JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST 199 1 VOL. 6 389 Hydride Generation Atomic Absorption Spectrometry From Alkaline Solutions Determination of Selenium in Copper and Nickel Materials* Torild Wickstrom and Walter Lund University of Oslo Department of Chemistry P.O. Box 1033 Oslo 3 Norway Ragnar Bye University of Oslo Department of Pharmacy P.O. Box 1068 Oslo 3 Norway Selenium is determined in copper and nickel materials without any interferences. Hydrochloric acid plus hydrogen peroxide are used to dissolve the samples. The solution is then made alkaline with sodium hydroxide in order to eliminate the interference from copper and nickel by precipitation of the corresponding hydroxides. Sodium tetrahydroborate is added to the alkaline solution in order to reduce selenium(iv) to the selenide ion and the solution is then filtered.The volatile selenium hydride is generated by acidification of the alkaline solution in a continuous flow system. The method was used successfully for the determination of selenium in three standard reference materials (SRMs) from the National Institute of Standards and Technology SRM 398 Unalloyed Copper V SRM 671 Nickel Oxide 1 and SRM 875 Cupro-Nickel 10 (CDA 706) (doped). The detection limit of the method was approximately 1 pg g-l. Keywords Hydride generation atomic absorption spectrometry; selenium in copper and nickel materials; removal of interfering metals as hydroxides Hydride generation atomic absorption spectrometry (HGAAS) has become an established technique for the determination of a range of elements.Arsenic antimony and selenium are probably the elements most frequently determined by this technique but there are many reports on the successful determination of bismuth germanium tin lead and tellurium. The generation of the volatile hydrides is normally carried out under strongly acidic conditions ( 1 - 10 mol dm-3) with sodium tetrahydroborate as the reducing agent. The acidic conditions are necessary in order to achieve both the effective formation and release of the hydride gases from the solution as the hydrides possess acid-base properties illustrated by the following equations for selenium hydride H2O H2Sek) * H2Se(aq) H2Se(aq)+H,0 =H,O++HSe- Ka,=1.3x HSe-+H,O *H30++Se2- Ka2=1.0x lo-" In a strongly acidic solution the dissociation of H2Se is suppressed.Normally the sample solution is acidified prior to the hydride generation as part of the sample preparation procedure but this might not always be advantageous. For example (z) if the chloride concentration of a sample is high (e.g. sea-water) the addition of acid can result in the reduction of selenium(v1) to selenium(Iv) thus losing the possibility of determining both species; (ii) if the sample is alkaline and contains the hydride-forming element in its lowest oxidation state (selenides and arsenides) the ele- ment may be lost as the volatile hydride upon acidification; and (iiz] it might be necessary to make an acidic sample solution alkaline in order to remove some interfering elements. Hence in all of these instances the reduction of the hydride-forming elements directly in neutral or alkaline solution would be advantageous.The principles of hydride generation from alkaline solutions have recently been described.' The method is based on the fact that the tetrahydroborate ion is a strong reductant in alkaline solution H2B03-+5H20+8e-+BH,-+80H- Eo= - 1.24V * Presented in part at the Fifth Biennial National Atomic Spectroscopy Symposium (BNASS) Loughborough UK 18th-20th July 1990. Tetrahydroborate is therefore capable of reducing the hydride-forming elements in alkaline solutions not to the gaseous hydride but to the corresponding anion e.g. for selenium 4Se032- + 3BH4- + 4Se2- + 3H2B03- + 3H20 This can be utilized by adding sodium tetrahydroborate directly to an alkaline sample solution. After reduction of a given element to the corresponding anion acid is added in order to form and release the gaseous hydride.The addition of acid is most conveniently carried out using a continuous flow generator but a batch technique can also be used. The serious interference from copper and nickel and other metal ions in the determination of selenium by the HGAAS technique has been well do~umented.~~~ Several approaches for minimizing such interferences have been suggested including the addition of various reagents such as thiourea4 and iron(1n) soluti~n.