Underpotential dissolution of metals under conditions of partial mass-transport control

Underpotential dissolution was studied over a wide range of concentrations, from complete diffusion control to surface control, assuming reversibility. Applicability of the Frumkin isotherm was assumed. Digital simulation was employed, showing the dependence of the parameters of the UPD, Epeak, jpeak and W1/2 on the concentration and the sweep rate. A strong influence of the Frumkin parameter, f, on the parameters of the UPD was demonstrated. Anodic dissolution under diffusion control shows no dependence of Epeak, jpeak on cbulk, at constant sweep rate. In contrast, under pure surface control the peak current is independent of concentration and the peak potential changes with concentration following the Nernst equation. A dimensionless parameter P was used, to allow easy demarcation of the regions of diffusion, mixed and surface control. Empirical equations were obtained for the values of Epeak, jpeak and W1/2 as a function of the experimental parameters, under conditions of pure diffusion control. The experimental system studied was lead on polycrystalline silver. The characteristic dissolution parameters of the UPD process were determined for a wide range of concentrations of PbCl2 in 0.1 M HCl (1 × 10−6 to 7 × 10−3 M), scan rates of 10–50 mV/s and initial fractional surface coverage ranging from 0.1 to 1. In the range accessible experimentally, very good agreement with the results of digital simulation was found, for f = −2.05 ± 0.1. The formal potential for UPD formation was found to be EUPD∘′=-293mVvs.Ag/AgCl/1MHCl, which is 88 mV positive with respect to the formal potential for OPD deposition of lead in the same system.