Trace Inorganics In Water - ACS Publications

15 Evaluation of Laboratory Methods for the Analysis of Inorganics in Water

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E . F. M c F A R R E N and R. J. L I S H K A Analytical Reference Service, National Center for Urban and Industrial Health, Public Health Service, U . S. Department of Health, Education, and Welfare, Cincinnati, Ohio

Recent studies conducted by Analytical Reference Service and involving the analysis of inorganics in water have been of methods for (1) cadmium, chromium, aluminum, iron, manganese, magnesium, lead, copper, zinc, and silver (Water Metals No. 3, Study 23); (2) arsenic, boron, selenium, beryllium, and vanadium (Water Trace Elements No. 2, Study 26); and (3) ammonia nitrogen, organic nitrogen, nitrate nitrogen, phosphates, and silicates (Water Nutrients No. 1, Study 27). A summation of the major findings of these three studies conducted to determine the precision and accuracy of some of the procedures given in "Standard Methods for the Examination of Water and Wastewater" is the subject of this paper.

A nalytical Reference Service ( A R S ) is a voluntary association of 302 laboratories responsible for the detection, identification, and meas urement of contaminants in the environment. These participants include 51 municipal, 88 state, 43 Federal, and 36 foreign agencies; 59 industries; and 25 universities. The primary purpose of A R S is to evaluate analytical methods by submitting samples of known composition to member laboratories for analysis by a specified method or methods. F r o m the analytical results obtained, the statistical parameters of precision and accuracy are calcu lated and used as measures of the acceptability or non-acceptability of the method(s). In this effort, A R S cooperates with organizations such as the American Public Health Association, the American Water Works Association, and the Water Pollution Control Federation to determine the suitability of methods for inclusion in "Standard Methods for the Exami253

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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TRACE INORGANICS IN WATER

nation of Water and Wastewater." Inasmuch as the statistical treatment of the data not only determines the precision and accuracy of the methods but also ranks the data and rejects values grossly in error, a coincidental benefit to the individual participant is that the program serves also as a check on their laboratory work. Since the inception of A R S in 1953, 27 studies involving more than 80 different methods have been completed. Of these studies, 18 have entailed determinations of substances in water, two in food, and four in air; and three had to do with radiochemistry. Eight of the studies have been repeated either to re-evaluate old methods or to determine the value of new ones. Recent studies involving the analysis of inorganics in water have been of methods for (1) cadmium, chromium, aluminum, iron, manga nese, magnesium, lead, copper, zinc, and silver (Water Metals N o . 3, Study 23); (2) arsenic, boron, selenium, beryllium, and vanadium (Water Trace Elements N o . 2, Study 26); and (3) ammonia nitrogen, organic nitrogen, nitrate nitrogen, phosphates, and silicates (Water Nutrients No. 1, Study 27). A summation of the major findings of these three studies constitutes the subject of this paper. Design of the Studies In the Water Metals No. 3 Study, 79 participating laboratories were instructed to dilute 5 m l . of a provided concentrated water sample to 1 liter and analyze for nine specified metals. The water sample was pre pared as a concentrated solution to ensure greater stability of the sample and to reduce shipping costs. The solution was shipped in sterile J E B tubes and consisted of sterile, distilled, deionized water which when diluted as instructed contained the concentration of metallic compounds indicated in Table I. Table I.

Composition of Water Metals Sample

Compound K A1 (S0 ) · 24H 0 Copper metal Fe(NH ) (S0 ) · 6H 0 KMn0 Zinc metal AgN0 Cadmium metal 2

2

4

4

2

4

3

Κ θΓ Ογ 2

2

Pb(N0 ) 3

2

4

2

4

2

2

Metal

mg./liter

Al Cu Fe Mn Zn Ag Cd Cr Pb

0.50 0.47 0.30 0.12 0.65 0.15 0.05 0.11 0.07



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The solid compounds were dissolved in excess nitric acid in order to reduce the final p H below 1.0. The extreme acidity was necessary to prevent precipitation and adsorption of the metals on the walls of the container. Repeated analysis in the A R S laboratory indicated that there was no change in the sample stored in a boro-silicate glass bottle. In their analysis for cadmium, lead, silver, and zinc, participants were instructed to use the procedures sent with the sample; these procedures now appear in the 12th edition of "Standard Methods for the Examina tion of Water and Wastewater." For the remaining metals (aluminum, copper, iron, manganese, and chromium), the participant was permitted to use the method of his choice. The method chosen often turned out to be the Standard Method. The design of the Water Trace Elements N o . 2 Study was similar to that of the Water Metals study in that a concentrate was provided and the participants were asked to dilute it from 5 m l . to a liter. It consisted of sterile, distilled, deionized water containing the con centrations indicated in Table II when diluted as instructed. Table II.

