses were taken from Mattauch et al. [20].

Table 21. Atom percents and atomic weights for the SRM samples.

Uncertainty components

Possible Mass systematic errors Possible

Overall^ spectrometric in correction systematic

limit of analytical factors and error in error error separated isotopes chemical analysis SRM 951 Boric Acid

Atomic weight 10.811756 ± 0.000053 ± 0.00002 ± 0.000023 ± 0.000009 Atom percent: 19.8270 ± 0.0054 ± 0.0021 ± 0.0024 ± 0.0009 ^^B 80.1370 ± 0.0054 ± 0.0021 ± 0.0024 ± 0.0009 SRM 952 Boric Acid Atomic weight 10.063267 ±0.000029 ± 0.000018 ± 0.000008 ±0.000003

Atom percent: 10b 94.9488 ± 0.0029 ± 0.0018 ± 0.0008 ± 0.0003 i^B 5.0512 ± 0.0029 ±0.0018 ± 0.0008 ± 0.0003

Nuclidic Masses: 10.012939, 11.009305 ^The overall limit of error is the sum of the 95% confidence limits for the ratio determinations and the terms covering effects of known sources of possible systematic error. 51
REFERENCES

[1] LeDuigou, Y.,and Lauer, K. P., Proc . Conf. HighPrecision Analysis of Substances of Interest to Nuclear

Energy, Bureau Central de Measures Nuclaires, Ic (1962)

pp. 89-102.

[2] Marinenko, G. and Champion, C. E., Manuscript in

preparation

.

[3] Roller, P. S., J. Am. Chem. Soc . , 50, 1 (1928).

[^] Marinenko, G. and Champion, C. E., Anal. Chem., 4l,

1208 (1969).

[5] Marinenko, G. and Taylor, J. K., Anal. Chem., 40_, 1645 (1968).

[6] NBS Technical News Bulletin, 47., No. 10 (I963) . [7] NBS Technical Note 425 (1967).

[8] Shields, W. R., Garner, E. L., and Dibeler, V. H.,

J. Res. NBS 66a (Phys. and Chem.), No. 1, 1 (1962).

[9] Shields, W. R., Murphy, T. J., Garner, E. L., and

Dibeler, V. H., J. Am. Chem. Soc, 84, 1519 (1962).

[10] Shields, W. R., Murphy, T. J., and Garner, E. L.,

J. Res. NBS 68a (Phys. and Chem.), No. 6, 589 (1964).

[11] Catanzaro, E. J., Murphy, T. J., Garner, E. L., and

Shields, W. R., J. Res. NBS 68A (Phys. and Chem.),

No. 6, 593 (1964).

[12] Shields, W. R., Murphy, T. J., Catanzaro, E. J., and

Garner, E. L., J. Res. NBS 70A (Phys. and Chem.),

No. 2, 193 (1966).

[13] Catanzaro, E. J., Murphy, T. J., Garner, E. L., and

Shields, W. R., J. Res. NBS 7OA (Phys. and Chem.),

No. 6, 453 (1966).

[14] Catanzaro, E. J., Murphy. T. J., Shields, W. R., and

Garner, E. L., J. Res. NBS 72A (Phys. and Chem.),

No. 3, 261 (1968).

[15] McMullen, C. C, Cragg, C. B., and Thode, H. G.,

Geochim. et Cosmochim. Acta 23_, l47 (196I).

.52
[16] Pinley, H. 0., Eberle, A. R., and Rodden, C. J.,

Geochim. et Cosmochlm. Acta 26, 911 (1962).

[17] Shlma, M., Geochim. et Cosmochlm. Acta 2J_, 911 (1963)

.

[18] Shields, W. R., Editor, NBS Technical Note 426, (I967).

[19] Shields, W. R., Editor, NBS Technical Note 277, (I966).

[20] Mattauch, J. H. E., Thiele, W., and Wapstra, A. H.,

Nuclear Physics 6j_, 1 (I965).

53

U. S. Depart^pei# of Commerce

MauriceSf. Stans APPENDIX 1

National Bj^Stl^ : Standards

A. V. As^fe^irector Ol^rttftcats of ^ml^^h

Standard Reference Material 951

Boric Acid

H3BO3, acidimetric assay, weight percent 100.00 ± 0.01 Absolute Abundance Ratio, ' ^B/' * B 0.2473 ± 0.0002

Boron- 10, atom percent 19.827 ±0.013

Boron-11, atom percent 80.173 ± 0.013 The atomic weight of the boron, calculated from the absolute abundance ratio using the nuclidic masses 10.0129 and 11.0093, is 10.812. This lot of boric acid was prepared to ensure material of high purity and homogeneity. As received, it was slightly deficient (approximately 0.01 percent) in moisture, but adjusts to a stoichiometric composition in about 30 minutes exposure to a normal room humidity

(approximately 35 percent relative humidity). Once adjusted to composition, the material is relatively insensitive (<0.01 percent) to moisture changes between 0 and 60 percent relative humidity, and absorbs only about 0.02 percent excess moisture in room temperature

humidities as high as 90 percent. The material cannot be heated as it decomposes with the

loss of considerable water. Assay was by coulometric titration of samples varying in size from 0.2 to 1.0 g of boric

acid, dissolved in 100 ml of a preneutralized solution IM in KCl and 0.75M in mannitol. The

inflection point of the potentiometric curve obtained from measurements with a glass-calomel electrode system was taken as the end point. The pH of the maximum

inflection point was taken as the end point. The pH of the maximum inflection point will vary from approximately 7.9 to 8.5 for the range of sample sizes given above, and the

titration must, therefore, be conducted in the absence of carbon dioxide or carbonates. The

indicated tolerance is at least as large as the 95 percent confidence level for a single determination of any sample in the lot of material, and the average essentially indicates a boron-hydrogen ion ratio of 1.0000, since separate examination shows the material contains

less than 0.001 percent of free strong acid.

