CODEPOSITION OF COPPER AND TIN IN THE ELECTROLYSIS OF SOLUTIONS OF CITRATE COMPLEXES

Experimental data are compared with the results of theoretical modelin g, which takes into account mobile chemical interactions in the diffus ion layer and the recharging process 2Cu(II) + Sn(II) reversible arrow 2Cu(I) + Sn(IV). In the acidic media, according to potentiometric dat a, the species SnL(2-) and SnLH(-) (where L is the tetravalent anion o f citric acid) predominate, and their logarithms of stability constant s are 15.65 +/- 0.05 and 19.5 +/- 0.1, respectively. The effective rat e 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 magnit ude. The potential of the Cu and Sn codeposition onset is also pH-depe ndent. This potential can be observed both in the region of the limiti ng current of the process Cu(II) + 2e --> Cu and in the rising branch of the current-potential curve for this process. This theoretical conc lusion is in agreement with the data on the composition of coatings ob tained at various potentials and solution pH.