Electrochemical reactions with protonations at equilibrium Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

The kinetics of the p-benzoquinone/hydroquinone Q/QH2 couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q−, kh3=1/6×10−3 cm s−1; QH.QH−, kh5=0.11 cm s−1; QH+QH., kh2⋍160 cm s−1; kh4 for the reaction QH2+.QH2 is in the range 0.5–4 cm s−1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H+e−H+e−, e−H+H+ e−, and e−H+e−H+ (reverse sequence during the oxidation).