458
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Interlaboratory Comparison of Determinations of Trace Level Petroleum Hydrocarbons in Marine Sediments L. R. Hilpert, W. E. May, S. A. Wise, S. N. Chesler, and H. S. Hertz” Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
Results of the determination of petroleum hydrocarbons at the pg/kg (ppb) level in marine sediments have been compared among eight laboratories. Values for concentrations of total extractable hydrocarbons scattered between 9 to 500 pg/kg and 49 to 6625 pg/kg for the two sites examined. Scatter of results for hydrocarbons in the gas chromatographic elution range, the most abundant aliphatic and aromatic hydrocarbons, and total polynuclear aromatic hydrocarbons (four rings and larger) were similar. Results for percent water and pristane/phyiane ratio were somewhat more consistent. Sample inhomogeneity and analysis uncertainty contributed to an observed intraiaboratory precision ( l a ) of *25 % for nine replicate analyses of one sediment sample. The data are discussed with regard to the reliability and comparability of current methods for environmental baseline measurements.
three laboratories (11). Results of an initial feasibility study consisting of an intercalibration of sediment analysis between two laboratories have recently been published (12). The results of an eight laboratory intercomparison exercise for the determination of hydrocarbons in two intertidal sediment samples from the Northeastern Gulf of Alkaska are described below. It was decided to intercalibrate on “real world” samples (i.e., samples containing hydrocarbons from natural sources and not “spiked”), recognizing that the mixture of chemicals in petroleum is highly complex and that the products of weathering and microbial degradation compound this complexity. It is also true that sample inhomogeneity may complicate intercalibration studies of a natural sample. If these problems can be controlled effectively, these data could be uniquely valuable in assessing the variability and reliability of current sediment hydrocarbon analyses from sample work-up through measurement and interpretation.
Analytical methodology for the determination of petroleum hydrocarbons in sediments is evolving a t a rapid rate. Studies on the fate of hydrocarbons that enter the marine environment from natural sources such as seeps or that are introduced through man’s activity in the form of pollution with fossil fuels have recently been reviewed (I,2 ) . Uptake by intertidal and benthic sediments is one such fate. Since the oil may then persist for years, resulting in continuous exposure of the marine ecosystem, measurement of petroleum hydrocarbon content in sediments must be an integral part of oil pollution ’ studies. Intensified research efforts arising from enr‘ironmental and public health questions have resulted in numerous methods for the measurement of hydrocarbons in sediments (3-8). T h e toxicity of petroleum is well documented for a number of different compound classes and specific compounds such as naphthalene, benzo[a]pyrene, and toluidine (9). Polynuclear aromatic hydrocarbons (PAH) have been studied extensively in recent years because of reported mutagenic and carcinogenic properties (IO). Environmental PAH concentrations must be monitored in order to assess potential human exposure. Analyses of environmentally significant molecules present a t trace levels are currently being performed in many laboratories, and the environmental analytical chemist is being called upon to report narrower confidence limits at lower levels of petroleum pollution. Ultimately, he must seek to extend t h e range of analysis to the sub-ppb level for accurate measurement and assessment of the hydrocarbon burden. For many of the environmental analyses, there is little or no knowledge of comparability of data from different laboratories and, in most cases, probably little knowledge of intralaboratory precision. In order that the data from diverse methods be meaningful and reliable, there must be a basis for intercomparability. Furthermore, unless the data can be p u t on a n equivalent basis, environmental standards can be neither set nor enforced. Farrington et al. have intercalibrated gas chromatographic analyses for hydrocarbons in spiked cod liver lipid extracts a n d tuna meal samples and found good agreement among
The intercalibration material consisted of two intertidal sediment samples from the Prince William Sound and Northeastern Gulf of Alaska. Two sites were selected for sampling: Hinchinbrook Island: 146O 41’ W, 60’ 21’ N; this site is at the ocean entrance to the Prince William Sound and is constantly being washed with water from the Gulf of Alaska. Katalla River: 144O 35’ W, 60° 11’N; this site is downstream from a known oil seep and provides samples with hydrocarbons known to be of petroleum origin. All samples were collected during low tide and stored in precleaned 1-gal. tin-plated steel cans. Samples were frozen immediately with dry ice and maintained in that state except for a brief period when the sediments were homogenized. The bulk sediment from each site was homogenized by mixing for 3 h in a specially modified cement mixer which had been cleaned with pentane prior to use. Subsamples (-350 g) of each sediment were removed from the rotating mixer with a stainless steel trowel and placed in 16-02, acid-washed,glass bottles. The bottles were sealed with plastic screw caps containing aluminum foil cap liners. These samples were refrozen immediately after packaging. Two bottles each of the Katalla and Hinchinbrook sediment samples were shipped frozen to each participating laboratory. The following data were to be obtained for each sample: 1. Total hydrocarbons in GC elution range (approximately C 10-C30). 2. Total extractable hydrocarbons. 3. Pristane/Phvtane ratio and the amount of each of these present. 4. Percent water. 5. Identities and amounts of the three most abundant aliphatic and three most abundant aromatic hydrocarbons. 6. Total polynuclear aromatic hydrocarbon (PAH) concentration (4 rings and larger). 7. Identity and amount of the most abundant PAH (4 rings or larger). The analytical methods employed by each of the participating laboratories are summarized briefly in Table I.