~-~ Interfering metals can also be removed by precipitation. Thus Welz and Melche? removed nickel by precipitation as nickel hydroxide. The procedure involved filtration before the solution was acidified diluted to volume and analysed in the conven- tional way.The procedure for removing interfering metals by precip- itation as the hydroxides is simplified by carrying out the reduction directly in an alkaline solution because the number of steps is then reduced. In this paper such a method is described for the determination of selenium in copper and nickel materials as an example of the usefulness of hydride generation from an alkaline solution. Experimental Apparatus and Operating Conditions A Perkin-Elmer Model 300 atomic absorption spectrometer and an Omniscribe (Houston Instruments Austin TX USA) Model D-5 1 16-2 recorder were used. The electrode- less discharge selenium lamp was operated at 196.0 nm. A continuous flow hydride generator (P.S. Analytical Seve- noaks Kent UK) was operated with the following settings delay 15s; measure 30s; and reset 20s.The gas-liquid separator of the hydride generator was connected to a 19 cm long T-shaped quartz tube which was heated in an air-acetylene flame using a 10 cm single slot burner. Argon (99.99%) was used as the purging gas; the flow rate was 0.5 lmin-l.390 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST 1991 VOL. 6 Peristaltic - Acid solution To AA ectrometer Waste I Filter Valve in sample position Fig. 1 Diagram of the continuous flow hydride generator used for alkaline sample solutions A schematic flow diagram is shown in Fig. 1. Compared with the conventional configuration the hydrochloric acid and tetrahydroborate solution streams have been inter- changed the tetrahydroborate solution now acting as a 'blank' instead of the hydrochloric acid.Sodium tetrahy- droborate solution was also added to the alkaline sample solution. Both the blank and the sample solutions contained 0.4% sodium tetrahydroborate. The concentration of so- dium hydroxide in the sample solution was approximately 0.2 mol dm-3. Hydrochloric acid (4 mol dm-3) was used to acidify the alkaline sample solution and to generate the hydrides. The diameter of the peristaltic pump tubing was 0.5 mm for the hydrochloric acid and 0.8 mm for the other two streams giving flow rates of 3.2 and 7.2 mlmin-I respectively. In order to prevent particles from entering the hydride system the pump tubing carrying the sample was equipped with a G4 glass filter (pore size 5-1 5pm) at the inlet end.Reagents and Samples All reagents were of analytical-reagent grade and only de- ionized water was used. Sodium tetrahydroborate solution (3% m/v) was prepared by dissolving NaBH (Fluka) in 0.1 r n ~ l d m - ~ sodium hydroxide solution (Merck). The solution was filtered before use. A 4 mol dm-3 hydrochloric acid solution was prepared by dilution of the concentrated acid (37O/o Merck). A 1 .OOO g 1-l SeIV solution was prepared by dissolving 3.3308g of Na2Se03-5H20 (Fluka) in water and diluting to 1000 ml. Standard solutions of 2.5-3Opg 1-1 SeIV were prepared by diluting this solution further. The hydrogen peroxide solution was 30% (Merck). Three Standard Reference Materials (SRMs) with certi- fied values for selenium were obtained from the National Institute of Standards and Technology (NET) (formerly the National Bureau of Standards Gaithersburg MD USA).The samples were SRM 398 Unalloyed Copper V SRM 671 Nickel Oxide 1 and SRM 875 Cupro-Nickel 10 (CDA 706) (doped). Procedure Dissolution of copper metal Transfer 1 g of copper metal into a 100 ml beaker add 10 ml of 12 mol dm-3 hydrochloric acid and heat to 100 "C on a water-bath. Add 7.5ml of hydrogen peroxide in 0.5ml portions allowing the evolution of the gas to subside each time before adding a new portion. During this treatment leave the beaker on the water-bath. After dissolution cool and transfer the clear solution into a 250 ml calibrated flask and dilute to volume with water. Dissolution of nickel oxide and copper-nickel alloy Transfer 1 g of sample into a 100ml beaker and add in sequence 1 ml of hydrogen peroxide 10 ml of water and 10ml of hydrochloric acid (37Oh).Heat the solution to 100 "C on a boiling water-bath and add 3ml of hydrogen peroxide in 0.5 ml portions allowing the evolution of gas to subside each time before adding a new portion. After dissolution cool and transfer the clear solution into a 250ml calibrated flask and dilute to volume with water. Precipitation of copper and nickel Transfer a 1O.OOml aliquot from the 250ml flask into a 50 ml calibrated flask and add 5 ml of 3 mol dm-3 sodium hydroxide solution slowly with continuous swirling. Warm the precipitate formed on a water-bath at 100 "C for 1 h. After cooling add 6 ml of 3Oh sodium tetrahydroborate solution and dilute to volume (50 ml).Filter the solution through quartz wool (without washing the precipitate) and analyse the filtrate for selenium by the proposed method. Calibration The quantification is either by means of a calibration graph or a standard additions graph. The calibration graph is obtained using standard selenium solutions to which sodium tetrahydroborate is added so that the concentra- tion is the same as in the sample solution (0.4%). Alternatively a standard additions graph is obtained by adding 0 1 2 3 and 4 ml of a 0.5 mg 1-l SeIV solution to five 10ml aliquots of the dissolved sample which have been transferred into 50 ml calibrated flasks. To each flask 5 ml of 3 mol dm-3 sodium hydroxide