ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
undertake such interlaboratory comparisons with sediment samples in the future, especially as new methods are developed and applied to environmental analyses. Such studies are needed to determine when different numbers generated by different laboratories using different methods are environmentally significant. If nothing else, the results of this intercomparison study should serve as a warning against overinterpretation of currently generated trace-level hydrocarbon determinations. The results should not be used as an argument against further intercomparison exercises, but should be used as encouragement for the continued development of the state-of-the-art of trace organic analysis. Ultimately, the goal of the National Bureau of Standards is to produce a Standard Reference Material with certified trace-level concentrations of environmentally significant organic compounds in a "real" matrix. Unfortunately, methods for preparing and certifying such a material have not yet been developed. Problems associated with sample homogeneity, stability, matrix effects, etc. must also be resolved before any such standard can become available. The low concentration of hydrocarbons anticipated in many pollution baseline studies necessitates the development of sensitive analytical techniques. Finally, some form of information exchange or intercomparison must exist among laboratories in order to assess the uncertainty of the data from these new analytical techniques.
ACKNOWLEDGMENT T h e authors thank P. D. LaFleur for his critical reading of this manuscript. T h e following laboratories and scientists participated with N B S in t h e intercomparison study: John A. Calder, Florida State University; Ronald A. Hites, M.I.T.; J o h n L. Laseter, University of New Orleans; William MacLeod, NOAA-Seattle; Steven J. Martin, Geochem Laboratories; Patrick L. Parker, University of Texas; and David Shaw, University of Alaska.
LITERATURE CITED (1) National Academy of Sciences, "Inputs, Fate, and Effects of Petroleum in the Marine Environment", A Report of h e Ocean Affairs Board, National
463
Academy of Sciences, Washington, D.C., 1975. (2) "Baseline Studies of Pollutants in the Marine Environment and Research Recommendations", Office of the International Decade of Ocean Exploration, National Science Foundation, Washington, D.C., 1972. (3) J. W. Farrington, and B. W. Tripp, ACS Symp. Ser., 18. 267-284 (1975). (4) D. G. Shaw, Environ. Sci. Technoi., 7, 740-742 (1973). (5) M. Biumer, and W. D. Snyder, Science, 150, 1588 (1965). (6) W. W. Younablood. and M. Blumer. Geochim. Cosmochim. Acta, 39, 1303- 1314 71975) (7) J W Farrington, and J G Quinn, Geochm Cosmochm Acta, 35, 735-741 (1971). (8) W. E. May, S. N. Chesier, S. P. Cram, B. H. Gump, H. S. Hertz, D. P. Enagonio, and S. M. Dyszei, J . Chromatogr. Sci., 13, 535 (1975). (9) K. Winters, R. O'Donnell, J. C. Batterton, and C. VanBaalen, Mar. Biol., 36, 269-276 (1976). (IO) L. Fishbein, W. G. Fhmm, and H. L. Falk, "Chemical Mutagens", Academic Press, New York, N.Y., 1972. 11) J. W. Farrington, J. M. Teal. G. C. Medeiros, K. A. Burns, E. A. Robinson, Jr., J. G. Quinn, and T. L. Wade, Anal. Chem., 48, 1711 (1976). 12) S. A. Wise, S. N. Chesier, B. H. Gump, H. S. Hertz, and W. E. May, in "Fate and Effects of Petroleum Hydrocarbons in Marine Ecosystems and Organisms", D. A. Wolfe, Ed.. Pergamon Press, New York, N.Y., 1976, pp 345-350. 13) J. W. Farrington, and P. A. Meyers, in "Environmental Chemistry", Voi. 1, G. Egiinton, Ed., The Chemical Society, Burlington House, London, 1975. 14) J. W. Farrington, personal communication. 15) J. S. Warner, Anal. Chem., 48, 578 (1976). 16) R. L. Glass, Lipids, 6, 919-925 (1971).
RECEIVED for review August 30, 1977. Accepted November 21, 1977. The authors acknowledge partial financial support from the Office of Energy, Minerals, and Industry within the Office of Research and Development of the 1J.S. Environmental Protection Agency under the Interagency Energy/ Environment Research and Development Program and the Bureau of Land Management through interagency agreement with the National Oceanic and Atmospheric Administration, under which a multiyear program responding to needs of petroleum development of the Alaskan continental shelf is managed by the Outer Continental Shelf Environmental Assessment Program (OCSEAP) Office. In order to specify procedures adequately, it has been necessary to identify commercial materials in this report. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necesarily the best available for the purpose.