(continued on reverse) The overall direction 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 February 28, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of August 9, 1968) 55
The abundance ratio was determined by single-filament solid-sample mass spectrometry

using the ion Na2B02. Mixtures of known ^ "B/^ ratio (at a 1:4, 1:1, and 4:1 ratio) were

prepared from high-purity separated isotope solutions and used as comparison standards. Correction was determined for the ^^O/^'^O ratio 'B/*°B ratio correction, -0.00079) by

measuring mass 91 using the high-purity boron- 11 separated isotope. The indicated

tolerances are at least as large as the 95 percent confidence limits for a single determination which includes terms for inhomogeneities in the material as well as analytical error. The material was prepared by the J. T. Baker Company of PhiUipsburg, New Jersey, for the Argonne National Laboratory. Separated isotopes were purified and solutions prepared

by K. M. Sappenfield and T. J. Murphy, coulometric titrations were made by G. Marinenko and C. E. Champion, mass-spectrometric measurements were made by E, J. Catanzaro and

E. L. Garner, Analytical Chemistry Division. The various procedures developed have been

published and are available in Special Publication 260-17.

56
U. S. Departito^ of Commerce

Maurice 'H- Stans APPENDIX 2

National Bkir^3j& <4 Standards

A. V. Afi^^^irector Standard Reference Material 952

Enriched Boric Acid

Absolute Abundance Ratio, ^ °B/^ ^ B . . . Boron- 10, atom percent Boron-11, atom percent H3BO3 , acidimetric assay, weight percent

18.80 ±0.02

94.949 ± 0.005

5.051 ± 0.005

. 99.97 ±0.02

The preparation furnished is an enriched boron- 10 boric acid. It is slightly contaminated by occluded mother-hquor but the assay value should make it useful by direct weighing for a "spiking" material for boron assays, as well as a useful material for the calibration of mass spectrometers. The atomic weight of the boron,

calculated from the absolute abundance ratio and the nucUdic masses 10.0129 and

11.0093, is 10.063. The abundance ratio was determined by single-filament solid-sample mass

spectrometry, using the ion Na2B02. Mixtures of known '°B/^^B ratio were

prepared from high-purity separated isotopes and used as comparison standards. Correction was determined for the ^^O/^'^O ratio C^B/^^B ratio -0.00079) by

measuring mass 91 using the high-purity boron-11 separated isotope. The indicated

tolerance is at least as large as the 95 percent confidence limits for a single determination which includes terms for inhomogeneities in the material as well as analytical error.

Details of the preparations and measurements are available in Special Publication -260-17. The material was prepared by the Oak Ridge National Laboratory. The material and the separated isotopes were purified and solutions prepared by K. M. Sappenfield and T. J. Murphy, coulometric titrations were made

by G. Marinenko and C. E. Champion, mass-spectrometric measurements were made

by E. J. Catanzaro and E. L. Garner, Analytical Chemistry Division. The overall direction 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 February 28, 1969 Office of Standard Reference Materials 57
APPENDIX 3

To clarify the titration procedure used in this work

a specific example Is considered. A sample of boric acid,

contained In a polyethylene boat and weighing 0.599207 g (vacuum corrected weight), was delivered Into the cathode

compartment of the coulometrlc cell containing 100 ml of

IM KC1-0.75M mannltol supporting electrolyte which had been

previously pretltrated to neutrality point (pH 6.98).

After passage of 935.20^ C of charge at 101.784 mA, the

current was Interrupted, the Intermediate cell compartments

were rinsed, and a final pH reading of 8.23^ was observed. A few drops of 0 . IN HCl were added to the cathode compartment

whereupon the pH of solution dropped to 7-567. Equal charge

Increments (O.IOOC) were passed through the cell at 10.178 mA and the pH was measured following passage of each Increment.

The data obtained Is shown In Table 1 and plotted In Figure 1. It can be seen that the Inflection point occurred at pH = 8.095 (point A on the graph). The pH at the completion

of passage of charge at high current was 8.23^ (point B on the graph), corresponding to an overt Itratlon of O.I78 C. Thus, 0.178 C must be subtracted, and the number of coulombs

equivalent to the delivered sample of boric acid was 935.026 C. By Faraday's law this corresponds to 9.69069 meq

of acid or the weight of boric acid of 0.599205 g. Since

0.599207 g of the acid was weighed out, the assay of the

material Is 99.9997 weight percent H3BO3.

58
Table 1. Determination of the end-point in coulometric titration of boric acid.

Charge, C

0.000

0.100 0.200

0.300

0.400 0.500

0.600

0.700 0.800

0.900

1.000 1.100

1.200 1.300

1.400

1.500 1.600

PH

7.567

7.615

7.669

7.729 7.792

7.865 7.942

8.025

8.110

8.195 8.272

8.341 8.413 8.480

8.542

8.599 8.651

apH/aC

0.48 0.54

0.60 0.63

0.73

0.77 0.83

0.85

0.85 0.77

0.69 0.72

0.67 0.62

0.57 0.52

59
0.4 0.8 1.2 1.6 CHARGE,

C

Figure 1. Determination of the End-Point in CouJometric Titration of Boric Acid, o- Points for pH=f(C); •- points for apH/aC = f(C)

A - Inflection point (ApH/\C)niax; B - Final pH of titration 60
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