solution are added slowly and the procedure described under Precipitation of copper and nickel is followed.Results and Discussion The dissolution of the samples with a mixture of hydro- chloric acid and hydrogen peroxide was preferred to oxidation with nitric acid because the excess of oxidant could then be removed easily by heating the solution. An excess of oxidant remaining would have resulted in the consumption of some of the tetrahydroborate solution added. The procedures for the copper metal and the nickel materials are somewhat different because it was found that the nickel materials reacted more vigorously with the acid than did the copper metal. In order to avoid the risk of losing selenium a milder treatment is used for the nickel materials. The dissolved sample was made alkaline with sodium hydroxide and sodium tetrahydroborate solution was added before the solution was diluted to volume.By using this procedure the subsequent filtration need not be quantitative which is advantageous because a quantitative filtration must involve a thorough washing of the precipi- tate. A washing step is time consuming and can lead to dilution of the sample solution. Apart from these considera- tions the tetrahydroborate solutions can as effectively be added after filtration and dilution to volume. Although not strictly necessary filtration of the solutions is recommended for two reasons. Firstly small particles of the precipitate can be introduced into the tubes of the hydride generator which involves the risk of a blockage. Secondly there is a possibility that the following unwanted reactions could take place after prolonged standing CuO(s) + Se2- + H20 3 CuSe(s) + 20H- Ni(OH),(s) + Se2- - NiSe(s) + 20H-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST 199 1 VOL.6 39 1 Table 1 Determination of selenium in NIST Standard Reference Materials; units rng kg-I Reference materials Certified value Found S* n t SRM 398 Unalloyed Copper 17.5 4 0.8 17.3 1.4 5 SRM 875 Cupro-Nickel 4 5.4 0.5 7 SRM 671 Nickel Oxide 2.0 _+ 0.3 2.1 0.5 6 * s= Standard deviation. t n=Number of parallel digestions. However within the time-scale of this procedure no indication of these reactions was observed. For filtering quartz wool was found to be preferable to a glass or paper filter because of simplicity and speed. Glass wool proved less satisfactory because not all the particles were retained by the filter. The G4 glass filter mounted on the inlet end of the tubing carrying the sample was used only as an extra safety precaution.The results of the analyses of three NIST SRMs are given in Table 1. There is very good agreement between the results obtained and the certified values. This is true even for SRM 875 as the value given by NIST for selenium in SRM 875 is based on three widely differing results 2.5 mg kg-l obtained by atomic absorption spectrometry; 4.3 mg kg-l obtained by spectrophotometry with diamino- benzidine as the reagent; and lomgkg-l also obtained by spectrophotometry. No further details about the methods were given in the certificate; it is surprising that NIST established a certified value based on the cited results. As a curiosity it can be mentioned that the mean value of these results is 5.6mgkg-l which is close to the value of 5.4 mg kg-l that we obtained.The good agreement between our results and the certified values demonstrates that selenium is not coprecipitated when copper and nickel hydroxides are formed. The validity of both the standard calibration graph and standard additions graph (see under Experimental) was tested for all three materials. It was found that the standard additions graph was always parallel to the calibration graph indicating that the interference from copper and nickel is completely eliminated in the proposed method. Therefore a calibration graph can be used for quantification. The slope of the calibration graph was 0.0048 absorbance 1pg-l. No significant blank value was observed. Based on a signal to noise ratio of 2 the detection limit in the final solution was estimated as approximately 1 ,ug l-l which using this pro- cedure corresponds to about 1 pgg-l in the solid sample. In this work a continuous flow system was employed for convenience. However a batch system might also be used; in this instance hydrochloric acid is added as the final reagent. The method of generating hydrides from alkaline solu- tions is a novel approach. As shown in the present paper it can represent a useful alternative to the conventional acid solution method. References Bye R. J. Autom. Chem. 1989 11 156. Welz B. and Melcher M. Analyst 1984 109 569. Hershey J. W. and Keliher P. N. Appl. Spectrosc. Rev. 1989 25 213. Bye R. Engvik L. and Lund W. Anal. Chern. 1983 55 2457. Welz B. and Melcher M. Analyst 1984 109 577. Bye R. Analyst 1986 111 11 1 . Bye R. Anal. Chim. Acta 1987 192 1 1 5. Welz B. and Melcher M. Anal. Chim. Acta 1983 153 297. Paper I /00313E Received January 22nd I991 Accepted March 26th 1991