VOLTAMMETRIC MEASUREMENT OF BIMOLECULAR ELECTRON-TRANSFER RATES IN LOW IONIC-STRENGTH SOLUTIONS

Numerical analysis of the sequential electron-transfer mechanism, A --> A+ + e- A+ --> A2+ + e-, demonstrate that the rate at which A can be oxidized to A2+ is inhibited in low ionic strength solutions by the diffusion-migration of chemically generated A+ away from the electrode surface. The species A+ is produced within the depletion layer by the homogeneous bimolecular reaction, A + A2+ --> k(b) 2A+, and is repelled from the electrode surface by the electric field in solution, thereby reducing the flux of A at the electrode surface. Steady-state voltammetric currents corresponding to two examples of this reaction mechanism, the oxidation of tetrathiafulvalene (TTF) and the reduction of tetracyanoquinodimethane (TCNQ), have been measured at platinum microdisk electrodes over a wide range of electrolyte concentrations (0-0.1 M). Analysis of the mass-transfer-limited currents provides a lower limit of 10(6) M-1 s-1 for the bimolecular reaction rate constants, k(b), for TTF + TTF2+ --> k(b) 2TTF+ and TCNQ + TCNQ2- --> k(b) 2TCNQ-.