The evaluation of rate constants for rapid electrode reactions using microelectrode voltammetry: virtues of measurements at lower temperatures

Abstract

The prospects of utilizing electrochemical kinetic measurements at lower temperatures for evaluating reliably the standard rate constants k s of rapid redox couples in non-aqueous media, specifically using microelectrode voltammetry, have been examined by means of digital simulations together with experiment. Cyclic voltammetric measurements were made between 200 and 300 K at a gold microdisk, for ferrocenium-ferrocene (Fc +/0), o-nitrotoluene 0/− and nitromesitylene 0/− redox couples in acetone, propionitrile and butyronitrile, at a gold microdisk with scan rates from 100 to 10 4 V s −1. The latter two couples were selected as examples of systems that exhibit rapid but yet still unambiguously measurable k s values at ambient temperatures; the more facile Fc +/0 system has been suspected to yield immeasurably fast electrode kinetics using conventional techniques under these conditions. Sample simulated voltammograms were generated between 200 and 300 K for a sequence of activation enthalpies Δ H ex ‡ for electron exchange, accounting also for known temperature-dependent diffusion coefficients, solution resistance and double-layer capacitance. As long as Δ H ex ‡ > 10 kJ mol −1, the use of diminished temperatures in such solvents is predicted to facilitate the reliable evaluation of k s. This expectation is in harmony with experiment: k s for Fc +/0 is apparently evaluated readily at lower temperatures, yielding Δ H ex ‡ = 20 kJ mol −1. The temperature-dependent electrode kinetic data for Fc +/0 are compared briefly with some expectations of contemporary theory.