NBSIR 75-662

Sulphur Dioxide Reference iiHaterials

J. K. Taylor and E. R. Deardorff

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

February 1975

Final Report

Prepared for The Environmental Protection Agency

Office of Research and Development

Research Triangle Park, North Carolina 2771

NBSIR 75-662

SULFUR DIOXIDE REFERENCE MATERIALS

J. K. Taylor and E. R. Deardorff

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

February 1975

Final Report

Prepa red for The Environmental Protection Agency

Office of Research and Development

Research Triangle Park, North Carolina 27711

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. PRELIMINARY INVESTIGATIONS 1

2.1 Dissolved Samples 1

2.2 Encapsulated Gas 2 2.3 Powder Samples 3

3. QUALITY CONTROL MATERIALS 5

3.1 Preparation of Samples 6

3.2 Data on Series I Samples 9

3.3 Data on Series II Samples . 10

3.4 Data on Long-Term Stability ..... 10

4. INVESTIGATION OF FREEZE-DRIED SAMPLES 11

5. CONCLUSION 12

6. ACKNOWLEDGMENTS 13

i

SULFUR DIOXIDE RI-FERENCE MATERIALS

ABSTRACT

The development of reference materials to serve

for quality control and performance evaluation of

measurements of sulfur dioxide by the pararosani line

method is described. Powder samples consisting of

sodium sulfite dispersed in mannitol can be prepared

to evaluate measurements for the concentration levels

of ambient interest. The samples are sufficiently

stable in routine use, with a service life of at least

three months. By the use of a series of five samples

of graded sulfite content, measurement errors greater

than five percent are significantly detected. A

detailed procedure for preparation of the samples is

included. Preliminary studies of an alternate method

for preparation of quality control samples, by a freezedrying process are also described.

1. INTRODUCTION

This report summarizes work to develop and produce test

materials for quality control of the pararosanil ine (PRA)

method for measurement of sulfur dioxide in the ambient air.

The requirements for such samples include ease and economy

of preparation, stability over a reasonable period of time,

and above all, the ability to test the performance of the

analytical measurements within the limits imposed by the

reliability of the method and by practical monitoring considerations . The requirements outlined above were considered to be

met by the use of a series of five samples with equivalent

SO2 contents in the range of 1 to 30 micrograms. The target

of five percent for homogeneity and stability over a threemonth period was considered to be acceptable. Cost ceilings

were not established, but it was understood that unit-cost

was an important consideration.

2. PRELIMINARY INVESTIGATIONS

2 . 1 Dissolved Samples

The simplest approach to preparation of test materials

was considered to be by means of liquid samples. Such

materials can be easily prepared in large quantities and

techniques are available for dispensing accurately measured

volumes into ampoules which are automatically sealed.

1
Experiments were carried out to investigate aqueous

solutions containing SO2 that would simulate the composition

of a typical absorber. It is well known that such solutions

have limited stability, decreasing in concentration at a rate of about 1.5 percent per day of storage. Accordingly,

studies were undertaken to increase their stability.

An attempt was made to preserve SO2-TCM ( tetrachloromercurate) solutions by sealing them in ampoules in a nitrogen atmosphere. The starting solution was carefully deaerated and then sealed into 10-ml ampoules that had been

filled with nitrogen which the in-coming liquid displaced.

They were immediately sealed while blanketed in nitrogen.

However, selected ampoules, analyzed by the PRA method,

showed a continued loss of SO2 , amounting to a 20 percent

decrease in 11 days.

It has been reported that mannitol is an inhibitor of

the decomposition of SO2 in solution, so this approach was

investigated [1]. Three TCM solutions were prepared containing

10 yg SO2/IO ml. One of these also contained 0.4 percent

mannitol. The solutions were analyzed periodically with

the following results:

, Loss after Loss after

5 days 86 days

Control, stored in refrigerator, 4°C 0% 4%

Stored at room temperature 4% 75%

With mannitol, room temperature 4% 75%

It is clear that mannitol was not effective as a stabilizer. Furthermore, the only satisfactory procedure

found was storage at reduced temperature which is not

feasible. Accordingly, further work to stabilize SO2 solutions was discontinued, in favor of other approaches.

