A study of the reduction of tris-oxalato Fe(III) from aqueous 1 M oxalate solution at a polycrystalline gold electrode

Abstract

The reduction of the Fe(III)-oxalate complex at a polycrystalhne gold electrode from a solution of 1 M K2C204 +0 05 M H2C204 has been studied by dc current and impedance measurements The results indicate a multi-step mechamsm of the CEC type, l.e the electron transfer is preceded and followed by a heterogeneous chemical reaction An EC mechamsm (preceding reaction not present or fast) can also fit the experiments if the individual transfer coefficient of the electron-transfer step is allowed to be 0 4 instead of 0.5 The conclusions are compared with earher results obtained at the dropping mercury electrodeElectrode reactions involving the transfer of a single electron between two solution-soluble redox components are usually expected to obey a relatively simple rate law describing the potential dependence of the rate of charge transfer. That is, the potential-dependent forward rate constant of reduction, k f, is expressed in terms of the potential-independent kinetic parameters ktsh and a by E=E ° kfexp[(zoF/RT)~p2]=ktsheXp[(-aF/RT)(E-~P2 -E° +(P2where q~2 is the potential of the outer Helmholtz plane (OHP), E o the formal standard potential and z o the charge on the oxxdized species in the solution.Obviously in the notation of eqn.(1) the standard heterogeneous rate constant k~h is the value of the left-hand side at E = E o. Concordance with eqn.(1) has indeed been observed for a number of redox couples of the type mentioned [1][2][3][4][5], and in addition the value of the transfer coefficient a is found to be close to 0.5, thus supporting theoretical views on the essentmls of charge transfer, for example the Marcus theory [6]. The tris-oxalato Fe(III)/tris-oxalato Fe(II) electrode reaction at the mercury electrode appears to be an exceptional case, especially concerning the latter aspect, the cathodic transfer coefficient being frequently reported to have an extreme value 0022-0728/84/$03 00