EXPERIMENTAL
RESULTS AND DISCUSSION The importance of establishing environmental baselines for hydrocarbon levels in sediments is well accepted; however, these baselines are meaningful only if one can assess the accuracy and precision of the data. This intercalibration
This paper not subject to U S . Copyright. Published 1978 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
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ANALYTICAL CHEMISTRY, VOL. 50, NO.
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Table 11. Homogeneity Studies on Intercalibration Materials, Results of Replicate Analyses Hinchinbrook Sediment Katalla Sediment
H-4
437 318 290
K-21
Hydrocarbons in GC range, a i k g 709 816 767
H-23
564 470 399 352
K-36
1071 7 28
K- 1
1226 1093 1175
Hydrocarbons in GC range, pg/kg
Bottle
H-39
Bottle
H-30 + H-31 homogenized
a
282 K-15 723 394 386 408 Averagea 418 i 124 (30%) Average (n= 1 2 ) ~ n indicates number of analyses. Precision expressed as the standard deviation ( l o ) .
602
9 1 0 i 231 (25%) ( n= 9)b
Table 111. Analyses of Hinchinbrook and Katalla Sediments“ Water, 5% Laboratory NBS 2
4.4 4.7 4.4
3
5.0
4 5 6 7 8 Range
Total extractable hydrocarbons, &gig
Hinchinbrook
-
*
0.1
5.8 4.3 4.3 4.79
15.4 i 5.7 14.3 i 2.1
-
4.3 - 15.4
Katalla 22.5 23.5 22.6 23.5 22.8
k
0.2
-
22.4 22.2 26.3 21.3 36.5 34.3
-
Hinchinbrook 0.22 4.4 6.2 24.3b 7.9
-
i i
6 2.1
21.3 - 36.5
-
Katalla
-
2.5 12.8 11.0 65.8 57.6
-
3.12 7.92 2.9 0.64 0.22 - 7.92
109 10.7 5.4 3.9
-
2.5 - 1 0 9
a Some laboratories supplied results of duplicate analyses. In such cases both results are presented in the Table. Where presented, precision is expressed as the standard deviation (lu). Laboratory 3 reported that this result is probably in error.
