Codeposition of copper and tin in the electrolysis of solutions of citrate complexes

Experimental data are compared with the results of theoretical modeling, which takes into account mobile chemical interactions in the diffusion layer and the recharging process 2Cu(II) + Sn(II) reversible arrow 2Cu(I) + Sn(IV). In the acidic media, according to potentiometric data, the species SnL(2-) and SnLH(-) (where L is the tetravalent anion of citric acid) predominate, and their logarithms of stability constants are 15.65 +/- 0.05 and 19.5 +/- 0.1, respectively. The effective rate constant (k(s)) calculated for the process CuL(2-) + 2e --> Cu + L(4-) to a significant degree depends on solution pH: in the pH interval from 3.5 to 11, k(s) drops as much as more than eight orders of magnitude. The potential of the Cu and Sn codeposition onset is also pH-dependent. This potential can be observed both in the region of the limiting current of the process Cu(II) + 2e --> Cu and in the rising branch of the current-potential curve for this process. This theoretical conclusion is in agreement with the data on the composition of coatings obtained at various potentials and solution pH.