SCANNING ELECTROCHEMICAL MICROSCOPY .9. THEORY AND APPLICATION OF THE FEEDBACK MODE TO THE MEASUREMENT OF FOLLOWING CHEMICAL-REACTION RATES IN ELECTRODE PROCESSES

The use of the scanning electrochemical microscope (SECM), in both the steady-state and chronoamperometric feedback modes, is proposed as a new method for measuring the kinetics of following chemical reactions in electrode processes. Particular attention is given to the E(r)C(i) mechanistic case (first-order irreversible chemical step following reversible electron transfer). Theory for the problem, relating the SECM feedback current to the tip-substrate distance, tip electrode radius, and rate constant for the chemical step, is developed numerically by using the alternating-direction implicit finite-difference scheme. The theoretical results demonstrate that both the chronoamperometric and steady-state feedback modes are sensitive techniques for accurate kinetic determinations, and that homogeneous chemical rate constants in excess of 2 x 10(4) s-1 should be accessible to measurement with current SECM technology. The theoretical predictions are verified experimentally with the measurement of the rate of deamination, in aqueous basic solution (pH 10.2-12.4), of oxidized N,N-dimethyl-p-phenylenediamine, produced at a platinum electrode.