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The early stages of the electrolytic deposition of silver onto vitreous carbon electrodes from ammonium hydroxide solutions have been investigated by the potential step technique. The analysis of the experimental current transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed by diffusion controlled growth of nuclei. It is shown that treatments that involve classifying the process as either instantaneous or progressive nucleation are not always adequate for the quantitative analysis of electrochemical nucleation phenomena. The nucleation kinetics parameters A (nucleation rate constant per site) and No (number density of active sites on the substrate surface) were estimated separately from the current transient maxima by two different approaches. Both quantities were found to vary with the potential and with the concentration of silver ions, except at very high overpotentials for silver deposition. The potential dependence of the nucleation rate A was interpreted according to the atomistic theory and in all cases it was found that the number of atoms in the critical nucleus (nk) was one over the entire potential range analyzed.

A method for the construction of predominance-zone diagrams of multicomponent and multiphase systems of chemical species in a single oxidation state, which consider the formation of mixed and polynuclear complexes (of the type MiLjXk) is presented. Such diagrams may be constructed by defining generalized species, complexation coefficients (with multiplicative structure) and generalized equilibria of complexation, dismutation, polynucleation, and phase formation, with their conditional constants associated. By having these diagrams for each oxidation state of the same component, multidimensional Pourbaix diagrams are constructed. The predominance-zone diagrams of*Eu(0), *Eu(II), and *Eu(III) in the presence of citric acid (H3Cit) and nitrilotriacetic acid (H3NTA) in aqueous solution, are constructed by using this approach.