Anal. Chem. 1985, 57, 1703-1706 (13) Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54, 2026-2029. (14) Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 5 4 , 1824-1825. (15) Mori. K.; Imasaka, T.; Ishlbashi, N. Anal. Chem. 1982, 5 4 , 2034-2038. (161 Mlvalshi. K.: Imasaka, T.: Ishlbashi. N. Anal. Chem. 1982. 5 4 , 2039-2044. Buffett, C. E.; Morris, M. D. Anal. Chem. 1983, 5 5 , 376-370. Carter, C. A.; Harris, J. M. Anal. Chem. 1984, 5 6 , 922-925. Sepaniak, M. J.; Vargo, J. D.;Kettler, C. N.; Maskarinec, M. P. Anal. Chem. 1984. 5 6 , 1252-1257. Pang, T. J.; Morris, M. D. Anal. Chem. 1984, 5 6 , 1467-1469. Leach, R. A.; Harris, J. M. Anal. Chem. 1984, 5 6 , 1481-1487. Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science 1978. 797, 183- 185. Fang, H. L.; Swofford, R. L. I n “Uitrasensitive Laser Spectroscopy”; Kliger, D. S.,Ed.; Academic Press: New York, 1983; Chapter 3. Buffett, C. E.; Morrls, M. D. Appl. Spectrosc. 1983, 3 7 , 455-458. Jansen, K. L.; Harris, J. M. “Abstracts of Papers”, 183rd National Meetlng of the American Chemical Society, Las Vegas, Aprll 1982; American Chemical Socletv: Washinaton. - DC.. 1982: Abstract ANYL 126. 1982, 21, (26) Sheldon, S. J., Knight, L. V.; Thorne, J. M. Appl. . . Opt. . 1663-1669. (27) Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 23, 476-481. (28) Carman, R. 0.; Kelley, P. L. Appl. Phys. Left. 1968, 72, 241-243.
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(29) Bevington P. R. “Data Reduction and Error Analysis for the Physical Sclences”; McGraw-Hill: New York, 1969; Chapter 3. (30) Casasent, D.; Psaltls, D.Proc. SOC.Photo-Opt. Inshum. €ng . 1978, 201, 107. (31) Meihuish, W. H. J. Res. Natl. Bur. Stand., Sect. A 1972, 76A, 547-560. (32) Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256-1261. (33) Currle, L. A. Anal. Chem. 1968. 40, 586-593. (34) Lee, W. H. I n “Progress in Optics”; Wolt, E., Ed.; North Holland: Amsterdam, 1978; Voc XVI. (35) Blodgett, J. A.; Athale, R. A.; Giies, C. L.; Szu, H. H. Opt. Left. 1982, 7 ,. 7-9. . . _. (36) Casasent, D.; Cheatham, L.; Fetterly, D. Appl. Opt. 1982, 27, 3292-3298. . ~ . ~ (37) Nieman, G. C.; Coison, S. D. J. Chem. Phys. 1978, 68, 2994-2996. (38) Perry, J. W.; Ryabov, E. A.; Zewail, A. H. Laser Chem. 1982, 7 , 9-15. (39) Scheeline, A., personal communication, University of Illinois at Urbana-Champaign, 1983.
RECEIVED for review November 5, 1984. Resubmitted April 8, 1985. Accepted April 8, 1985. This material is based upon work supported by the National Science Foundation under Grant CHE82-06898.
Radiotracer Error-Diagnostic Investigation of Selenium Determination by Hydride-Generation Atomic Absorption Spectrometry Involving Treatment with Hydrogen Peroxide and Hydrochloric Acid Viliam Krivan* and Kilian Petrick Sektion Analytik und Hochstreinigung, Universitat Ulm, Oberer Eselsberg N-26, 0 - 7 9 0 0 Ulm, Federal Republic of Germany
Bernhard Welz and Marianne Melcher Department of Applied Research, Bodenseewerk Perkin-Elmer & Co. GmbH, 0-7770 Uberlingen, Federal Republic of Germany
The radiotracer technique was used to find out the sources of systematic errors of a hydride generation AAS procedure for the determlnatlon of selenium in wastewater and other environmental samples. The Investigated procedure involves sample decomposition with a mixture of sulfuric acld and hydrogen peroxide, and reduction of Se(V1) to Se( I V ) by boiling with 5 M hydrochloric acid. Satisfactory recoveries are obtained in the decomposition (99.7 f 2%), reduction 98 f 1%), and hydration (95 f 3%) steps. Serious errors can be introduced via the the-dependent back oxidatlon of Se(1V) to Se(V1) by the residual chlorine produced in the reduction step. Removal of the chlorine from the solutlon is necessary If the hydratlon does not follow the reduction step immediately
.
