Kinetics of Sn and Co codeposition in citrate solutions

The model of mass transport taking into account mobile chemical interaction s in the Nernst-type diffusion layer is used for the estimation of surface concentration of Sn(II) and Co(II) citrate complexes. The analysis of the s olution composition indicates that the complexation degree of Sn(II) is muc h higher than that of Co(II) in the ligand-deficient slightly acid media. T his fact makes it possible to choose optimal concentrations of solutions wh erein equilibrium potentials of Sn and Co are close. The kinetics of SnLH- reduction in cobalt-free solutions may be described quantitatively within t he framework of the model with cathodic alpha = 0.7 and the exchange curren t density i(o) = 6 mA cm(-2). A similar mechanism provides the best fit for a partial process of Co(II) reduction in solutions containing Sn(II). The effect of Co(II) on the kinetics of Sn(II) reduction may be interpreted on the assumption that SnL2- species may also be treated as an electrically ac tive complex. (C) 2000 Elsevier Science Ltd. All rights reserved.