SCANNING ELECTROCHEMICAL MICROSCOPY .20. STEADY-STATE MEASUREMENTS OFTHE 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 subs trate in a thin-layer cell arrangement. The technique developed can be used to study electrode reactions which are among the fastest known a nd were previously accessible only from transient measurements. The ab sence of complications in the measurements associated with solution re sistance and charging current, typical for relaxation techniques, and the availability of a simple method of data analysis allow determinati on of reliable values of kinetic parameters. The standard rate constan t, k-degrees, for ferrocene oxidation in acetonitrile at a Pt electrod e (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 ultramicroelec trodes. Two types of analytical approximations describing steady-state quasi-reversible voltammograms at a disk-shaped SECM microtip electro de in proximity to a conductive substrate are presented along with the exact solution in the form of a two-dimensional integral equation. Th e equivalence of these approximations at small tip-substrate separatio ns is demonstrated. In general, this approach c an be applied to the d etermination of k-degrees, when the dimensionless parameter k-degrees d/D is less than 5 (where D is the diffusion coefficient).