Hydride-generation atomic absorption spectrometry has become one of the most important techniques for the determination of traces of selenium (1). Its main advantages are the high sensitivity, the small number of interfering elements, and the absence of background attenuation in the atomization step. The valency state of the analyte element, however, is of principal importance, because selenium hydride can be
formed essentially only from tetravalent selenium. For this reason, hexavalent selenium must be reduced to its tetravalent state prior to the hydride generation. Boiling with 5 to 6 M hydrochloric acid is most frequently used for this purpose (2). In the analysis of wastewater or other environmental samples, the determination of selenium by the hydride-generation technique may be influenced by matrix constituents (3)as well as by the chemical form in which selenium is bound. Heating with sulfuric acid and hydrogen peroxide in a flask equipped with a reservoir and a reflux condenser has been used for the destruction of organic constituents in wastewater and similar samples ( 4 ) . In a systematic investigation involving this decomposition procedure (5), apparent losses of selenium were observed even when matrix-free reference solutions were processed. In addition, these losses showed a large scatter between nearly 0 and 40%. Volatilization of selenium was unlikely because all work was done in an essentially closed system with an intensive reflux condenser. The recovery for selenium was reproducibly around 100% whenever the decomposition was carried out in the presence of trivalent iron and one or both of the other metals, copper and/or nickel. In the present work, 75Sewas used as radiotracer to reveal these unidentified losses of selenium, whereby all essential steps of the procedure, Le., the decomposition, the hydride
0003-2700/85/0357-1703$01.50/00 1985 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
generation and the hydride transport, were examined. The modification of the decomposition procedure proposed on the basis of the radiotracer experiments has also been tested by hydride-generation AAS measurements. EXPERIMENTAL SECTION Instrumentation. The decomposition apparatus described in ref 4 was slightly modified for these experiments. It comprises a double-neck round-bottom flask (volume 250 mL), condensate reservoir with vapor bypass tube, and a reflux condenser. The condensate reservoir with reflux condenser is fitted to the central neck of the flask; the reservoir is closed by a stopcock at the bottom with a 9 mm bore. The side neck is used for a gas purge inlet. A Perkin-Elmer MHS-10 mercury/hydride system was used for the radiotracer experiments. The system is described in detail elsewhere (6). A 100-mL gas wash bottle was mounted behind the MHS-10 to absorb the selenium hydride. A mrinually operated PTFE valve had to be installed between the reaction flask and the reservoir for the reducing agent to avoid any solution being forced back into the reductant reservoir. These modifications made a reaction time of 15 s necessary for applying the same volume of reducing agent (6 mL) as in the unmodified system. A well-type NaI(T1) scintillation detector (3 in. X 3 in.) coupled to a Berthold single-channel analyzer with automatic sample chahger was used for y-counting in the energy region 100-420 keV. A counting error below 1% was achieved by the choice of an appropriate counting rate and counting time in each experiment. A Perkin-Elmer Model 4000 atomic absorption spectrometer, equipped with an electrodeless discharge lamp for selenium, operated at 6 W from an external power supply, was used for the AAS measurements. Selenium was determined at the 196.0-nm line using a 2.0-nm slit width. A Perkin-Elmer MHS-20 mercury/hydride system operated with a quartz tube temperature of 900 "C, and time settings of 35 s for PURGE I, 8 s for REACTION, and 30 s for PURGE I1 were used to generate and atomize selenium hydride. The MHS-20 and its operation are described in detail elsewhere (7). Reagents. The radiotracer experiments were carried out with a commercial tracer 75Sein form of selenious acid in 0.2 M HC1 (NEN Chemicals, Boston, MA), with a nominal specific activity of 58.4 Ci-g-l. The radionuclide purity is given as 99.9999%. The decomposition experiments were carried out with 5 pg of selenium carrier and the hydration experiments were carried out with 80 ng of selenium carrier. Sodium tetrahydroborate solution (3% w/v) was prepared from sodium tetrahydroborate powder (Riedel-de-Haen AG, Seelze/ Hannover, FRG) by dissolving in deionized water and stabilizing with 1% (w/v) sodium hydroxide. The solution was filtered before use. Standard