In general, the voltammeteric current from a mixture of redox-active substances will not be the sum of the currents that each substance would produce at the same concentration independently

At the potential range where both decamethylferrocene (dMeFc) and ferrocene (Fc) are oxidized with rates controlled by linear diffusion, electrogenerated Fc+ radicals diffusing outwards from the electrode react quantitatively (K23 ⁰c=5.8 x 108) with dMeFc diffusing towards the electrode and produce Fc and dMeFc+. That reaction replaces dMeFc with faster diffusing Fc and the total mass-transfer limited current from the mixture is increased by ~10%. Analogous observations are made when mass-transfer is controlled by convective-diffusion as in RDE voltammetry. Similar results have been obtained with another, and for all practical purposes randomly selected pair of redox- active substances, [Co(bipy)3]2+ and N-methylphenothiazine (MePTZ); reaction of MePTZ+ with [Co(bipy)3]2+ replaces the latter with the faster diffusing MePTZ and the current increases by ~20%. The experimental voltammograms have been simulated numerically and the role of the rate constant of the homogeneous reaction and of the diffusion coefficients of all species involved have been studied in detail. Importantly, it was found that even within a given redox system the dependence of the mass-transfer limited current on the bulk concentrations of the redox-active species is expected to be non-linear. These findings are discussed in terms of their electroanalytical implication