Adsorptive stripping voltammetry of phenylthiohydantoin and methylthiohydantoin derivatives of amino acids in the presence of copper(II) ions at a hanging mercury drop electrode

In the absence of Copper(II)—and even in its presence, if accumulation has been effected at positive potentials vs. Ag/AgCl—phenylthiohydantoin and methylthiohydantoin derivatives of amino acids gave a peak at −0.60 V (pH 8.0) owing to reduction of the mercury salt formed upon accumulation. At negative accumulation potentials, the presence of copper(II) affected the adsorptive stripping voltammograms markedly, including the lowering of the detection limits. When accumulation was effected at −0.3 V, the thiohydantoin derivatives gave single peaks in the range −0.4 to −0.6 V owing to reduction of copper(II) accumulated as copper(II) complexes. Determinations could be made readily at the 5 × 10−9 M level, accumulating at −0.3 V (pH 8.0) or −0.1 V (pH 4.5): calibration plots were rectilinear up to 8 × 10−8 M. Accumulation at −0.1 V (pH 8.0) in some cases gave additional minor peaks, but glycine derivatives were exceptional in giving a well-defined peak at a considerably more negative potential. This peak appeared to be due to the accumulation of a different copper(II) complex, and evidence was obtained for the interconversion of these complexes in the surface. Mutual interferences by competitive adsorption were observed.