AN EQCM STUDY OF CORROSION AND COMPLEXATION AT ELECTRODE SURFACES - OXIDATION OF SILVER IN THE PRESENCE OF 4,4'-BIPYRIDYL

The oxidation of Ag in the presence of 4,4'-bipyridyl (Bpy) has been s tudied by combined cyclic voltammetric and mass measurements (derived from the electrochemical quartz crystal microbalance). A Ag(I)-Bpy com plex is formed at the electrode surface, but observed mass changes are found to result from the superposition of a mass loss (from release o f Ag+) and a mass gain (from complex precipitation). Altering the cond itions of the experiment (scan rate, upper potential Limit and bipyrid yl concentration) can switch the balance between the two processes and lead to a net mass loss or a net mass gain; at slow scan rates at a b ulk concentration of 0.25 mM bipyridyl, the balance between the proces ses switches twice, even in the narrow potential range before complex reduction. The principal cause of these changes is the slow accumulati on of Ag+ in the diffusion layer as not all the Ag+ released is comple xed. At higher concentrations of bipyridyl (2 mM) mass changes due to complex formation are dominant until the potential is extended such th at the surface concentration of bipyridyl is significantly depleted an d electrode corrosion makes the principal contribution to the mass cha nge.