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- demonstrates 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, 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 tetrathlafulvalene (TTF) and the reduction of tetracyanoqulnodimethane (TCNQ), have been measured at platinum mierodisk electrodes over a wide range of electrolyte concentrations (0–0.1 M). Analysis of the mass-transfer-lmited currents provides a lower limit of 106 M−1 s−1 for the bimolecular reaction rate constants Kb for TTF + TTF2+ → 2TTF+ and TCNQ + TCNQ2 → 2TCNQ-. © 1991, American Chemical Society. All rights reserved