The silver(I)-copper reaction in acetonitrile-water mixtures

Abstract

An electrochemical and kinetic study of the silver(I)-copper displacement (cementation) reaction in acidified acetonitrile-water mixtures is reported. The cathodic polarization curves for the reduction of silver(I) were found to be those of a fast electrochemical reaction. From the variation of limiting current with rotation speed, the diffusion coefficient of silver(I) as a function of acetonitrile concen- tration was determined. The anodic oxidation of copper gave a Tafel slope of 60 mV per decade of current and was rotation-speed-dependent. It is suggested that the slow step in the reaction mechanism is the diffusion of copper(I) from the electrode surface. At all acetonitrile concentrations investigated, the two polarization curves intersected at a point where the current is fixed by the diffusion of silver(I); this suggests that the displacement reaction will be diffusion-controlled, which was confirmed by kinetic studies. Provided the silver deposit was not too thick, there was good agreement between the measured kinetic rate and that calculated from the silver(I) limiting-current measurements. However, when the deposit became too thick, some increase in reaction rate due to surface roughening was observed.