A coulostatic double-pulse method for a fast electron-transfer process followed by a fast chemical reaction

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Abstract

A new technique is proposed for measuring the kinetic parameters of electrochemical electron-transfer reaction whose reductant or oxidant species is quite unstable and undergoes an irreversible first-order reaction in solution. The principle of this procedure is in quick generation of the unstable reactant and quick measurement of overpotential before decomposition occurs. The working electrode placed in a solution containing one of the reactants is polarized at a potential where no current flows. Then, by applying a coulostatic pulse to the cell, the potential of the working electrode is stepped to a potential beyond the reversible half-wave potential determined in advance by a separate experiment. At the very moment when the decaying potential reaches a potential chosen near the reversible half-wave potential, the second coulostatic pulse with reversed polarity is applied to the cell. Analysis of the potential vs. time curve after the end of the second pulse is performed in the same way as in the modified coulostatic pulse method. The technique makes it possible to measure the standard rate constant for the electron-transfer reaction involving an unstable reactant whose lifetime is as short as 10−5 s. It is applied to the reaction Co(en)33++eCo(en)22++en (en=ethylenediamine) in acidic and basic solutions. The standard rate constant of 0.5 cm s−1 and the transfer coefficient of 0.5 are obtained in the presence of a follow-up reaction whose first-order rate constant is as large as 104 s−1. They are in excellent agreement with the values obtained under the experimental conditions where no follow-up reaction occurs.

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