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Determination of Selenium in Copper by Hydride Generation/Atomic Absorption Spectrometry after Electrolytic Removal of Copper Ragnar Bye Department of Chemistry, University of Oslo, Box 1033 Blindern, Oslo 3, Norway Methods for determination of small concentrations of selenium in copper by atomic absorption spectrometry have generally been beset with problems. Although it has been claimed that selenium can be determined directly in the nitric acid sample solution by graphite furnace atomic absorption spectrometry ( I ) , most authors have found it necessary to include a separation step (2-5). This has most frequently been done by coprecipitation of the selenite ions, either with iron(II1) hydroxide (2,3) or lanthanum hydroxide ( 4 ) or with arsenic after reduction of selenite to elemental selenium (5). As such, methods require strict control of the p H of the solution, and filtration and a subsequent dissolution of the precipitate seem unattractive due to the risk of loss/contamination. Even more important, the use of iron(II1) hydroxide might easily lead t o erroneous results owing t o the severe spectral interference from iron when a deuterium source is used as background correction for selenium (6, 7). As to the hydride generation/atomic absorption technique, it is well-known that copper is one of the most seriously interfering elements. Several attempts have been made to minimize this, all of them having in common that they could only compensate for minor amounts of copper (8,9). However, the ratio of copper-selenium in copper samples is about lo5, and none of the proposed methods are able to compensate for such concentrations. The methods cited above (2-5) are all based on isolation of the selenium from the sample solution. They are therefore dependent on absolute, or a t least a reproducible, recovery of the isolated selenium. However, it might be that isolation of the interfering ion instead, Le., copper, from the sample solution is a more attractive approach, as the recovery of such a process should be less critical for the selenium result. A method, based on this idea, is presented in this paper. The copper is removed electrolytically from the sample solution by using the traditional electrogravimetric method for the determination of copper. T o prevent simultaneous electrodeposition of the selenium, it is necessary to ensure complete oxidation of tetravalent selenium to the hexavalent state prior to the electrodeposition. Se(V1) is not electrochemically reducible (10, 11). After the electrolysis is terminated, the platinum electrode is removed, and the selenium in the solution is reduced back to the tetravalent state with hydrochloric acid, thus becoming available for determination by the hydride generation technique.
EXPERIMENTAL SECTION Reagents. A 1ppm selenium(1V)solution was prepared from selenous acid (p.a.). A 5% (w/v) solution of potassium permanganate was prepared by dissolving 5 g of KMnO, (pea.)in water and diluting to 100 mL. H202(3%, v/v) was made by diluting 30% (v/v) H20z(p.a.1 10 times. All acids were of p.a. quality and were not diluted unless otherwise stated. Solid ammonium nitrate was of pea.quality. Instrumentation. A Perkin-Elmer 300 atomic absorption spectrometer with a selenium electrodeless discharge lamp was operated at 196.0 nm, slit width 2 nm. A Radiometer Servograph REC 51 strip-chart recorder was operated at 5 mV full scale. A Perkin-Elmer MHS-10 hydride generation system and a 10-cm single-slotburner were used as recommended by the manufacturer. An acetylene-air flame was employed with flow rates of 3.5 and 20 L m i d , respectively. Argon (99.99%) was used to purge the generating system. Procedure. To 0.4 g of copper, accurately weighed in a 150-mL beaker, add 8 mL of nitric acid (1 + 1). Cover the beaker with
a watch glass, and after the sample has dissolved (a few minutes), add 15 mL of water and 2-3 mL of sulfuric acid. Heat the beaker on a hot plate at 150 “C for 45 min. Dilute to the 75-mL mark on the beaker, and add potassium permanganate solution dropwise until the solution is definitely purple (2-3 drops are sufficient when analyzing “pure” copper). Place the beaker on the hot plate, and let the solution boil for 5 min. Add hydrogen peroxide solution dropwise until the purple color disappears and the solution regains its faint blue color. Boil again for 5 min. Let cool, and rinse the watch glass into the beaker and dilute to the 125-mL mark with water. Add 2 g of ammonium nitrate and electrolyzethe solution, with stirring, at a current of 1.5 A (at a voltage of about 2.5 V) by using a platinum gauze cathode and a platinum spiral anode. After 90 min, lower the beaker from the electrodes, and rinse these with a small volume of water into the beaker. Add 1mL of formic acid, cover with a watch glass, and boil for 5 min. After some cooling, rinse the watch glass with water and transfer the solution into a 400-mL beaker. Wash the 150-mL beaker with small volumes of water and five 25-mL portions of hydrochloric acid. The total volume in the 400-mL beaker should now be about 300 mL. Cover the beaker with a watch glass, and heat on a boiling water bath for 2 h. Let cool, rinse the watch glass, and transfer the solution to a 500-mL volumetric flask, which is filled to volume with water. Transfer 5-mL aliquots of the solution to the hydride generation vessel and perform the hydride generation/atomic absorption determination. Repeat the procedure after addition of suitable volumes (1100wL)of standard selenium(1V) solution. Finally, calculate the concentration of selenium in the sample from the results of these standard additions.
