Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 51 / a

Make heterogeneous electron transfer fast relative to delivery of reactant, usually by applying sufficient overpotential so that its surface concentration approaches zero. Use excess supporting electrolyte to suppress migration. For purely diffusional transient transport, suppress uncontrolled convection; for a rotating disk electrode, controlled convection supplies the bulk while diffusion across the thin interfacial boundary layer limits the current. A limiting current that no longer increases with driving potential is the characteristic test; a large current alone is not sufficient.

The midpoint potential $E_m$ characterizes the redox couple under specified solution conditions: it is the potential at which oxidized and reduced forms have equal activities, or equal concentrations in the corresponding formal-potential convention. For a reversible cyclic voltammogram it is estimated by $(E_{pa}+E_{pc})/2$. It approximates the formal redox potential when the usual reversibility and diffusion assumptions apply; it is not the potential halfway along a scan chosen arbitrarily.