Indirect Determination of Selenium in Sodium Selenate Wladyslaw Reichel" and Meyer Lallouz Canadian Copper Refiners Limited, Montreal East, Quebec, Canada H2Y 3H2
A method for the determination of Ses+ in sodium selenate, based on the stoichiometric reduction of hexavalent selenium to the tetravalent state with hydrochloric acid, has been developed. A calculated excess of As3+ is added to the dissolved sample. Liberated chlorine oxidizes As3+ to As5+ and the excess As3+ is titrated with standard potassium bromate. Se4+does not interfere. The accuracy of the method was evaluated using high purity sodium selenate to which calculated amounts of Se4+ were added. Average recovery of See+ was 99.98%. The standard deviation was 0.023% at 41.38% See+ concentration.
An increasing demand for purer sodium selenate, particularly by drug manufacturers, has become apparent in recent 0003-2700/78/0350-0463$01.00/0
years. Therefore the precise and accurate determination of Se6+has become imperative. Gravimetric analysis ( I ) is not sufficiently accurate, since moisture retained by selenium causes high results. Common volumetric methods ( 2 ,3 ) are subject to interferences and unacceptable errors. These procedures are not specific for Se6+and require corrections for interfering ions, including Se4+. Barabas and Bennett ( 4 ) developed a differential potentiometric method for the determination of selenium in refined selenium wii,h acceptable precision. However, a correction for Se4+is mandatory, a step which introduces an error. T h e same limitation can be observed in the differential AAS procedure of Reichel ( 5 ) . Kolthoff and Elving ( 2 ) suggest a reaction in which hexavalent selenium is quantitatively reduced to the tetravalent state on reaction with hydrochloric acid: H,SeO,
+
2HC1--* H,SeO,
1978 American Chemical Society
+ C1, +
H,O
(1)
464
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Table I. Recovery of Selenium6+from Synthetic Standards Sodium Equivalent Added sodium Se6+,g selenite, g selenate, g" 1.7000 0.7103 0.3000 1.8000 0.7521 0.2000 1.9000 0.7939 0.1000 1.9800 0.8189 0.0200 1.9900 0.8272 0.0100 1.9960 0.8323 0.0040 " Sodium selenate used was 99.98% pure. On the basis of this reaction a single vessel technique has been devised in which a calculated excess of trivalent arsenic (as As203) is oxidized to the pentavalent state by chlorine liberated upon the reduction of selenium. ASCI, t C1,
- ASCI,
Equivalent Se4+ 0.1370 0.0913 0.0457 0.0091 0.0046
C
where A is the mass in grams, of As3' added; R is the mass in grams, of As3' found, and C is the mass in grams, of the sample. The ratio of the atomic weights of selenium to arsenic is 1.0539. RESULTS A N D DISCUSSION Several samples of commercial grade sodium selenate were analyzed and found to contain between 40.65 and 41.68% Se". T h e amount of As3+ added varied with the purity of the sample, such that excess arsenic ranged between 10 and 15 mg. Greater excesses of arsenic resulted in higher titrations and decreased accuracy. I n t e r f e r e n c e s . Strong oxidants such as nitric acid, permanganate, and chromate, or strong reductants such %? organic matter and stannous chloride seriously interfere. Te6+reacts in the same manner as Se6+ and also interferes; Te", Ag+,
Recovery, % 99.99 100.02
99.99 99.96 100.01
99.97
Table 11. Precision Study Data % Se6+
41.39 41.39 41.41 41.36 41.34 41.36 41.39 41.39 41.40
T h e reaction is carried out under reflux and is completed within one hour. T h e optimum excess As3+ established by preliminary analysis is then titrated with standard potassium bromate. High accuracy is achieved since only a small portion of t h e originally added arsenic is titrated.
