Steady-state voltammetry at a microdisc electrode in the absence of excess supporting electrolyte for reversible, quasi-reversible and irreversible electrode kinetics

Abstract

The steady-state voltammetry for a one electron reduction, A + e⁻ ⇌ B, is studied at a microdisc electrode in the absence of excess supporting electrolyte. For the first time, the full voltammetric waveshape is numerically simulated. Using a combination of theory and experiment, the voltammetry is investigated as a function of two variables: the concentration of the supporting electrolyte and the electrochemical rate constant. The ‘hemispherical approximation’ (in which a microdisc is assumed to be a hemisphere of the radius) is shown to be valid under weakly supported conditions, for a range of electrochemical rate constants . The simulations were used, in conjunction with the Debye–Hückel theory, to rationalise the experimental steady-state voltammetry of two aqueous redox couples: hexaammineruthenium ([Ru(NH₃)₆]³⁺/[Ru(NH₃)₆]²⁺) and hexachloroiridate ([IrCl₆]²⁻/[IrCl₆]³⁻) (each with varying levels of KCl supporting electrolyte). This investigation provides evidence for ion pairing between [IrCl₆]²⁻/[IrCl₆]³⁻ and K⁺ from the supporting electrolyte. No observable ion pairing occurs between [Ru(NH₃)₆]³⁺/[Ru(NH₃)₆]²⁺ and Cl⁻.