SUCCESSIVE ELECTRON-TRANSFERS IN LOW IONIC-STRENGTH SOLUTIONS - MIGRATIONAL FLUX COUPLING BY HOMOGENEOUS ELECTRON-TRANSFER REACTIONS

Analysis of successive heterogeneous electron transfer reactions requi res consideration of the coupling of molecular fluxes by homogeneous e lectron transfer reactions. We demonstrate that bimolecular electron t ransfer between members of the redox system that differ by +2 in oxida tion state (i.e., reproportionation) can strongly influence the magnit ude of transport-limited currents when ion migration contributes to mo lecular transport. A theoretical description of the problem is develop ed in the limit of infinitely fast homogeneous electron transfer and i dentical diffusion coefficients. An analytic solution is presented tha t allows calculation of voltammetric limiting currents at microelectro des for any stepwise n-electron transfer as a function of the ratio of redox and supporting electrolyte concentrations. The results demonstr ate that migrational flux coupling of homogeneous electron transfer re actions must be included in the reaction mechanism to obtain even qual itatively 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-electro n reductions of Ru(bpy)(3)(2+) in acetonitrile + toluene solutions. (C ) 1997 Elsevier Science S.A.