Determination of fast electrode kinetics facilitated by use of an internal reference

The concept of using an internal reversible reference process as a calibration in the determination of fast electrode kinetics has been developed and applied with the technique of Fourier transformed large amplitude ac voltammetry to minimize the influence of errors arising from uncertainties in parameters such as electrode area (A), concentration (C), diffusion coefficient (D), and uncompensated resistance (Ru). Since kinetic parameters (electron transfer rate constant, k0, and electron transfer coefficient, α) are irrelevant in the voltammetric characterization of a reversible reaction, parameters such as A, C, D, and Ru can be calibrated using the reversible process prior to quantification of the electrode kinetics associated with the fast quasi-reversible process. If required, new values of parameters derived from the calibration exercise can be used for the final determination of k0 and α associated with the process of interest through theory-experimental comparison exercises. Reference to the reversible process is of greatest significance in diminishing the potentially large impact of systematic errors on the measurement of electrode kinetics near the reversible limit. Application of this method is demonstrated with respect to the oxidation of tetrathiafulvalene (TTF), where the TTF0/• process is used as a reversible internal reference for the measurement of the quasi-reversible kinetics of the TTF•/2+ process. The more generalized concept is demonstrated by use of the Fc0/+ (Fc = ferrocene) reversible process as an internal reference for measurement of the kinetics of the Cc+/0 (Cc+ = cobaltocenium) process. Via the internal reversible reference approach, a k0 value of 0.55 cm s-1 was obtained for the TTF•/2+ process at a glassy carbon electrode and 2.7 cm s-1 for the Cc+/0 one at a carbon fiber microelectrode in acetonitrile (0.1 M Bu4NPF6).