Electrocrystallization of mercury, silver and palladium

Abstract

The kinetics of electrodeposition, on to inert electrodes of vitreous carbon and pyrolitic graphite, of deposits of Ag, Pd and Hg from solutions of their perchlorates, chlorides and nitrates respectively, are described for potentiostatic conditions. For Hg, which requires no lattice incorporation, the slow step is diffusion-controlled growth of three-dimensional nuclei. For Ag, the deposition occurs by two-dimensional growth of plates formed on the top of plates so as to produce three-dimensional nuclei. The slow step appears to be incorporation into the Ag lattice of Ag neutral atoms, the concentration of which is determined by a fast electrochemical pre-equilibrium. This means the measurements can not be interpreted in terms of α, and i_o for the electrochemical step alone. The constant for the lattice incorporation step at the reversible potential is about 5 × 10^–5 moles cm^–3 sec^–1 for a 1.5 × 10^–2 M solution of silver nitrate. For Pd, deposition occurs by three-dimensional nucleation and growth. In contrast to silver, the slow step is the electrochemical reaction preceding crystal growth. The rate constant at the reversible potential is about 3.3 × 10^–9 moles cm^–2 sec^–1, and α= 0.6 for a 5.12 × 10^–2 M palladous chloride solution.