SCANNING ELECTROCHEMICAL MICROSCOPY .20. STEADY-STATE MEASUREMENTS OF THE FAST HETEROGENEOUS KINETICS IN THE FERROCENE/ACETONITRILE SYSTEM

The application of the scanning electrochemical microscope (SECM) for steady-state measurements of fast heterogeneous kinetics is described. The method is based on the determination of the steady-state current vs potential curve for an electrode reaction at an ultramicroelectrode (radius a), held in close proximity (distance d) to a conductive substrate in a thin-layer cell arrangement. The technique developed can be used to study electrode reactions which are among the fastest known and were previously accessible only from transient measurements. The absence of complications in the measurements associated with solution resistance and charging current, typical for relaxation techniques, and the availability of a simple method of data analysis allow determination of reliable values of kinetic parameters. The standard rate constant, k-degrees, for ferrocene oxidation in acetonitrile at a Pt electrode (a = 1.08 mum) was found to be 3.7 +/- 0.6 cm/s, i.e., 2-4 times the values determined from fast scan cyclic voltammetry at ultramicroelectrodes. Two types of analytical approximations describing steady-state quasi-reversible voltammograms at a disk-shaped SECM microtip electrode in proximity to a conductive substrate are presented along with the exact solution in the form of a two-dimensional integral equation. The equivalence of these approximations at small tip-substrate separations is demonstrated. In general, this approach c an be applied to the determination of k-degrees, when the dimensionless parameter k-degrees d/D is less than 5 (where D is the diffusion coefficient).