Composition of Water Trace Elements Sample

Compound

Metal

mg./liter

SeO BeS0 · 4 H 0

Se Be

0.02 0.25

a

4

2

Β

H3BO3

As 0 NH V0 2

3

4

3

As V

0.24

0.04 0.006

Copies of procedures for the analysis of arsenic, selenium, and beryl lium were also provided. Procedures for the other two metals (boron and vanadium ) were suggested, and references to their appearance in the literature were provided. Fifty-nine laboratories participated i n this study. In the study of water nutrients, three samples containing various concentrations of silicate, phosphate, ammonia nitrogen, organic nitrogen, and nitrate nitrogen were provided, and each participant was requested to do a single analysis for each element in each of the three samples. The solutions were shipped in sterile, sealed glass ampoules and consisted of sterile, deionized water containing the concentrations indicated i n Table III when diluted as instructed. The chloride was added as an interference in the nitrate determina tion and the p H was adjusted to 1.5 with sulfuric acid to preserve the samples. In the determination of silica, the participants were to analyze the samples by either the colorimetric molybdosilicate method or the heteropoly blue method. Phosphates were to have been determined by



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the aminonaphtholsulfonic acid method or the stannous chloride method (with or without extraction). For nitrogen analysis, participants were given a choice of the phenoldisulfonic acid method, brucine method, or modified brucine method. Ammonia nitrogen was to have been de termined either by distillation followed by nesslerization, distillation fol lowed by titration, or direct nesslerization. Finally, organic nitrogen was to be determined by either boiling to remove ammonia followed by kjeldahl digestion and nesslerization, boiling followed by kjeldahl diges tion and titration, or total kjeldahl minus ammonia. A l l of these pro cedures, except the modified brucine, are Standard Methods, which were to be evaluated in this study. Table III.

Composition of Water Nutrients Samples mg./liter

Compound

Nutrient

Na Si0 · 9H 0 (NaP0 ) KN0 NH C1 Glutamic acid NaCl 2

3

3

3

4

2

e

Si0 P0 Ν (nitrate) Ν (ammonia) Ν (organic) CI (interference) 2

4

Sample 1

Sample 2

Sample 3

5.00 10.00 1.00 0.20 1.50 10.00

15.00 5.00 1.00 0.80 0.80 200.00

30.00 0.50 1.00 1.50 0.20 400.00

In addition, a few selected laboratories were requested to analyze the samples for nitrate by the chromotropic acid method, and another group was to determine any or all of the nutrients by autoanalyzer. A total of 110 laboratories participated in the water nutrients study. Treatment of the Data Before statistical parameters were developed, the mean of the results reported by each participant in the water metals and water trace ele ments studies were plotted on normal probability paper to determine the distribution. Values showing a gross deviation from the normal distri bution were then rejected as nonrepresentative because of errors in calculation, dilution, or other indeterminate factors and were not used in subsequent calculations. For the water nutrients study, a somewhat more sophisticated, and more objective, computer-programmed technique was used for rejection of outliers. As verified by plotting of the data on probability paper, however, the results were about the same. The mean and standard deviation as used in this report have their usual meaning. The standard deviation expressed as a percentage of the mean (relative standard deviation) is used as a measure of the precision of the results. To obtain a measure of the accuracy of the results, the mean


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Laboratory Methods

error expressed as a percentage ( relative error ) of the true value is used. The mean error is equal to the difference between the mean of the series of test results and the true value.


Results As can be seen i n Table I V , neither the aluminon (2) nor spectrographic (23) method gave satisfactory results in the determination of aluminum. In fact, on the basis of this and previous studies, it seems safe to state that no satisfactory procedure exists for aluminum. Table IV.

Summary of Data on Aluminum, Copper, and Iron

Method

No.

Mean

Std. Dev.