selenium(1V) stock solution (1000 mg/L) was prepared by diluting a Titrisol solution (Merck, Darmstadt, FRG) to the appropriate volume with deionized water. Hydrochloric acid (32 or 37%, Merck), sulfuric acid (95%, Fluka, Buchs, Switzerland), hydrogen peroxide (30%, Merck), and sodium hydroxide (Merck) were of analytical reagent grade. Sample Decomposition. A 50-mL water or wastewater sample, 5 mL of sulfuric acid, and 5 mL of hydrogen peroxide are placed into the round-bottom flask of the decomposition apparatus. After the mixture was heated to boiling for 30 min, the PTFE cock is closed and the condensate (about 40 mL) collected in the reservoir until the sulfuric acid starts to give off fumes. After cooling, 25 mL of hydrochloric acid (32% w/v) is added to the condensate in the reservoir and the contents are brought back into the flask and heated for another 15 min to boiling to reduce the hexavalent to tetravalent selenium. During the boiling, the flask is flushed with a nitrogen stream of 5 L/h. After cooling, the decomposition solution is transferred into a 250-mL volumetric flask and the apparatus washed with a minimum of deionized water. In order to obtain 5 M HC1 resulting solution, 92 mL of hydrochloric acid (32% w/v) is added to the decomposition solution transferred to a 250-mL volumetric flask and made up to volume with deionized water. A 5-mL portion of this solution (corresponding to a l-mL water sample) is added to 35 mL of hydrochloric acid (16% w/v) in the reaction flask of the hydride system for the determination of selenium.
Determination of the Activity. For all radiotracer experiments a gas wash bottle filled with 50 mL of 1% (w/v) sodium hydroxide solution was mounted behind the hydride-generation system to absorb the selenium hydride. An appropriate volume of the solution was pipetted from the reaction flask of the hydride system or from the sodium hydroxide absorber solution in the gas wash bottle before and after hydration for counting. After execution of the activity counting, the solution was always returned into the reaction flask or the wash bottle. An experimentally determined correction factor was used for the increase in volume (by 6 mL) in the hydration step. To check whether adsorption of selenium on the walls is taking place, both the decomposition flask and the hydration flask were counted before and after the execution of the process. RESULTS AND DISCUSSION Hydration Efficiency. After the execution of the hydration, 95 f 3% of the selenium activity was found in the sodium hydroxide absorber solution. Up to 3% of the selenium was adsorbed on the walls of previously unused reaction flasks, but only less than 0.5% after about 20 hydrations. Adsorption on the silicone tubing and on other parts of the apparatus was difficult to quantify (different geometry), but it was at the level of 1% or less. A loss of 3 to 7% of the original activity was found after summing all activities. No activity was found in a second wash bottle with sodium hydroxide absorber solution behind the first one, showing that selenium hydride is absorbed quantitatively in the first wash bottle. The adsorption on the reaction flask and the tubing is probably due to elemental selenium which is formed by decomposition of selenium hydride. No detectable adsorption of selenium(1V) (less than 1%) on polypropylene or glass was found even within one week in separate experiments. Sample Decomposition. The purpose of the digestion of the sample with sulfuric acid/hydrogen peroxide is the decomposition of the organic compounds and the oxidation of the lower valency states of selenium. The final heating step with 16% (w/v) hydrochloric acid is used to reduce the hexavalent selenium to its tetravalent state. The recovery of selenium in these two steps was also investigated by the radiotracer technique. The decomposition procedure was carried out with model solutions containing 5 pg of selenium in 50 mL of deionized water. The activity of the solution as well as of the digestion flask was measured before and after the decomposition. The obtained mean recovery (n = 13) of 99.7 f 2.2% in the solution proves that no selenium is lost during the decomposition. Back Oxidation of Se(1V) to Se(V1). Preliminary experiments indicated that the yield of selenium in the hydride generation was strongly dependent on the time that elapsed between reduction and hydration. If the hydration was carried out immediately (about 20 min) after the reduction, a residual activity of 7 f 0.5% was found in the hydride-generation flask after the hydration. The residual activity was the higher the later the hydration was carried out after the reduction step, and increased to 80 to 95% after about 1week. We were able to prove that this increase of the nonreactive selenium with time was due to a slow back oxidation of tetravalent selenium to the hexavalent state by chlorine formed in larger quantities in a reaction between hydrochloric acid and hydrogen peroxide. The redox reaction between selenite and chlorine
