Microelectrode voltammetry for the simultaneous reduction of labile complexes of two divalent cations in aqueous solutions with and without supporting electrolyte

The steady-state voltammetric behaviour for the reduction of mixtures of labile complexes of two divalent cations was studied in aqueous solutions containing different concentrations of supporting electrolyte. Mixtures of lead(II) plus cadmium(II) salts, and mercuric(II) plus lead(II) salts were investigated using a mercury-coated platinum and a naked platinum microelectrode, respectively. For the mixture containing lead plus cadmium salts, in solution, with no deliberately added supporting electrolyte, the steady-state limiting current of cadmium increased much more than expected, as a consequence of the increased rate of mass transport of lead which reduces at less negative potentials. Conversely, the steady-state limiting current of lead was not affected by the presence of the more easily reduced mercury ions. The different behaviour observed for the two mixtures was due to the inhibition of the migratory component in the mass transport of mercury(II), which formed labile complexes with chloride and hydroxide ions to a considerable extent. In solutions containing supporting electrolyte, the overall steady-state limiting currents, measured in the presence of the more negative reducible ions, depended on the supporting electrolyte to metal ion concentration ratio, provided that the complexation of the metal ions was negligible. The experimental data agreed with those obtained from theoretical equations derived from already known treatments, and adapted to the solutions investigated here. (C) 1998 Elsevier Science S.A. All rights reserved.