Mean Error

Relative Relative Std. Dev. Error

A l u m i n u m 0.50 mg./liter 0.10 0.26

38.2 38.7

20.0 52.0

0.090

0.04

17.5

8.5

0.495 0.470 0.448

0.058 0.021 0.079

0.03 0.00 0.02

11.7 4.4 17.6

6.0 0.0 4.2

0.460 0.400 0.477 0.557

0.016 0.056 0.011 0.125

0.01 0.07 0.01 0.09

3.4 14.0 2.3 22.5

2.1 14.9 2.1 19.1

0.04 0.05 0.04 0.02 0.03 0.00

25.5 24.2 62.4 32.2 1.8 26.4

13.3 16.6 13.3 6.6 10.0 0.0

Aluminon ° Spectrograph

44 4

0.400 0.235

Cuprethol" Sodium diethyl dithiocarbamate Bathocuproine Neocuproine Zinc dibenzyl dithiocarbamate Spectrograph Atomic absorption Polarograph

25

0.514

12 7 6 4 3 3 2

0.153 0.091

Copper 0.47 mg./liter

rt

α

Iron 0.30 mg./liter 1, 10-Phenanthroline° Thiocyanate Tripyridine Spectrograph Bathophenanthroline Atomic absorption α

44 11 6 5 3 2

0.337 0.352 0.260 0.280 0.327 0.295

0.086 0.085 0.162 0.090 0.006 0.078

" Standard Method.

In the determination of copper, the best results were obtained b y the bathocuproine ( 8 ) procedure, although the zinc dibenzyl dithiocarba mate (28) and atomic absorption (1) procedures produced essentially the same precision and accuracy. The bathophenanthroline (25) and the 1,10-phenanthroline (9) pro cedures produced the best results i n the determination of iron, except that the precision of the 1, 10-phenanthroline procedure was rather poor.


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Five different methods for the determination of manganese were studied (Table V ) . The colorimetric procedure using persulfate (JO) to oxidize manganous compounds to permanganate gave good accuracy, but the precision was only fair.


Table V .

Summary of Data on Manganese, Silver, and Cadmium

Method

No.

Mean

Std. Dev.

Mean Error

Relative Retotive Std. Dev. Error

Manganese 0.12 mg./liter Persulfate* Periodate" Spectrograph Atomic absorption Formaldoxime

33 14 4 3 3

Dithizone* Spectrograph

14 3

0.118 0.150 0.113 0.127 0.180

0.031 0.054 0.022 0.025 0.069

0.00 0.03 0.01 0.01 0.06

26.3 36.0 19.4 19.6 38.4

0.0 25.0 8.3 8.3 50.0

0.00 0.06

61.0 73.5

66.6 40.0

0.00 0.01

24.5 68.0

0.0 20.0

Silver 0.15 mg./liter 0.049 0.090

0.030 0.066

Cadmium 0.05 mg./liter Dithizone Polarograph e

44 4

0.053 0.040

0.013 0.034

" Standard Method.

For the determination of silver, neither the dithizone (19) nor the spectrographic (18) method was found to be satisfactory. In the determi nation of cadmium, however, the dithizone (5) method gave excellent accuracy, but poor precision. In the determination of chromium (Table V I ) , none of the methods studied gave very good results. The accuracy of the permanganate (6) and alkaline hypobromite (7) procedures were only fair, and the pre cision of the permanganate method was rather poor. Atomic absorption (27) appeared to be the best method for the de termination of lead. The results obtained on zinc indicated that all methods were reason ably good. Unfortunately, the high acidity of the sample attacked the stopper of the J E B tube and leached zinc from the rubber. Because of the many variables involved, such as the position in which the tube was stored and the length of time that the solution was in contact with the rubber, the amount of contamination was indeterminate. The accuracy and precision of the zinc data were, therefore, questionable and are not reported here. As indicated in Table V I I , only one method for each of the trace ele ments was studied; namely, the method suggested.


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Table VI.

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259

Summary of Data on Chromium and Lead

Method

No.

Mean

Std. Dev.