2.2 Encapsulated Gas .

A novel method of preparing SO2 reference samples,

consisting of enclosing the pure gas in precision-bore

capillaries, has been described by R. C. Paule [Anal. Chem.

44 1537 (1972)]. The gas is confined between mercury drops

in each end of a capillary, so that it is essentially at

atmospheric pressure. Tape seals on each end prevent accidental loss of the contents. The volume of the gas is determined by measurement of the capillary bore and the distance

between the mercury drops. The technique for preparation

and measurement is described in Paule 's paper.

2
Ten tubes were prepared at each of three nominal levels,

as follows: 5, 10, and 20 yg SO2. To analyze the contents,

one tape seal was removed and the tube was held vertically

with the lower end beneath the surface of 10 ml of TCM solution in a 25-ml volumetric flask. The tape seal was then

removed from the upper end, whereupon both mercury and SO2

flowed into the solution, after which the latter was determined by the PRA method.

Analytical results on 10 tubes selected at random are

given in the following table.

Tube no.

SO2 by measurement SO2 by analysis

1 - 1 4.0 4.22

3 - 2 5.68 5.86

1 - 3 9.62 9.30

2 - 3 9.13 8.70

4 - 3 9.91 10.08

5 - 3 9.34 9.76 2-5 20. 18 20. 70 3-5 21.37 21.60 4-6 21.86 21.63 6-6 21.39 20.68

The results show a good degree of reliability of standards prepared by this technique. Such standards have the

advantage of a high degree of simulation with samples

collected in the field. The main disadvantage is the relatively high cost of preparation since they must be individually calibrated.

2 . 3 Powder Samples

The feasibility of producing solid samples containing

chemically equivalent amounts of SO2 was investigated and

found to be a satisfactory approach. An initial search was

made for a solid inert diluent in which to disperse sodium

sulfite. Such a diluent must not only be inert to sodium

sulfite or other chemical equivalents of SO2, but also it

should not interfere with subsequent determinations by the

pararosaniline method. Because of the earlier experience

with mannitol, this was investigated and found to be useful.

It was further shown that large excesses of mannitol did not

interfere with the PRA method.

Mannitol-sodium sulfite dispersions were prepared in

the following manner. ACS reagent grade D-mannitol was put

through a fine-mesh screen to break up any lumps. The

required amount of powdered sodium sulfite was added and

3
mixed thoroughly. In the initial experiments, mixing was by

ball milling, using plastic balls. This was later replaced

by a V-tube blender. Details are given in Section 3,1.

It was originally planned to fix the composition of the

mixture so that the analyst could weigh a small portion and

dissolve in 10 ml of the TCM solution, A solid mixture consisting of 83.6 mg Na2S03 in 150 g of mannitol was prepared.

This mixture is equivalent to 28.3 yg SO2/IOO mg sample.

Samples of 50 to 80 mg of this were taken and analyzed with

the following results:

Sample no. SO2 Ug/100 yg

Average

23.8

34,2

16,9

23.1

33,0

26.2

While the average value was near the expected composition, the deviations between samples were too large. Accordingly, the mixture was twice reblended and analyzed after

each trial. Average values of 32.3 yg/100 mg and 29,8 yg/

100 mg were obtained, respectively, with between-sample

deviations as before. It was concluded that the limit of

homogeneity for a 100-mg sample had been reached and that

larger samples would be required to achieve the requisite

uniformity. To test this hypothesis, four 0.8-g samples were

weighed and dissolved in 100 ml of TCM in a volumetric flask,

Aliquots of 5 ml and 10 ml were then analyzed with the

following results.

Grand average

Standard deviation

5 ml aliquot 10 ml aliquot

Sample no. SO2, yg/100 ml

^ 1 : . 25,.2 26,.4

2 25,.5 25,.4

3 24,.0 25,.1

4 23,.0 23.,4

1.23

Three large mixtures were prepared containing 17.0,

30.0, and 7.0 yg SO2/IOO mg and labelled A, B, and C,

respectively. These were analyzed initially, and periodically, to investigate their stability. Initial analyses

were made on 10 ml aliquots of four 1-g samples which were

dissolved in 100 ml of TCM solution. The results are given

below:

. 4
Sample

number-aliquot

Mixture, SO2 , yg/100 mg aliquot

A B

1 a 17.21 31. 2 1 b 16.94 31.6

2 a 16.63 29.6

2 b 16.46 30. 0

3 a 17.21 29.7

3 b 17.05 30.

