SPECIATION OF SELENIUM IN NATURAL-WATERS AND SNOW BY DPCSV AT THE HANGING MERCURY DROP ELECTRODE

A procedure is described for the speciation of selenium in natural wat ers. According to this procedure the original concentrations of Se(IV) , Se(IV) + Se(VI), and Se(IV) + Se(-II) in the sample can be determine d in turn. Differential pulse cathodic stripping voltammetry (DPCSV) w as used according to the method proposed by U. Baltensperger and J. He rtz (Anal. Chim. Acta, 1985, 172, 49-56) to measure the concentration of Se(IV), the only electrochemically active species. By properly sele cting pH and reagent concentrations, the following reaction steps were accomplished under UV irradiation: (i) decomposition of organic compo unds that are generally present in natural waters and hinder the DPCSV determination of Se(IV) (pH around 1.6, 50 mu l of 30% H2O2 in a 60-m l sample); (ii) quantitative reduction of Se(VI) to Se(IV) at pH 10; ( iii) quantitative oxidation of Se(-II) compounds and organic interfere nts to Se(IV) in the presence of H2O2 (50 mu l was added to a 60-ml sa mple 2 mM in Na2B4O7). Various tests were performed in natural and syn thetic seawater samples. In step 1, spikes of humic acid (up to 20 mg liter(-1)), benzene (80 mu g liter(-1)), and Triton X-100 (10 mg liter (-1)) were completely destroyed in 30 or 100 min depending on the irra diating device used. Trimethylselenonium ion and DL-selenomethionine, representative of selenocompounds in natural waters, also did not oxid ize to Se(IV) at the nanogram per liter level. In step 2, no losses in Se(IV) spikes were observed after irradiation at pH 10, which means t hat Se(VI) reduction does not proceed toward oxidation states lower th an +4. In step 3, spikes of standard solutions of trimethylselenonium and DL-selenomethionine were completely recovered as Se(IV), whereas S e(IV) spikes were not lost during the reaction step. Compared with oth er procedures reported in the literature, the present procedure requir es a lower UV absorbed power per sample, a shorter time of irradiation , and a lower reagent concentration. This, in addition to the use of a 16-bit ADC sampling board and a suitable data processing program, all owed a Se(IV) concentration of 0.16 ng liter(-1) to be measured with a n overall coefficient of variation as low as 16 (among parallel sample s). Two different irradiation devices are described. (C) 1998 Academic Press.