INFLUENCE OF ARSENIC(III) ON THE CATHODIC STRIPPING VOLTAMMETRY OF SELENIUM(IV)

Arsenic(III) can act as an interference in the selenium(IV) determinat ion in acid solution by cathodic stripping voltammetry on the hanging mercury drop electrode in the experimental conditions generally used f or this purpose (formation of HgSe at a preconcentration potential of -0.2 V (Ag/AgCl/Cl- sat. electrode and measuring the stripping peak cu rrents at -0.45 V). However, in the presence of Arsenic(III), Selenium (IV) could be preconcentrated at -0.45 V on the HMDE, involving the ac cumulation of As2Se3 layers. Then in the cathodic stripping step a sha rp peak is obtained, As2Se3 is reduced into AsH3 and H2Se. This peak c urrent is proportional to selenium(IV) concentration in solution with a constant arsenic(III) concentration and is significantly larger than the analogous HgSe stripping peak obtained without arsenic(III). To d evelop a selenium determination method in the presence of arsenic(III) several parameters were studied and optimum experimental conditions w ere established. The sensitivity of the calibration plots depend on th e arsenic(III) concentration. In the presence of 500 mu g(-1) arsenic( III) the detection limit and quantification limit were 0.4 and 0.5 mu gl(-1) selenium concentration, respectively. This methodology was test ed and applied to selenium determination in white clover, a certified reference material (CRM 402) obtaining very satisfactory results.