Influence of protons on electrochemical behaviour of the system quinone-hydroquinone in dichloromethane

Additions of strong acids to a solution of quinone in dichloromethane lead to protolytic equilibria which have a marked influence on its electrochemical behaviour. In the presence of free protons, the electroactive species is the monoprotonated form of quinone, QH+, and at higher acid concentrations the species QH22+ is formed with the acidity constant pK(QH+/QH22+) = 1.2. The overall equilibrium EC reaction scheme necessarily involves also the protonated forms of hydroquinone, at least the species QH3+ which is, in contrast to the inactive hydroquinone, electroactive on the dropping mercury electrode in the medium of dichloromethane.