1

4 a 16.82 30.2

4 b 17.08 30.3

Average 16.93

2.d. 0.27 1.0

Additional samples from mixtures A, B, and C were analyzed

periodically with the following results:

Days after preparation

1

7 10

15

19

26

42

Mixture, SO2, pg/lOO mg aliquot

A B C

16.9

16.5

16.4

16.6

16.5

30.3

30.2

30.0

30.5

29.3

7.6

7.0

The behaviour observed over a 6-week period indicated that

the samples would be sufficiently stable during the required

three-month period.

3. QUALITY CONTROL MATERIALS

As a result of the foregoing investigation, the quality

control materials were chosen to consist of vials containing

known amounts of sodium sulfite dispersed in mannitol. To

minimize uncertainties due to homogeneity and packaging of

fixed amounts of the powders, it is necessary to provide a i-g sample from which a solution aliquot is then used for

the measurement. Two sets of such samples were prepared for

EPA. In each case five vials, each containing a specified

amount of sodium sulfite (SO2 equivalent), were provided as

a unit of issue. Ampoules of TCM solution were also provided with the first set but were not included in the second

because they were deemed unnecessary.

The recommended procedure for use of the samples is as

follows. By means of a clean funnel, transfer the contents

of a vial into a clean 50-ml volumetric flask with TCM solution. Successive rinses with small volumes of TCM solution

are made to insure complete transfer before dilution to

S
exact volume. The TCM solution is that normally used as a reagent for routine SO2 sampling. An aliquot of 5 ml of

the resulting solution is analyzed for SO2 in the conventional manner.

The general procedure used at NBS for analysis was as

follows. The 5 ml aliquot was transferred to a 25-ml volumetric flask. A 5-ml portion of TCM solution was added.

Accurate additions of 2 ml of 0.2 percent formaldehyde solution and 5 ml of pararosani line solution were made followed

by dilution with distilled water to the exact volume. After

30 minutes, the absorbance of the solution was measured at

548 nm, using 1-cm cells, with distilled water as a reference.

The SO2 content was calculated, using a calibration curve

prepared in the manner described in the EPA reference method

[2].

3 . 1 Preparation of Samples

The starting materials for each lot were ACS reagent

grade chemicals having the following specifications.

D-mannitol

2 5 Specific rotation [ajp

Insoluble matter

Residue after ignition

Loss on drying at 105°C

Acidity (as CH3COOH)

Heavy metals (as Pb)

NaaSOa, Anhydrous

Assay (NaaSOa) min.

Insoluble matter

Free alkali (as NazCOa)

Chloride

Heavy metals (as Pb)

+23.5°

0.0011

. 009%

. 004%

,001%

.0001%

98.51

0.0051

.051

.011

.001%

The sodium sulfite was ground in a ball mill for seven

successive 5-minute periods, with stirring in between. It

was then passed through a 60-mesh sieve before use.

Portions of the materials as indicated below were

placed in a twin V-shaped dry blender for mixing. Each

clear-plastic V-section accepted a charge of about 300 g and

rotated at a speed of about 24 rpm. In the rotation process,

the contents are alternately divided and combined. The V's

were tapped with a wooden block to prevent material from

adhering electrostatically to the walls.

6
Analysis of the blends showed that uniformity had been

attained after two hours. The several batches at each level

were combined in a large glass jar and intermixed by rotation. Analysis of the individual batches and the total lot

showed no significant difference, , The importance of thorough mixing cannot be overemphasized. The critical steps in blending appear to be

elimination of all lumps and initial reduction of each component to an equivalent particle size. Anhydrous materials

of this kind tend to clump and stick to the walls of the

blender, thereby decreasing mixing efficiency. Mechanical

shock (tapping the blender) was used to minimize this problem. It is possible that the use of metal vees would overcome this problem but this would not make it possible to

observe the mixing and recognize clumping should it occur.

It is also believed to be important to minimize the

moisture content of the samples. This is made evident by

the increased stability of powder samples over that of solutions. Accordingly, the use of the highest grade chemicals

with minimum water content is recommended. Mixing and

bottling should be carried out in relatively dry atmospheres

and the vials should be stoppered with tight closures.