Mean Error


Chromium 0.11 mg./liter Permanganate Alk. hypobromite Hex. chromium Spectrograph Polarograph

31 7 5 4 3

0

a


0

0.092 0.089 0.010 0.145 0.130

0.044 0.023 0.012 0.053 0.020

0.03 0.03 0.10 0.04 0.03

47.8 25.9 120.0 37.6 18.4

27.2 27.2 91.0 36.3 27.2

0.006 0.023 0.015

42.1 62.4 8.2

8.5 32.8 21.4

Lead 0.07 mg./liter Dithizone Spectrograph Atomic absorption

43 3 2

0

0.076 0.093 0.085

0.032 0.058 0.007

° Standard Method.

Table VII. Method

Summary of Data on Trace Elements No.

Mean

Std. Dev.

Mean Error


Selenium 0.020 mg./liter

3, 3' diaminobenzidine

35

Aluminon

32

0

0.021

0.0044

0.001

21.2

5.0

Beryllium 0.250 mg./liter 0.247

0.0176

0.003

7.13

12.0

Boron 0.240 mg./liter Curcumin Silver diethyl dithiocarbamate

30

0.240

0.0547

0.000

22.8

0.0

0.000

13.8

0.0

0.000

20.0

0.0

Arsenic 0.040 mg./liter 0

46

0.040

0.0055

Vanadium 0.006 mg./liter Gallic acid rt

22

0.006

0.0012

Standard Method.

The aluminon (24) procedure for the determination of beryllium produced the best precision and the curcumin (4) procedure for boron, the poorest. In general, all of the methods (3, 25, 20) were found to be acceptable. In the determination of silica, only about half of the participants digested the sample with sodium bicarbonate, because the sample ap peared to contain no insoluble material. Unfortunately, the conditions present in the sample converted most of the silica i n the sample to a molybdate-unreactive state. Although these results are not reported here,



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it is interesting to note that the molybdate reactive silica concentration appeared to be about the same in all samples (slightly more than 1 mg./liter), even though the total silica content was quite different. The remaining half who d i d digest the sample with bicarbonate obtained nearly equivalent results by using either the colorimetric molybdosilicate (16) or heteropoly blue ( 17) method except at the lower concentrations (Table V I I I ) , where poor results were obtained by the heteropoly blue method. Table VIII. Method

No.

Summary of Data on Silica Mean

Std. Dev.

Mean Error

Relative Std. Dev.

Revive Error

0.4 0.2 3.7

14.3 27.2 16.4

7.8 3.0 95.3

0.6 0.4 13.5

8.4 18.0 35.2

4.2 2.9 95.0

3.0 1.5 28.2

7.7 4.9 43.4

9.8 5.1 94.0

Sample 1, 5.0 mg./liter Molybdosilicate Heteropoly blue Autoanalyzer °

19 11 3

4.6 4.8 1.3

0.66 1.32 0.21

Sample 2,15.0 mg./liter Molybdosilicate Heteropoly blue Autoanalyzer

19 11 3

Molybdosilicate Heteropoly blue Autoanalyzer

20 10 3

β

14.4 15.4 1.5

1.20 2.78 0.53

Sample 3, 30.0 mg./liter

β

1

27.0 28.5 1.8

2.09 1.38 0.78

Undigested.

In the determination of phosphate, almost half of the participants neglected to hydrolyze the sample and obtained low results since the samples contained hexametaphosphate. These results are not included in Table IX. As can be seen in Table IX, the stannous chloride (14) method without extraction was the most popular and gave the best results. Good results were also obtained with the aminonaphtholsulfonic acid (13) and auto analyzer methods except at the lower concentration, where poor results were obtained. A l l three samples contained the same amount of nitrate (1.0 mg./liter) but increasing concentrations of chloride; namely, 10, 200 and 400 mg./liter. The best results were obtained with the modified brucine (21) method (Table X ) , although nearly equal precision and accuracy were obtained by the chromotropic acid (26) method. A l l nitrate meth ods, however, failed to produce accurate results in the presence of a mod erately high chloride concentration.


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Table IX.

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Laboratory Methods

Summary of Data on Phosphate Mean

Std. Dev.

Mean Error

Rehtive Rehtive Std. Dev. Error

Method

No.