%Se6+ =
+
0.0018
(2)
EXPERIMENTAL Apparatus. A 500-mL conical flask with a ground joint. attached to a 50-cm water-cooled condenser and a combination hot platemagnetic stirrer were used. Reagents. All reagents should be of analytical grade. Distilled or deionized water should be used. The reagents used were: Potassium bromide solution, 2 % w/v; Sodium hydroxide solution, 2.5 M; Methyl orange solution, aqueous, 0.1% w / v (free of sediment); Arsenic trioxide, powder form; and Potassium bromate solution, 0.02 N. Procedure. Weigh 2 f 0.0002 g of the sample in triplicate into the 500-mL flasks, each containing 1.0470 g arsenic trioxide. Carry a reagent blank throughout the procedure in which sodium selenate and arsenic trioxide are omitted. Add 10 mL of 2.5 M sodium hydroxide solution and mix. Heat gently with frequent agitation until the arsenic trioxide is completely dissolved. Cool and add 50 mL concd HCI. Connect immediately to the water condenser and reflux for 1 h on the hot plate at a temperature of 89 i 4 "C. When reflux is completed, wash down the condenser with 50 mL of distilled water into the flask. Disconnect the flask, introduce a magnetic stirring bar into the solution. add 2 drops of KBr solution and 2 drops methyl orange. Be consistent in the addition of methyl orange. Add the same number of drops to the blank. Raise the temperature to 85 k 5 "C and maintain it throughout the titration. Use one of the triplicatk samples to determine the approximate volume of potassium bromate titrant needed to reach the end point. Titrate the remaining two samples to within 2 to 3 mL of the end point. Complete the titration dropwise until the color of methyl orange is discharged. (lorrect the result with that obtained on the reagent blank. Calculations. The selenium concentration is given bl( A - B ) X 1.0539 X 100
Se6
found, g 0.7102 0.7523 0.7938 0.8186 0.8273 0.8321
41.39 41.34 41.35 41.34 41.37 41.37 41.39 41.40 41.38
n = 18 Av, 41.38% Std dev, 0.023% Re1 std dev 0.054%
Zn2+.Sb"+: Pb": Na+! Cr", Cu2+,Fe3+,V4+,Sn4+,and Nit' did not interfere u p to 0.01%. T h e presence of Cr3+, Cu2+, Fe"', V", and Ni2+in higher concentration than 0.01% results in highly colored solutions, obscuring the detection of the end point. This interference study was performed on synthetic samples of sodium selenate spiked with the above elements a t their various oxidation states. It is, however, most unlikely that the interfering ions will be present in high purity sodium selenate, particularly those in their lower oxidation state. Effect of Hydrochloric Acid Concentration. Optimum conditions were reached by addition of concentrated HC1 to the dissolved sample to result in a final 10 M HC1 solution. In more dilute solutions the reduction of selenium was slow and required longer refluxing time. Effect of T e m p e r a t u r e a n d Reflux. Arsenic losses occurred over a wide range of temperatures. Volatilization of arsenic chloride resulted a t temperatures as low as 50 "C. T h e use of a water cooled condenser prevented losses of arsenic a t temperatures of up to 95 -100 "C. At higher temperatures, severe losses occured even under reflux. Although losses appeared to be minimized at lower temperatures, the reaction was found to be complete only at or above 85 "C. Accuracy a n d Precision. The accuracy of'the procedure was evaluated hy recovery experiments on synthetic samples of pure sodium selenate to which calculated amounts of Se4+ were added. Near-theoretical recoveries were obtained (Table I). The precision was evaluated on a sodium selenate sample with a mean Se6+ concentration of 41.88%. The standard deviation for a set of 18 results was 0.023% with relative standard deviation of 0.054% (Table 11). O t h e r Applications. The method with slight modification was successfully applied to the determination of selenium in sodium selenite. In a preliminary step, Sed+is oxidized to Se6+ with hydrogen peroxide in the presence of excess sodium hydroxide. The indirect bromate procedure is then followed. LITERATURE CITED ( I ) N. Howell Furman. "Standard Method of Chemical Analysis", Sixth ed , Volume I, Robert E. Krieger Publishing Company, Huntington, N.Y., 1975, p 928. ( 2 ) I . M. Kolthoff and P. J. Elving, "Treatise on Analytical Chemistry ', Part 11, Volume 7, Interscience Publishers, New Yo&, N.Y , 1961, pp 175-179. (3) Silvio Barabas and W. Charles Cooper, Anal. Chem , 28, 129 (1956) (4) Silvio Barabas and Peter W. Bennett. Anal. Chem., 35, 135 (1963). (5) Wladyslaw Reichel, Anal. Chem., 43, 1501 (1971). RECEIVED
5, 19-77,
for review September 5, 1977. Accepted December