Batch composition of Lot 1 August, 1972

Series 5000

305.5 g

0.1559 g

Manni tol

NasSOs

Series 4000

305.5 g

0.1217 g

Manni tol

NaaSOa

Series 3000

154.9 g

150.1 g

Manni tol

Series 5000 mix

Series 2000

245.0 g

60.0 g

Manni tol

Series 5000 mix

Series 1000

280,0 g

24.6 g

Manni tol

Series 5000 mix

7
The second lot was made in a similar manner except the

concentration levels were changed. Accordingly it was more

convenient to make a batch which was used only as a starting

material for four of the Series samples.

Batch Composition omposi t ion of Lot II

t

Feoruary , 1974

Bulk Mix

305.5 g

0.156 g

Manni tol

NazSOs

Series 5000

305.5 g

0.075 g

Manni tol

NaaSOs

Series 4000

175 g

125 g

Manni tol

Bulk mix

Series 3000

205 g

95 g

Mannitol

Bulk mix

Series 2000

254 g

46 g

Mannitol

Bulk mix

Series 1000

281 g

18.7 g

Mannitol

Bulk mix

The powders were dispensed into 1-oz amber vials using

the equipment and facilities of the NBS Office of Standard

Reference Materials. A Matur automatic dispensing apparatus

was used, provided with a stainless steel hopper and a precision screw dispenser. Check samples were taken at intervals and minor adjustments were made as needed to obtain a uniform weight of sample. The vials were closed with polyethylene insert seals having an inverted hat shape. Aluminum

tear-seals were crimped over these to indicate that the vial

had not been opened prior to use.

Random samples were selected from each concentration of

the production run and were further randomized prior to

analysis. Analysis was made by the same procedure as

8
recommended above. The calibration curve used was obtained

by the method described in the Federal Register [2],

3 . 2 Data on Series I Samples

In addition to analysis for SO2 content, each of the

analytical samples of Lot I was weighed to determine the

statistical variation of sample size. The analytical data

could then be reported as \ig SO2/IOO mg sample.

The data were statistically analyzed by Dr. John Mandel

of the NBS. Table 1 is a summary of the statistical analysis.

The first grouping lists the average weight, the 95 percent

confidence interval for the average, and the standard deviation for the weight of individual bottles. The second

grouping lists the average analytical value for a 1/10 solution aliquot, expressed as pg S02/aliquot, together with the

95 percent confidence interval for the average and the standard deviation of a single determination. These estimates

of error include both analytical uncertainty and variation of

composition between bottles, of which the latter is considered to be the major contribution. The values listed in

this grouping are those pertinent to the recommended method

for use of the samples. The third grouping lists similar

information, in which the SO2 content is calculated on the

basis of a given weight of sample. The average assays in

the third grouping are larger than those of the second

grouping by the factor 100 . The standard

Average Sample Weight

deviations of a single determination are only slightly

smaller for the third grouping as compared with the second

grouping indicating that the larger source of variation is

the homogeneity of the samples, rather than the sample

weight. In fact the relation

s S02/aliquot = /^s ~TTT^~*~Ts SO2/IOO mg)-^

mg ^ ^ weighf^ ^ mg ^

is valid to a high degree.

Table 1. Suntary of analytical values, lot I

Weight, g SO|/aliquot UK SOi/lOO mg

95 t 95 1 95 I Series

c . i

.

s c . i

.

s c.i. s average average single average average single average average single

1000 0.934 .014 .029 1.34 .08 .18 1.43 .08 .19 2000 .940 .013 .027 3.94 .19 .40 4.18 .16 .35 3000 .954 .016 .034 11.49 .32 .69 12.04 .27 .57 4000 .938 .017 .035 17.85 .46 .97 19.04 .33 .72 5000 .951 .012 .024 23.02 .46 .94 24.21 .37 .78

9
3 , 3 Data on Series II Samples

The analytical data for Lot II are summarized in table

2. Because the variation of weight of sample had been found

to be of minor significance, the data were analyzed on the

basis of yg S02/aliquot, according to the recommended mode

of use. For some unexplained reason, the variation between

samples of the 2000 Series was much larger than that found

for the other series. However, this variation should cause

no serious problems in the use of the samples for quality

control purposes.

