Channel-electrode voltammetry. Waveshape analysis of the current–voltage curves of EC2 and DISP2 processes

Abstract

Theory is presented for the shape of ‘reversible’ current–voltage curves measured at a channel electrode for the cases where the substrate reacts via either a DISP2 or EC₂ process. It is shown that conventional (mass-transport-corrected) Tafel analysis of measured voltammetric waves leads to significantly curved Tafel plots and this non-linearity permits, in principle, the deduction of the second-order rate constant describing the following reaction. Additionally, the half-wave potential is found, in the case of a reduction, to be shifted anodically to an extent which is flow-rate dependent. Again this can be used to glean mechanistic/kinetic information. In the case of the DISP2 mechanism the kinetics can also be deduced by studying the effective number of electrons transferred as a function of solution flow rate. An appropriate working curve for this is presented. It is further shown that at slow flow rates and with fast-following second-order kinetics a reaction-layer approximation may be applied to the channel electrode to allow the analytical solution of both the EC₂ and DISP2 problems.

Experimental verification of the DISP2 theory was obtained through studying the reduction of the dye fluorescein at high pH (0.1 mol dm^–3 NaOH) at illuminated (390 nm) silver electrodes. The waveshape analysis and the variation of both the half-wave potential and the effective number of electrons transferred with flow rate were all in excellent agreement with theory, and a rate constant is deduced.