Nonadditive Voltammetric Currents from Two Redox-Active Substances and Electroanalytical Implications

At the potential range where both decamethylferrocene (dMeFc) and ferrocene (Fc) are oxidized with rates controlled by linear diffusion, electrogenerated Fc.+ radicals diffusing outward from the electrode react quantitatively (K23(degree)C = 5.8 × 108) with dMeFc diffusing toward the electrode and produce Fc and dMeFc.+. That reaction replaces dMeFc with Fc, whose diffusion coefficient is higher than that of dMeFc, 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 MePTZ, which diffuses faster, and the total current increases by ~20%. The experimental voltammograms have been simulated numerically and the role of (a) the rate constant of the homogeneous reaction, (b) the relative concentrations, and (c) the diffusion coefficients of all species involved have been studied in detail. Importantly, it was also identified that within any given redox system the dependence of the mass-transfer limited current on the bulk concentrations of the redox-active species is expected to be nonlinear. These findings are discussed in terms of their electroanalytical implications.