exercise was conducted to determine the adequacy of analytical procedures for hydrocarbon determinations in sediments and to indicate the uncertainty with which results from different laboratories may be compared. The current. most commonly used analytical approach for determining hydrocarbons in sediments involves an organic solvent extraction, saponification, and column or thin-layer chromatography to isolate the hydrocarbons (1,13). Within this general scheme, however, there exists a variety of analytical methods. Since the “true” or “actual” values cannot be verified with current state-of-the-art methodology, one cannot conclude which is the “best” method or result. It is imperative, however, that one be cognizant of the limitations of each method; knowledge of how a procedure compares with others is extremely important when environmental decisions with far reaching economic and social consequences are to be made. Examination of t h e Hinchinbrook and Katalla sediments showed both to be predominantly fine to medium grain sand. Homogeneity studies on t h e two sediments were conducted by t h e National Bureau of Standards utilizing the dynamic headspace sampling technique previously described (8). The results of these studies are summarized in Table 11. T h e relative standard deviation for the Katalla sediment (910 Fg/kg h 2 5 % , n = 9) is slightly better than that for the Hinchinbrook sediment (420 Wg/kg f 30%, n = 12). An
internal standard of phenanthrene was added t o both sediments at the 20 kg/kg concentration level; an average of 83% was recovered from the Karalla sediment, while only 41% was recovered from the Hinchinbrook sediment. Mesitylene, naphthalene, and trimethylnaphthalene also exhibited similar recovery behavior from the two sediments. The Hinchinbrook sediment thus appears t o have greater affinity for hydrocarbons than the Katalla sediment. Intercomparison Results. Table I11 contains the results of percent water analyses for t h e two sediments. T h e agreement is generally good with the exception of high results from lab No. 7, which accounts for the large standard deviations (Hinchinbrook = 6.7 f 4.3% HzO, Katalla = 25.3 f 5.2% HzO). However, this uncertainty or even larger uncertainties have no significant effect on the remaining data, which are reported on a dry weight basis. T h e amount of extractable hydrocarbons obtained for the sediments is reported in Table 111. Laboratories 5 , 6, and 8 dried the samples (freeze dried or otherwise) prior t o extraction. T h e drying process results in some loss of hydrocarbons (up to Czo,depending on the procedure, temperature, etc.) from the mixture of hydrocarbons to be measured. Farrington (14) has suggested a n alternative method to circumvent this loss which employs a headspace analysis of the sediment, followed by freeze drying and solvent extraction.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MAP,CH 1978
Table IV. Hinchinbrook Hydrocarbons in GC range, pg/kg Lab oratory
Aliphatic
Unsat./ aromatic
Total
Pristane, -phvtane ratio
NBS (headspace)
420 + 1 2 0
0.9
NBS (extraction)
250
0.8
144 103
2.59
?
0.2
___
Most abundant hydrocaybons, pgikg
Aliphatic
Aromatic Me-Naph 2 C,-Naph 3. C,-Naph I.
C,, 16 I1 2 C,, 1 5 2 8 c,, 1 2 1 1 c,; 7
10
54 57
21 23
3
4 5
4
140 93
75 80
-
-
9 26
35 12
6
7 8
loo? 500 ? -
50
40
i-
30
600
400
?
100
2.05 2.18 -
44 38
-
15.9 34.1
-
140 900
-
-
Cz540
c,, 20 C?, 1 9 Prist 4 c,, 4 c,, 4 c,, 4
-
1.67
2.11 3.6
c,, 1.1
3
C,, 30 c,, 1 2 c,, 1 2 C,, 5 c,, 5
c,, 5
C,-Naph l . 3 Phen 0.7 C,-Fluor 0.2 unk 7 unk 3 unk 3
C,-Naph 2.7 Phen 0.7 2-Me-Naph 0.3 unk 3 unk 3 unk 3 -
C,, 1.4 c,, 1.3
i
-
c,, 7
C2,6 2
461
C,, 4.7 c,, 4.5 c,, 3.3
-
C,, 0.39 C,, Prist 0.38 C,, C,, 0.32 C,, Prist
unk 6
-
unk 3 unk 2
-
unk 1
unk 1.8 unk 1.5 unk 1.2
unk 0.6 unk 0.5 unk 0.4
-
2.9
1.9 1.8 1.6
Range
9-300 4-400 15.9-900 0.8-3.6 Some laboratories supplied results of duplicate analyses. In such cases both results are presented in the Table. Laboratory 7 submitted a summary of multiple analyses on each bottle of sediment. All precision data is expressed ai the standard deviation ( l o ) . In the Table, unk is used as an abbreviation for unknown; C, represents n-alkane containing x carbons; Prist is pristane; Me is methyl; Naph, Phen, and Fluor are naphthalene, phenanthrene, and fluoranthene, respect.ively. A dash (-) is used when results were not supplied by a participating laboratory. a
