MODELS FOR MEDIATED REACTIONS AT FILM-MODIFIED ELECTRODES - CONTROLLED ELECTRODE POTENTIAL

Models have been developed previously for redox reactions mediated in films on electrode surfaces. As developed by Saveant and coworkers for steady state rotating disk voltammetry, a redox species, P, confined in the film is electrolyzed at the mass transport limited rate to form Q. Q is able to undergo reaction with a species, A, present in soluti on and able to permeate the film, to regenerate P and form product B; A + Q -->k(1) B + P. The reaction may occur either in the bulk of the film or at the film-solution interface. Here, the models are modified to include control of the electrode potential such that the concentrat ion of P and Q at the electrode surface are parameterized by the Nerns t equation. As long as no P and Q are lost from the film, the Nernstia n condition will govern most electrode systems at steady state. Contro l of the electrode potential allows (1) simplified determination of th e kinetic characteristics for some reaction schemes, and (2) evaluatio ns where a second electrolysis occurs at a potential sufficiently clos e to the redox potential of P/Q that there are advantages in not apply ing potential sufficient to electrolyte P at the mass transport limite d rate. This includes reactions where the formal potentials for P/Q an d A/B are within 200 mV of each other as well as cases where a larger applied potential exceeds the solvent limit or leads to film decomposi tion. Equations are also provided for the electrolysis of A to B at th e electrode surface as governed by the Nernst equation. Methods for ev aluating experimental data are outlined.