Medium effects for very fast electron transfer reactions at electrodes:: the [Ru(NH3)6]3+/2+ system in water

The reduction kinetics of [Ru(NH3)(6)](3+) was studied at Au(111) and Au(100) single-crystal ultramicroelectrodes in dilute perchloric acid electrolytes. Both heterogeneous rate constants and experimental transfer coefficients varied with the crystallographic orientation of the gold surface. The value of the heterogeneous rate constant at Au(111) was significantly larger than that at Au(100). The experimental transfer coefficients also increased but in the opposite order. Standard rate constants at both electrodes increased with an increase in electrolyte concentration. Using double-layer data obtained in 0.01 M HClO4, it is shown that the true transfer coefficient for this reaction is 0.5 within experimental error. The effective charge on the reactant which has a nominal charge of +3 is close to +1. The latter result reflects the distribution of charge within the polyatomic reactant.