Ion-pairing kinetics investigated using nanometer-size Pt electrodes

The kinetics of ion pairing of IrCl63- with K+, Ca2+ and tetraethylammonium (TEA+) were investigated by steady-state voltammetry at disk and hemispherically shaped Pt electrodes. A general square-scheme mechanism is employed that allows for redox reactions of both free and ion paired IrCl63- and IrCl62-. Numerical simulations of the voltammetric response employing the ion pairing square scheme suggest that the dissociation rate constant of the reactant ion pair can be determined from the voltammetric wave shape recorded at very small electrodes. No ion-pair kinetic limitation was discernable in K+ and Ca2+ solutions using Pt hemispherical electrodes with radii as small as 13 nm, indicating that ion-pair dissociation in these solutions occurs at rates greater than 107 s−1. However, the voltammetric wave in 1 M TEA+ displays a shoulder at electrodes with radii less than 500 nm, consistent with a slower rate of ion-pair dissociation. The appearance of the shoulder is predicted by the numerical simulations and corresponds to a direct observation of the oxidation of both free and ion paired IrCl63- in the same solution.