Heterogeneous reaction rate constants by steady-state microelectrode techniques and fast scan voltammetry

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Rate constants of fast electron transfer reactions of hexachloroiridium, hexaammine ruthenium, aminoferrocene, tetracyanoquinodimethan(TCNQ), ferrocenecarboxylic acid and ferrocene, which have been determined equivocally by fast voltammetry, are attempted to be evaluated with steady-state voltammetric measurements at ultramicroelectrodes. This work is motivated by the experimental feasibility of high current density at ultramicroelectrodes without effects of capacitive currents or solution resistance. The theory of mass transport mentions that current densities by steady-state voltammetry at electrodes 10μm and 1μm in diameter correspond, respectively, to 2.1 Vs-1 and 210 V s-1 scan rates at large electrodes. However, no potential shift is observed even at electrodes 0.01μm in diameter for the six species. This fact implies that the heterogeneous rate constants are at least larger than 10 cm s-1. Fast scan voltammetry is carried out at a large electrode for scan rates less than 10 V s-1. Peak-to-peak potentials increase with an increase in the scan rates. This variation is fitted to the mass transport equation complicated by the electrode kinetics for the rate constant of 0.01-0.1 cm s-1. The inconsistency between the two methods is ascribed partly to solution resistance and partly to the resistance relevant to the electric double layer at fast scan voltammetry. The contribution of the solution resistance is demonstrated from the facts that the peak currents vary linearly with the peak potentials, that the slope of the linearity is independent of kinds of redox species, and that the slope corresponding to the conductivity has a linear relationship with concentration of supporting electrolyte.

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