Table 2. Summary of analytical values

,

lot II

Series n Average

s Single

0.'0 cv

yg SOa/aliquot

s_ 951 c.i

X average

1000 10 1.628 0.071 4.4 . 022 .05

2000 . 9

10

19

4.034

3.904

3.966

.335

. 254

.294

8.3

6.5

7.4

.110

. 080

.067 .14

3000 . 10 8. 096 .320 3.9 .10 .23

4000 10 10.499 .436 4.2 .14 .32

5000 10 12.197 .242 2.0 . 077 .17

2

^ Data on Long-Term Stability

While quality control samples made as described in this

report were deemed adequate for short-term use, there was a need to evaluate their long-term stability. Because elaborate storage treatment is not feasible, the useful service

life was evaluated under normal conditions, namely in air at

room temperature.

Samples were randomly selected from stock and analyzed

at intervals over a period of approximately one year with

the results given in table 3. Tabulated are the average

values found on each date for the number of samples, n, and

the standard deviation of the mean, s. It is clear that the

samples progressively decreased in SO2 content. However,

the losses are essentially uniform within each series,

judged by the reasonable constancy of the coefficient of

variation. The average decrease in SO2 content was 1.4

10
percent/month for all of the samples except the 1000 series

which was approximately twice that value.

Table 3. Stability of series II samples

2- 74 6-74 8-74 1-75

series s— s— s—

n average X n average X n average X n average

1000 10 1.628 .022 5 1. 229 .057 4 1.099 .054 1 1.046

2000 19 3.966 .067 4 3.572 .172 4 3.412 .145 1 3.268

3000 10 8.096 .100 S 7.780 .122 3 7.393 .104 1 6.765

4000 10 10.499 .140 5 10.121 .268 4 9.909 .302 1 8.595

5000 10 12.197 .077 5 11.365 .380 4 11.623 .271 1 10.098

In view of the progressive but regular decrease of concentration, it would be practical to use the samples for a period of at least one year, provided periodic analyses were

made on randomly-selected samples. Alternatively, a method

of storage other than in air at room temperature might extend

the service life. However, other types of samples should be

investigated if long-term stability is a requirement,

4. INVESTIGATION OF FREIiZE -DRI ED SAMPLES

The feasibility of preparation of quality control

samples by a f reeze-drying technique was investigated for

several reasons. The advantages of dispensing liquid samples

have already been mentioned. The f reeze-drying of such

samples followed by sealing in vacumn or in an inert

atmosphere would also eliminate tlie reaction with air and/or

water which is believed responsible for the instability of the

powder samples.

Accordingly an experimental lot of freeze-dried samples

was prepared for evaluation. A solution of TCM containing

275 ug NaaSOa per ml (equivalent to 139.7 yg SO2 per ml) was

prepared and 0.100 ml aliquots were dispensed into ampoules,

using an automatic pipet. The samples were quick-frozen

while rotating in an ethanol-dry ice mixture to encourage

distribution of the solution as a film on the sides of the

ampoule. The ampoules were connected to a vacuum system

through a cold-trap, evacuated two hours, and then flame

sealed. A total of 45 tubes were prepared in batches of

three or four.

Tubes prepared as described above were analyzed periodically over a 10-month period. The results are given in

table 4. The within-sample variation of the analytical

results, amounts to 0.30 pg. Since this represents both the

analytical error and sample variation, the latter is considered to be quite acceptable.

11
Table 4. Stability of f reeze-dr ied samples

Date SO2 found

,

Average s 3/18/74 13.36,

13.39,

13. 26,

13. 29,

13.36,

12.96,

13.

13.

13.

59

19

02

13.27 .19

5/8/74 12.95

13.11

13.11, 13. 21 13.10 .11

6/24/74 12.39,

12.19,

11 . 54

11.47,

11.74,

11.

11.

31

38 11.72 .42

: : 12.73,

11.65

12. 76, 12. 40 12. 39 .54

1/3/75 - ' 12.68,

^ . 12.39,

12.81,

12. 52,

12.

12.

39

45

12. 54 .17

The apparent changes of composition with time are not

readily explainable. It is difficult to believe that the

SO2 decreased and then increased. It is more likely that

this represents analytical error. If the results obtained

on June 24, 1974 are ignored, the samples can be considered

to have experienced a decrease of SO2 content of about 8

percent/year, but this is only a conjecture.

