Coupled electron- and proton transfer processes in the reduction of alpha-[P2W18O62](6-) and alpha-[H2W12O40](6-) as revealed by simulation of cyclicvoltammograms

Quantitative analysis of the complex problem of coupled electron- and proto n-transfer steps during reduction of the polyoxo anions alpha-[P2W18O62](6- ) and alpha-[H2W12O40](6-) in aqueous NaCl (0.5 M) has been achieved by sim ulation of cyclic voltammograms (Rudolph, M.; Reddy, D. P.; Feldberg, S. W, Anal, Chem. 1994, 66, 589A) over wide ranges of anion concentration, pH, a nd scan rate. Since there are too many unknown parameters to attempt a ones tep global form of simulation, a systematic, stepwise approach has been ado pted by progressively accessing regimes of increasing voltammetric complexi ty. This protocol allows experimental behavior in each system over 5 orders of magnitude in proton concentration to be simulated by estimation of thre e protonation constants combined with experimentally determined reversible halfwave potentials for the two one-electron processes involved, Fast elect ron transfer and protonation kinetics are assumed. The importance of the va lues chosen for the diffusion coefficients of the proton and polyoxo anion species is considered. The simulations account for the fact that pairs of o ne-electron processes coalesce to give an apparent two-electron process in the pH range 1-6 for reduction of both anions.