Quantitative Voltammetry in Weakly Supported Media. Chronoamperometric Studies on Diverse One Electron Redox Couples Containing Various Charged Species: Dissecting Diffusional and Migrational Contributions and Assessing the Breakdown of Electroneutrality

Chronoamperometric transients are recorded in different quantities of supporting electrolyte, from full to self-support, for one electron transfer reactions of redox systems carrying diverse charges, and are studied to dissect migrational and diffusional contributions to mass transport under different conditions (applied overpotential, electrode radius, kinetics and charge of electroactive species). The one-electron redox systems studied are the reductions of Ru3+ (hexaammineruthenium(III)/(II)), Co3+ (cobaltocenium/cobaltocene), and Fe3+ (hexacyanoferrate(III)/(II)). The mass transport (diffusion and migration) is simulated using the Nernst−Planck−Poisson (NPP) model, with good agreement between experiment and theory. The effect of the charge of the electroactive species on its mass transport to the electrode is considered when only small amounts of supporting electrolyte are added to the solution. The mass transport to the electrode is increased by “attractive” migration when a positively charged electroactive species is reduced or a negatively charged species is oxidized. Conversely, when a positively charged electroactive species is oxidized or a negatively charged species is reduced, the migration is “repulsive” and the mass transport to the electrode is decreased.