Introduction to a kinetics-sensitive double-step voltcoulometry

As an alternative to the numerous state-of-the-art versions of voltammetry, a kinetics-sensitive double-step voltcoulometry is introduced. The transie nt current flowing in response to a potential step across the electrochemic al cell is integrated and simultaneously processed by a deliberately select ed time-domain "cascade" filter, while scanning the applied potential. In c ontrast to the widely used sampling scheme of sampling the transient curren t just before and in the end of the excitation pulse, three values of the t ransient charge are sampled in the interval between subsequent excitation p ulses. Each measuring period is preceded by a single measurement of the ste ady-state current with the excitation pulse being switched off. The latter measurement makes it possible to actively compensate the parasitic charge a cross the feedback capacitor of the integrator, due to the steady-state cur rent, while storing the steady-state current data. The goal of introducing the third sampling event resides in discriminating the kinetics of the tran sient charge via the parameter beta that enters the time dependence of the transient charge Delta q proportional to t(beta). In general, our filtering scheme is capable of eliminating both any constant or linear components in the transient charge (beta=0, 1). Moreover, any superlinear transient char ge (beta > 1) is detected as a peak of the Delta q versus potential plot wi th its sign being opposite to that one of a sublinear redox reaction (0 <be ta < 1). This enhanced performance of our double-step voltcoulometry is doc umented by a series of experiments on aqueous solutions of ascorbic acid us ing a carbon fiber microelectrode as the working electrode. Finally, how to assess the degree of reversibility of the redox reaction and deduce the ti me constant RC of the double-layer charging current is shown. (C) 1999 Amer ican Institute of Physics. [S0034-6748(99)04309-9].