H2Se03+ C12 + H20 F.i H2Se04 + 2HC1
(1)
is an equilibrium reaction, Using standard heats of formation for the compounds involved in 1 M aqueous solution and neglecting the heats of dilution for selenic and selenious acids from 1 M to almost infinite, and for hydrochloric acid from 5 M to 1M, an enthalpy value for this reaction, AH, of -144.9 kJ.mol-' is obtained a t 25 "C. This means that reaction 1is
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 t
Table I. Percentage Distribution of Selenium of Tetravalent and Hexavalent Forms under Different Experimental Conditions experimental conditions
percentage yield Se(1V) Se(V1)
described reduction procedure at 100 "C, in equilibrium described reduction procedure at 25 O C , in equilibrium H2SeO3in solution saturated with Clz, standing overnight HzSeOsin 5 M HC1 saturated with Clz at 25 "C, in equilibrium HzSe03in HzOzsolution (100 mg/L) at 25 "C after 2 weeks described procedure, boiling with q C l for 15 min under Nz stream, after standing for 3 weeks
25
50
75
1705
95
5
3
91
1.5
98.5
3
97
100
0
I
\ \
97
\
\
-2
3
1
0
25
\ , 50
75 TIME ( h )
Figure 2. Comparison of the experimental (-) and theoretical (- - -) curves for the dependence of In ([Se(IV)]/[Se(IV]]")on time.
100
125
150
'
TIME (hl
Figure 1. Decrease of the Se(IV)concentration with time after the reduction of Se(V1) to Se(1V) with hydrochloric acid. exothermic in the direction from left to right at room temperature, and the equilibrium would be shifted to the left at higher temperatures. The results of our experiments (Table I) show that reaction 1 is strongly dependent on the temperature and that it virtually is not hindered kinetically. The equilibrium concentration for selenate is around 5% a t 100 "C, but it is almost 97% a t room temperature. When an aqueous solution of selenious acid is saturated with chlorine at 25 "C and left standing overnight, only 1 to 2% of the selenium can be hydrated, whereas 98 to 99% remain in the residue, i.e., are converted into selenic acid. Figure 1 shows the change of the selenium(1V) concentration with time after the reduction with hydrochloric acid. From the diagram it can be seen that after 1 week, the concentration of tetravalent selenium had decreased to 6%. This value remained constant with a deviation of f3% over an additional 4 weeks so that it can be considered as the equilibrium concentration. Twenty minutes after the reduction, 93% of the initial tetravalent selenium concentration was found in the solution. Five additional experiments resulted in curves similar to that shown in Figure 1,but with some variations in their slopes and in the equilibrium concentration, which can be explained with changes in the chlorine concentrations. The half times were found to be between 0.5 day and 2 days. In an additional experiment, a 5 M hydrochloric acid solution of selenious acid was saturated with chlorine a t 25 OC,and for this system an equilibrium concentration of 3 f 1% Se(IV) was obtained after 2 days, and the half time was 1 h. The chlorine concentration in the decomposition solution depends upon several parameters, the most important of which
are (i) the decomposition of the hydrogen peroxide which can be catalyzed by dust particles, traces of heavy metals, etc., (ii) the boiling time, and (iii) the extent of the disproportionation reaction of chlorine with water to form HCl and HOCl. The chlorine concentration is the only variable in equilibrium reaction 1as the hydrochloric acid concentration is constant. Assuming that reaction 1 is pseudo first order in both directions (high excess of Clz and Cl-), a plot of In ([Se(IV)]/ [Se(IV)]O)vs. time should give a linear course. As can be seen from the semilogarithmic plot in Figure 2, based on experimentally measured selenium(1V) concentrations, the course of this dependence corresponds to the model of a pseudofirst-order reaction only up to about 12 h after the reduction of the selenium to the tetravalent state; Le., the chlorine concentration can be considered as constant for only about this time period. Afterward, due to decreased chlorine concentration, the speed of the reaction, which is proportional to [Clz], becomes slower. By use of a test with a saturated solution of titanium sulfate in 2 M sulfuric acid, it was found that the sample solutions contained 1-10 g/L hydrogen peroxide before and a nondetectable concentration (
