Influence of arsenic(III) on the cathodic stripping voltammetry of selenium(IV)

Arsenic(III) can act as an interference in the selenium(IV) determination in acid solution by cathodic stripping voltammetry on the hanging mercury drop electrode in the experimental conditions generally used for this purpose (formation of HgSe at a preconcentration potential of -0.2 V (Ag/AgCl/Cl- sat. electrode and measuring the stripping peak currents 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 accumulation of As2Se3 layers. Then in the cathodic stripping step a sharp peak is obtained, As2Se3 is reduced into AsH3 and H2Se. This peak current 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 develop a selenium determination method in the presence of arsenic(III) several parameters were studied and optimum experimental conditions were established. The sensitivity of the calibration plots depend on the 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 tested and applied to selenium determination in white clover, a certified reference material (CRM 402) obtaining very satisfactory results.