Losses of volatile hydrocarbons would be minimized with such a procedure and a broader molecular weight range of compounds could be analyzed. Data obtained for the Hinchinbrook sediment, including hydrocarbons in the GC range, pristane/phytane ratio, and t h e most abundant aliphatic and aromatic hydrocarbons are presented in Table IV; analogous data for the Katalla sediment are shown in Table V. T h e results of measurement of hydrocarbons in the GC range vary widely among the eight laboratories; the agreement is better for the Katalla sediment than for the Hinchinbrook sediment. This variability, which exists even for laboratories employing similar extraction and/or work-up procedures, may be partially a result of the different manner in which the gas chromatographic quantification was carried out. GC analysis of the saturated or aliphatic fraction of t h e sediment extract usually produces a chromatogram with an unresolved complex mixture of alkanes and cycloalkanes with a wide range of molecular weights. Quantitative data based solely on resolved chromatographic peaks differs from that in which a contribution from the unresolved “envelope” is considered. Studies were conducted a t NBS in which a sediment sample was headspace-extracted and analyzed by capillary column gas chromatography. The resulting chromatogram was quantified both on the basis of resolved peaks only, and resolved peaks plus a contribution from the unresolved envelope. Values for t h e hydrocarbon concentration showed a variability as high as 300%. In cases where quantitation was based on an external standard, the percent recovery for each component of the standard must be known. Warner (15)has shown that diethyl ether extraction recoveries for naphthalene, dimethylnaphthalene, and biphenyl from spiked marine organisms may be as low as 40% for concentrations below 0.1 wg/g. T h e
addition of an internal standard prior to any analysis step would seem logical in order to correct for such losses. T h e internal standard should contain both aliphatic and aromatic components characteristic of the molecular weight range and concentration of compounds to be analyzed in the samples. Losses during sample work-up are compensated for by a similar loss of the internal standard. T h e sample must be analyzed with and without the internal standard, however, to ensure that components in the standard are not also present in the sample; or if they are present, their contribution can be taken into account. The underlying assumption in methods involving an internal standard is that the standard is incorporated into, and equilibrated with, the sample matrix. This may or may not be the case, however, and errors may result. Values for the most abundant aliphatic hydrocarbons in the Katalla sediment (Table V) show t h a t the headspace extraction recovered the volatile, lower molwular weight components, C9-Cll, which may be lost during the sample drying step or the solvent concentration step required in methods employing an organic extraction. Sample extracts were saponified to reduce the problem of 5eparating hydrocarbons from lipids coextracted from the sediments by laboratories 3, 6, 7 , and 8. These compounds may co-elute or overlap with peaks of interest on certain chromatographic systems (14). Methods which d o not remove these polar compounds may be expected to give results for hydrocarbon content which are high. Even when saponification is carried out, there is a potential problem of transesterification with the potassium hydroxide-methanol extraction usually used. Methyl esters of fatty acids may be produced at concentrations which are significant when analyzing for hydrocarbons a t the ppm level (16). Farrington has noted that saponification in the presence of 25% water will reduce transesterification considerably (3). Laboratory
462
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Table V. Katalla Sediment‘, Hydrocarbons in GC range Hg/kg Laboratory
Aliphatic
Unsat./ aromatic
Total
NBS (headspace)
910
NBS (extraction)
2700
t
230
Pristane/ phytane ratio
610 880
120 130
730 1010
3
1940 1420
530 710
2470 2130
4
-
-
-
-
5
3454 6625
401 4 17
3855 7042
2.81 2.69
196 49.4
7
2 0 0 t 200 4 0 0 i 200
