MICROELECTRODE VOLTAMMETRY FOR THE SIMULTANEOUS REDUCTION OF LABILE COMPLEXES OF 2 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 sol utions containing different concentrations of supporting electrolyte. Mixtures of lead(II) plus cadmium(II) salts, and mercuric(II) plus lea d(II) salts were investigated using a mercury-coated platinum and a na ked platinum microelectrode, respectively. For the mixture containing lead plus cadmium salts, in solution, with no deliberately added suppo rting electrolyte, the steady-state limiting current of cadmium increa sed much more than expected, as a consequence of the increased rate of mass transport of lead which reduces at less negative potentials. Con versely, the steady-state limiting current of lead was not affected by the presence of the more easily reduced mercury ions. The different b ehaviour observed for the two mixtures was due to the inhibition of th e migratory component in the mass transport of mercury(II), which form ed labile complexes with chloride and hydroxide ions to a considerable extent. In solutions containing supporting electrolyte, the overall s teady-state limiting currents, measured in the presence of the more ne gative 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.