Successive electron-transfers in low ionic strength solutions. Migrational flux coupling by homogeneous electron transfer reactions

Analysis of successive heterogeneous electron transfer reactions requires consideration of the coupling of molecular fluxes by homogeneous electron transfer reactions. We demonstrate that bimolecular electron transfer between members of the redox system that differ by +2 in oxidation state (i.e., reproportionation) can strongly influence the magnitude of transport-limited currents when ion migration contributes to molecular transport. A theoretical description of the problem is developed in the limit of infinitely fast homogeneous electron transfer and identical diffusion coefficients. An analytic solution is presented that allows calculation of voltammetric limiting currents at microelectrodes for any stepwise n-electron transfer as a function of the ratio of redox and supporting electrolyte concentrations. The results demonstrate that migrational flux coupling of homogeneous electron transfer reactions must be included in the reaction mechanism to obtain even qualitatively accurate predictions of the current. The theory is found to be in good agreement with values of voltammetric currents measured at an inlaid 11.2-mu m radius Pt microdisk for the two- and three-electron reductions of Ru(bpy)(3)(2+) in acetonitrile + toluene solutions. (C) 1997 Elsevier Science S.A.