Stannous chloride Aminonaphtholsulfonic acid Stannous chloride with extraction Autoanalyzer

33

9.50

0.816

0.50

8.5

5.0

18

9.16

0.374

0.84

4.0

8.4

4 5

9.19 9.50

0.448 0.401

0.81 0.50

4.8 4.2

8.2 5.0

Stannous chloride Aminonaphtholsulfonic acid Stannous chloride with extraction Autoanalyzer

33

4.81

0.515

0.19

10.7

3.8

20

4.79

0.635

0.21

13.2

4.2

6 5

6.07 4.45

2.830 0.168

1.07 0.55

46.6 3.7

21.2 11.2




Sample 3, 0.5 mg./liter Stannous chloride Aminonaphtholsulfonic acid Stannous chloride with extraction Autoanalyzer

32

0.52

0.099

0.02

19.1

4.0

20

0.65

0.314

0.15

48.3

30.0

6 5

0.76 0.51

0.258 0.195

0.26 0.01

33.9 38.3

52.0 2.0

The determination of ammonia by direct nesslerization (12) and by autoanalyzer (22) produced good accuracy and precision (Table X I ) , except at the lower concentration, in samples containing no interference, but distillation ( I I ) introduced a substantial error. Regardless of the method used, the measurement of organic nitrogen was poor (Table X I I ) , and, in fact, at a concentration of 0.20 mg. per liter, all were unacceptable. Conclusions On the basis of this and previous studies, no satisfactory procedure exists for determining aluminum. In the determination of copper the best results were obtained by the bathocuproine procedure. Likewise, the bathophenanthroline procedure produced the best results in the de termination of iron. Manganese was best determined by persulfate oxida tion of manganous compounds to permanganate. The dithizone procedure gave good results in the determination of cadmium, but poor results for silver. In fact, none of the methods studied gave good results for silver,


262

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Table X. Method

I N O R G A N I C S

I N

W A T E R

Summary of Data on Nitrate

No.

Mean

Std. Dev.

Mean Error



Sample 1, 1.0 mg./liter plus 10 mg./liter C I Phenoldisulfonic Brucine Modified brucine Autoanalyzer Chromotropic acid

46 23 17 8 5

0.62 1.02 0.94 1.09 0.97

0.461 0.289 0.053 0.179 0.080

0.38 0.02 0.06 0.09 0.03

74.4 28.2 5.5 16.4 8.1

38.0 2.0 6.0 9.0 2.6

57.9 13.8 7.9 16.6 1.2

31.0 1.0 0.0 11.7 0.0

53.8 16.8 9.2 15.9 9.2

19.0 29.0 26.0 42.0 38.0

Sample 2, 1 .0 mg./liter plus 200 mg./liter C I Phenoldisulfonic Brucine Modified brucine Autoanalyzer Chromotropic acid

46 21 17 8 4

0.69 1.01 1.00 1.12 1.00

0.399 0.140 0.080 0.186 0.013

0.31 0.01 0.00 0.12 0.00

Sample 3, 1 .0 mg./liter plus 400 mg./liter C I Phenoldisulfonic Brucine Modified brucine Autoanalyzer Chromotropic acid Table XI. Method Distillation w i t h nesslerization Distillation w i t h titration Direct nesslerization Autoanalyzer Distillation w i t h nesslerization Distillation with titration Direct nesslerization Autoanalyzer Distillation w i t h nesslerization Distillation w i t h titration Direct nesslerization Autoanalyzer

45 22 17 8 5

0.81 1.28 1.26 1.42 1.38

0.436 0.217 0.117 0.227 0.128

0.19 0.28 0.26 0.42 0.38

Summary of Data on Ammonia Nitrogen No.

Mean

Std. Dev.

Mean Error


Sample 1, 0,.20 mg./liter 44

0.22

0.100

0.02

46.3

10.0

21 20 7

0.24 0.20 0.28

0.167 0.077 0.094

0.04 0.00 0.08

69.8 38.1 33.8

20.0 0.0 40.0

Sample 2, 0,.80 mg./liter 42

0.73

0.155

0.07

21.2

8.7

20 20 7

0.76 0.80 0.79

0.218 0.090 0.084

0.04 0.00 0.01

28.6 11.2 10.6

5.0 0.0 1.2

Sample 3, 1 .50 mg./liter 42

1.44

0.261

0.06

18.0

4.0

21 21 7

1.46 1.49 1.46

0.315 0.174 0.148

0.04 0.01 0.04

21.6 11.6 10.1

2.6 0.6 2.6


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Table XII.