RESULTS AND DISCUSSION The Standard Reference Material 498 “Unalloyed Copper Cu V from the National Burea of Standards with a certified value for selenium of 14 f 4 pgg-l was analyzed 5 times. The following results were obtained 12.4, 11.6, 11.8, 12.9, and 13.7 g-g-l, giving a mean value of 12.5 and one standard deviation of 0.9 pgg-l in good agreement with the certified value. The blank value was determined by going through the procedure without a copper sample, giving typically 2 pgg-l. The values above were obtained after subtraction of the blank. The sample is dissolved in nitric acid followed by boiling after addition of sulfuric acid. This is to ensure removal of the nitrogen oxides formed during the dissolution, as these are probably the reason for the interferences that have been attributed to nitrate (12). The addition of potassium permanganate causes a rapid oxidation of tetravalent selenium to the hexavalent state. Any tellurium, antimony, and arsenic present will thereby be oxidized to their highest oxidation states too. The use of potassium permanganate precludes the use of sample dissolution mixtures containing hydrochloric acid, as it would be oxidized to chlorine by the permanganate. Hydrogen peroxide, used to destroy excess of permanganate, is known to reduce hexavalent selenium to tetravalent under certain conditions (13) but causes no trouble with this procedure as described. Before the electrolysis, ammonium nitrate is added to the solution as a “depolarizer” in order to eliminate hydrogen formation a t the cathode and to control the potential. Any nitrogen dioxide formed during electrolysis is decomposed by heating the solution with formic acid. The purpose of the heating step with hydrochloric acid is to reduce the hexavalent selenium to the tetravalent state (13). Only the tetravalent state is reducible to selenium hydride by sodium borohydride. There are few other effective re-
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ductants for this purpose. Besides, hydrochloric acid solutions are very suitable for hydride generation. Of other hydride-forming elements, tellurium will be reduced to the tetravalent state by hydrochloric acid, and the proposed method should therefore be applicable for this element too. This should also be the case for antimony and arsenic provided the hydrochloric acid also contains some iodide, as this is necessary to reduce these elements from pentavalent to the trivalent state. With minor modifications the method should also be useful for copper alloys as brass (Sn02 formed during dissolution must be filtered off; lead should end up on the anode and thus be removed). For copper-nickel alloys, however, nickel will remain in the solution during electrolysis and interfere seriously during the subsequent hydride formation step. However, a method for complexing a large amount of nickel in such analyses is to be published (14);but the method has not yet been tested on real samples.
Registry No. Se, 7782-49-2; Cu, 7440-50-8. LITERATURE CITED (1) Haynes, 8. H. A t . Absorpt. News/. 1979, 18, 46. (2) Ohta, K.; Suzuki, M. Anal. Chim. Acta 1975, 7 7 , 288. (3) Siu, K. W.; Berman, S. S. Anal. Chem. 1984, 5 6 , 1808. (4) BBdard, M.; Kerbyson, J. D. Can. J . Spectrosc. 1976, 21, 64. (5) Muir, M. K.; Anderson, T. N. A t . Spectrosc. 1982, 3 , 149. (6) Manning, D. C. At. Absorpt. News/. 1976, 1 7 , 107. (7) Fernandez, F. J.; Beaty, M. M. Spectrochim. Acta, Part 8 1984, 398, 519. (8) Bye, R.; Engvik, L.; Lund, W. Anal. Chem. 1983, 5 5 , 2457. (9) Vijan, P. N.: Leung, D. Anal. Chlm. Acta 1980, 120, 141. (10) Holak, W. J . Assoc. Off. Anal. Chem. 1976, 5 9 , 650. Adeloju, S. B.; Bond, A. M.; Hughes, H. C. Anal. Chim. Acta 1983. 148, 59. (12) Campell, A. D. Pure Appl. Chem. 1984, 5 6 , 645. (13) Bye, R. Talanta 1983, 30,993. (14) Bye, R. Analyst (London) 1985, 110, 85.
