Scratched silver electrodes in perchloric acid solution

In an earlier investigation[l] we showed that analysis of the charge densities passed on freshly generated silver surfaces in 1 M KOH reveals two distinct types of oxidised monolayer formed in different ranges of potential, both formed anodically from the metal. Charge densities of 15OpC cm2 after re-equilibration of the mechanically perturbed surface were measured at potentials some 600 mV below the reversible potential for bulk Ag,O formation. At higher potentials a second plateau charge density of 3OOflC cmW2 was observed, still at potentials below that at which bulk Ag,O formation can occur. Adsorbed surface species of AgOH and Ag,O were proposed respectively for these two plateau charge densities, the differing densities of Ag+ in each monolayer accounting for the different amounts of oxidation that had occurred. These processes were also discernible in potential pulse and rapid potentiodynamic sweep experiments, showing that they were not exclusive properties of the scratch surface. The present communication describes the behaviour of scratched silver electrodes in l.OM HCIO, solutions, both in the presence and absence of dissolved Ag’. A mechanism for steady state dissolution is described. The technique used to scratch potentiostatically controlled specimens was described in earlier papers[l-51. The shallow scratch, 2 mm long by 4Opm wide, is generated on a rotating disc electrode with a diamond stylus, the entire electrode surface being exposed to the electrolyte. The scratch occupies 0.1% of the total electrode area and is generated at 2 m s‘. The ohmic potential drop for the scratched electrode surface has been calculated[6]. The parameters measured by these experiments are the bare surface current density i,, and the total charge density 4,. required to bring the current on the scratch to its steady state (or equilibrium) value. The electrolyte was de-aerated l.OM HCIO, with or without the presence of 0.0033 M Ag’ added as AgC104. Experiments were performed at 293 + 2 IL The results are shown in Fig. 1. The bare surface mixed potential is 355 mV (nhe). In the range 300+ 100 mV (nhe) i, and q. are both independent of E, at 0.46Acme2 and 100 pC cm-’ respectively. This charge is somewhat lower than that obtained in