The freeze-drying technique definitely appears to be a promising approach and merits further study. Equipment is

presently available to freeze-dry the contents of vials or

ampoules in large quantity and to insert closures under

vacuum or inert gas atmospheres. Such a procedure would

reduce the cost of preparation of freeze-dried samples.

5. CONCLUSION

A technique has been developed for the preparation of

samples suitable for quality control and performance

evaluation of sulfur dioxide measurements by the pararosaniline method. Mixtures of sodium sulfite dispersed in

anhydrous mannitol can be blended so that 1-g samples have

adequate homogeneity for simulating a 24-hour sample at all

levels of ambient monitoring. An alternate approach,

employing freeze-dried standards, also appears promising.

The purposes of quality control and performance evaluation are best served by the use of multiple samples covering a concentration range. This approach reduces the homo12
geneity requirement for the samples and also provides information on the nature of any analytical error. A single

sample can only indicate whether the "right" answer was

obtained on that particular sample within its homogeneity

limits and the known variance of the measurement system.

Statistical considerations make it clear that only relatively

large discrepancies will be significant. Dual samples of

substantially the same concentration can be used to differentiate between measurement and sample biases but such samples

must have compositional uncertainties (inhomogenei ty) smaller

than the measurement variances. Such samples have the same

limitations as single samples in the identification of the

source and nature of the analytical error.

The use of several samples, spanning a concentration

range provides information on the nature of the analytical

error, whether additive or multiplicative. Random variation

of individual samples from the lot-average can be tolerated

and these can be in excess of the measurement variance, provided a sufficient number of samples is used. The lotaverage composition must be bias-free to evaluate the accuracy of a measurement system.

6. ACKNOWLEDGMENTS

Several members of the NBS staff have collaborated in

certain aspects of this work and their contributions are

gratefully acknowledged. Rolf A. Paulson conducted much of

the preliminary investigations. John Matwey planned and

supervised the bottling of the powder samples with the

assistance of Clyde Wachter and Robert Stewart of the NBS

Office of Standard Reference Materials. Dr. John Mandel

performed statistical analyses of the data and provided

invaluable guidance and counsel on the interpretation of the

analytical measurements.

7. REFERENCES

[1] Fritz Ephraim, "Inorganic Chemistry", Fourth Ed., p 553

(Revised 1943).

[2] Federal Register, Vol. 36, No. 84, Part II, Friday,

April 30, 1971.

USCOMM NBS DC

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U.S. DEPT. OF COMM.

BfBLIOGRAPHIC DATA

SHEET

1. imULK ATION OR Rl-I^OR 1 NO. NBSIR 75-662

2. Gov't Accession

No.

3. Recipient's Accession No.

4, TITI.I- AND SUin ri Li; Sulfur Dioxide Reference Materials

5. Publication Date February 1975

6. Performing Organization Code

7. AU rHOK(S)

J. K. Taylor and E. R. Deardorff

8. Performing Organ. Report No.

9. PKRFORMINd ORGANIZATION NAMF, ANO ADDRKSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10, Pro)ect/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organizatu>n Name and ( omplcte Address (Street, City, Stale, ZIP)

The Environmental Protection Agency

Office of Research and Development

Research Triangle Park, North Carolina 27711

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPl.KMKNTARY NOI i:S

16. ABSTRACT (A 200-word or less factual summary oi most si^ificant ir\(ormation. U document includes a significant

bibliography or literature survey, mention it here.)

The development of reference materials to serve for quality control and

performance evaluation of measurements of sulfur dioxide by the

pararosaniline method is described. Powder samples consisting of

sodium sulfite dispersed in mannitol can be prepared to evaluate measurements for the concentration levels of ambient interest. The samples are

sufficiently stable in routine use, with a service life of at least

three months. By the use of a series of five samples of graded sulfite

content, measurement errors greater than five percent are significantly

detected, A detailed procedure for preparation of the samples is

included. Preliminary studies of an alternate method for preparation

of quality control samples, by a f reeze-dry ing process are also

described.

17. KEY WORDS (six to twelve entries; alphabetical order, capilali ze only the first letter of the first key word unless a proper name; separated by semicolons)

Air pollution; chemical analysis; pararosaniline method; quality control; reference materials; sulfur dioxide

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