A capacitance study of the silver(I)/copper displacement reaction

Metal displacement (cementation) reactions are usually diffusion controlled and conform to first order kinetics. When a displacement reaction is carried out in a batch reactor, positive deviations from a first order rate plot are commonly observed once the deposit becomes sufficiently thick. It has been suggested that these rate enhancements are the result of changes in deposit structure resulting in increases in surface area. Another theory argues that the rate enhancements are due to turbulence associated with increasing roughness of the deposit. In this paper, an attempt is made to discriminate between these two possibilities by measuring the capacitance of a copper electrode immersed in a silver(I) solution. It was found that no anomalous capacitance changes were observed corresponding to the region in which rate increases were observed. This suggests that turbulence effects, rather than area effects, are responsible for the increased reaction rate. Capacitance measurements were also conducted for the reaction between silver(I) and copper in the presence of cyanide which caused the silver to deposit in a thin uniform layer on the copper surface, thus blocking the displacement reaction. It was found that the capacitance changed very little with time under these circumstances.