Method Boiled, Kjeldahl, titration Boiled, Kjeldahl, nesslerization Kjeldahl minus ammonia by nesslerization Autoanalyzer Boiled, Kjeldahl, titration Boiled, Kjeldahl, nesslerization Kjeldahl minus ammonia by nesslerization Autoanalyzer Boiled, Kjeldahl, titration Boiled, Kjeldahl, nesslerization Kjeldahl minus ammonia by nesslerization Autoanalyzer

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Summary of Data on Organic Nitrogen No.

Mean

Std. Dev.

Mean Rehtive Rehtive Error Std. Dev. Error

Sample 1, 1.5 mg./liter 30

1.16

0.633

0.34

54.7

22.6

26

1.36

0.586

0.14

43.1

9.3

16 3

1.44 1.82

0.660 0.766

0.06 0.32

45.9 42.1

4.0 21.3


0.83

0.373

0.03

44.8

3.7

26

0.70

0.364

0.10

52.1

12.5

16 3

0.87 0.69

0.458 0.101

0.07 0.11

52.6 14.6

8.7 13.7


0.34

0.360

0.14

104.4

70.0

26

0.31

0.290

0.11

94.8

55.0

15 3

0.34 0.49

0.235 0.406

0.14 0.29

68.8 82.7

70.0 145.0

and only fair results were obtained by the alkaline hypobromite pro cedure for chromium. The few participants who used atomic absorption spectroscopy achieved excellent results for copper and the best results for lead. In general, poor results were obtained for all the metals by the spectrographic method. In the determination of the trace elements, selenium, beryllium, boron, arsenic, and vanadium, all of the methods studied were found to be acceptable. In the study of water nutrients, nearly half of the participants ob tained poor results in the analysis of silica because they failed to digest the sample with sodium bicarbonate to convert it all to molybdate-reactive silica. Similarly, in the analysis of phosphates, only about half of the par ticipants hydrolyzed the sample to convert the polyphosphates to ortho phosphate and hence, the remainder obtained low results. In silicate



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analysis, the best results were obtained by the colorimetric molybdosili cate method. The aminonaphtholsulfonic acid method produced good results i n analysis for phosphates. The best results were obtained for nitrates by the modified brucine method, although nearly equally good results were obtained b y the chromotropic acid method. A l l methods failed to produce accurate results i n the presence of moderately high chloride concentrations. The determination of ammonia by direct nessleri zation produced the best results. None of the methods studied were sat isfactory for the determination of organic nitrogen. Literature

Cited

(1) Allen, J. E., Spectrochim. Acta 17, 459 (1961). (2) American Public Health Association, "Standard Methods for the Ex amination of Water and Wastewater," 12th ed., p. 53, Am. Public Health Assoc., New York, New York, 1965. (3) Ibid., p. 56. (4) Ibid., p. 61. (5) Ibid., p. 67. (6) Ibid., p. 124. (7) Ibid., p. 126. (8) Ibid., p. 133. (9) Ibid., p. 156. (10) Ibid., p. 173. (11) Ibid., p. 187. (12) Ibid., p. 193. (13) Ibid., p. 231. (14) Ibid., p. 234. (15) Ibid., p. 251. (16) Ibid., p. 261. (17) Ibid., p. 264. (18) Ibid., p. 267. (19) Ibid., p. 271. (20) Fishman, M . J., Skougstad, M . W., Anal. Chem. 36, 1643 (1964). (21) Jenkins, D., Medsker, L. L., Anal. Chem. 36, 610 (1964). (22) Kamphake, L. J., Hannah, S. Α., Cohen, J. M . , Water Research 1, 205 (1967). (23) Kopp, J. F., Kroner, R. C., J. Appl. Spectroscopy 19, No. 5, 155 (1965). (24) Luke, C. L., Campbell, M. E . , Anal. Chem. 24, 1056 (1952). (25) Smith, F. G., McCurdy, W . H., Diehl, H., Analyst 77, 418 (1952). (26) West, P., Ramachandran, T. P., Anal. Chim. Acta 35, 317 (1966). (27) Willis, J. B., Anal. Chem. 34, 614 (1962). (28) Wilson, A . L., Analyst 87, 884 (1962). RECEIVED April 24, 1967.


Trace Inorganics In Water - ACS Publications

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