Autocatalytic processes of copper(II) and silver(I) reduction by cobalt(II) complexes

A thermodynamic analysis of equilibria in systems Cu-Cu-II-Co-II-Co-III-ethylenediamine and Ag-Ag-I-Co-II-Co-III-NH3 was carried out, and the conditions of Cu-II and Ag-I reduction to metal by Co-II were predicted. Copper(II) reduction should occur at pH 4-11 (50 degrees C) and silver reduction at pH 6-12 (20 degrees C). The solution pH and ligand (En or NH3) should have the largest effect on the reduction processes. The autocatalytic copper and silver deposition was observed in solutions where Delta E of Cu-II-Cu or Ag-I-Ag and Co-III-Co-II couples reaches values of -200 to -400 mV, the lower pH limit of metal deposition being higher than predicted by calculations. The highest metal deposition rate (up to 6 mu m/h for Cu and 3 mu m/h for Ag) in the absence of considerable metal formation in the solution bulk was obtained in solutions with predominating Co-II complexes CoEn(2)(2+), Co(NH3)(5)(2+) and Co(NH3)(6)(2+). The autocatalytic metal deposition process proceeds at overvoltage of -5 to -30 mV from the Cu-II-Cu equilibrium potential and -40 to -50 mV from that of Ag-I-Ag. (C) 1999 Elsevier Science Ltd. All rights reserved.