14 4.2
-
8
210 53.6
3 0 0 t 300 8 0 t 10
-
3.71 2.38
500 480
-
Aliphatic
Aromatic
1.9 t 0.02 C,, 32 i 11 C,, 25 i 7 C, 23 + 1 3 1.7 c,, 6 6 c,, 6 6 Prist 58 3.27 C,, 42 C,, 9 8 3.27 C 24 C,, 6 0 Pi%t 2 3 C,, 57 C,, 180 Prist 110 3.55 6.32 C15 180 C,, 100 Prist 1 4 0 unk 9 0
2
6
Most abundant hydrocarbons, pg/kg
3.25 3.10
-
C,, 7 5 C,, 1 9 4 C,, 57 C,, 1 7 9 C,, 5 3 C,, 1 3 8 c,, 19.9 c,, 4.3 C,, 18.7 C,, 4.1 C,, 13.7 C,, 3.9 C,, + Prist 47 C,, 36 C;, + Phyt 2 1 Prist 28.8 Prist 19.9 C,, 22.7 C,, 16.2 C,, 22.7 C,, 15.5
Me-Naph 1 6 t 5 C,-Naph 1 5 j 3 C,-Naph 1 4 i 4 C,-Naph 54 C,-Naph 27 Me-Naph 24 Phen 9.7 Phen 9.1 C,-Naph 7.1 C,-Naph 7.4 2-Me-Naph 5.3 2-Me-Naph 5.7 unk 50 unk 72 unk 50 unk 58 unk 40 unk 29
-
unk unk unk unk unk unk
19
14 8 2.9 2.5 2.1
-
unk unk unk unk unk unk
14 13 5 0.8 0.7 0.7
-
Range 49.4-6625 4.2-710 53.6-7042 1.7-6.32 a Some laboratories supplied results of duplicate analyses. In such cases both results are presented in the Table. Laboratory 7 submitted a summary of multiple analyses on each bottle of sediment. All precision data is expressed as the standard deviation ( l o ) . Abbreviations are the same as in Table IV, in addition Phyt is phytane. A dash (-) is used when results were not supplied by a participating laboratory. Table VI. Polynuclear Aromatic Hydrocarbons in Sedimentsa, Hinchinbrook Sediment Laboratory NBS 2
Katalla Sediment
Total PAH, pg/kg, Total PAH, wg/kg, 4 rings and larger Most abundant PAH, pg/kg 4 rings and larger 5
4
t
0.5
-
3.8
chrysene 0.3 pyrene 0.08 pyrene 0.1 chrysene 1 Me-pyrenes and Me-fluoranthenes 1
40 t 2 10 8.6 74
Most abundant PAH, pg/kg Me-chrysene 3 Me-pyrene 3.9 Me-pyrene 3.2 Me-pyrenes and Me-fluoranthenes 28
Average
4.4 t 0.85 33.2 t 3 1 ( n = 2)“ ( n= 4) Results of duplicate analyses are presented for Laboratory 2. All precision data is expressed as the standard deviation A dash (-) indicates no results were supplied. n indicates the number of values averaged. (lo).
1 (NBS) employed high-performance liquid chromatography (HPLC) t o remove t h e polar biogenic compounds in the extraction procedure, but not in the headspace procedure (see Table I). It was found that an HPLC clean-up of headspace sampled sediment resulted in no change in the results of the GC analysis. This result indicates that these interfering compounds were removed from the sample matrix during solvent extraction only and not during headspace sampling. Values for relative amounts of pristane and phytane are sometimes used to differentiate natural sources of hydrocarbons such as biogenic hydrocarbons from petroleum-based pollutants (5). Experimental values for the pristane/phytane ratio (Tables IV and V) are in sufficient agreement to answer this question. Results for the polynuclear aromatic hydrocarbon (PAH) content of the samples are presented in Table VI. Only three of the eight laboratories involved in the intercomparison submitted results for PAH concentrations. It seems clear from this limited response t h a t this higher molecular weight fraction, which may be the most critical in terms of toxicity, carcinogenicity, and persistence, cannot be easily determined
by gas chromatography alone. Laboratories 2 and 4 both found the methylpyrenes to be t h e most abundant PAH (4 rings and larger) in the Katalla sediment. Laboratory 4 identified methyl-substituted pyrenes and fluoranthenes, and chrysene in the Katalla sediment by comparison of their mass spectra with known standards. NBS used HPLC and fluorescence emission spectroscopy to identify methylchrysene as the most abundant PAH in t h e Katalla sediment.
CONCLUSIONS T h e results of this study indicate the high variability of state-of-the-arthydrocarbon analyses on “real world” sediment samples. Unlike intercalibration on spiked samples where a substrate is added to a matrix a t a suitable concentration and assumed to be incorporated into and equilibrated with t h e matrix, intercalibration on real samples requires no such assumption. In setting environmental baselines, use of inaccurate and imprecise consensus values is always a danger; we feel the intercomparison data for a common (homogeneous) sediment sample are a necessary addition to such baseline data. We hope other laboratories will be encouraged t o
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)