Received for review December 10,1984. Accepted February 11, 1985.
Semiautomated Method for Determination of Selenium in Geological Materials Using a Flow Injection Analysis Technique C h r i s C. Y. C h a n
Geoscience Laboratories, Ontario Geological Survey, Ministry of Natural Resources, 77 Grenville Street, Toronto, Ontario, Canada Following the development of an automated method (1)for the determination of Se in rocks using hydride generation and atomic absorption techniques, an effort has been made to improve sensitivity and reduce analysis time. The new approach is based on the adoption of a nonsegmented stream (2, 3), instead of an air-segmented stream, for performing chemical analysis in a continuous flow system in conjunction with the use of hydride generation and AAS techniques. A flow injection module was utilized for insertion of a sample segment into a continuous flowing stream. Since air bubbles are not required as an agent for mixing and segmenting the solutions, narrow tubing can be used throughout the entire flow system. This offers several advantages: (1) the flow volume being miniaturized allows the transportation of sample to be completed more rapidly; (2) the sample zone is well defined with little dispersion a t the boundary; (3) the sample segment is not being diluted by the carrier solution; (4) the volumes of sample segments are extremely reproducible; and ( 5 ) the sample and the reagent solutions can be well mixed within the narrow stream. Beacuse of these flow characteristics, the signal response is not only rapid and precise but the resolution and peak height are improved. This paper describes the new method. The method permits the accurate determination of Se in geological materials a t levels as low as 5 ppb with a rate of more than 50 digested samples per hour. Se values on 40 international geological reference samples are reported. EXPERIMENTAL S E C T I O N Instrumentation. A Varian Model AA6 atomic absorption spectrometer was equipped with a Model 9176 strip-chart recorder. A Technicon Sampler-I1 and a Proportioning Pump-I were used to sample and propel the solutions. A flow injection module (Model No. 1000-600, Lachat Chemical, Inc.) was utilized for sample injection. In performing flow injection analysis, samples are alternately pumped into and flushed out of the sample loop that is mounted on the valve of the flow injection module. The
connections of these components and the analytical system are shown in Figure 1. A gas-liquid separator was installed to separate the hydride from the waste solution. An impinger partly filled with concentrated sulfuric acid served to remove moisture and to homogenize the hydride-argon mixture. The quartz tube, 16 cm long and 10 mm i.d., with a 10 cm long inlet tube (3 mm i.d.) fused into it at the center, was wound with a 22-gauge chrome1 A heating wire and insulated with a layer of wrapped asbestos string. It was mounted on the burner of the spectrometer. The temperature of the quartz tube atomizer was controlled at 850 *20 "C by a variable transformer. Reagents. All chemicals used were reagent grade, and water was distilled from glass. Mineral acids were hydrofluoric acid (48%),perchloric acid (60%),nitric acid (70%), and hydrochloric acid (38%). Digestion Mixture. In a polyethylene bottle mix hydrofluoric acid, perchloric acid, nitric acid, and water in the ratio of 4:4:1:1, respectively. Reducing Solution. Dissolve 5 g of sodium borohydride and five pellets of sodium hydroxide in 500 mL of water. Store in a refrigerator when not in use. The solution is stable for at least a week at 4 "C. Masking Reagent. Dissolve 1g of 1,lO-phenanthroline in 100 mL of 0.1 M HC1. Stock Se Standard Solution, 1000 wg/mL. Dissolve 0.100 g of powdered selenium in 100 mL of 10% nitric acid. Working Se Standard Solutions. Prepare 0.25,0.50, 1.0, 2.0, and 4.0 ng/mL solutions by serial dilution of the stock standard solution with 3.6 N HC1. Decomposition of Samples. Digest 0.200 g of rock sample with 5 mL of digestion mixture in a 30-mL Teflon beaker on a hot plate for about an hour until white fumes of perchloric acid appear and the volume of the contents reduces to ca. 1 mL. Cool and add ca. 2 mL of water and 4.5 mL of concentrated HC1. Heat the contents t o just under boiling for several minutes to reduce the Se that is in the + 6 oxidation state to the + 4 state. Cool and transfer the contents to a test tube calibrated at 15 mL. Make up to volume with water. Seal the test tube with a piece of Parafilm, and mix the solution thoroughly. The concentration of HC1 in the sample solution is now 3.6 N (30% v/v). Prepare
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