Photochemical generation of semiquinone intermediates at a rotating semi-transparent disc electrode

Abstract

Light from a mercury lamp shone through a semi-transparent electrode into a solution containing a mixture of anthraquinone and its corresponding hydroanthraquinone generates semiquinones inside the diffusion layer of the electrode. The semiquinone species can either be oxidized or reduced on the electrode. The rate constants for their electrochemical reactions and for their homogeneous recombination can be measured. At the potential of zero photocurrent the electrochemical rate constants for the reduction and oxidation of the semiquinone are equal. At this point the free energies for the first and second transition states in the electrochemical reduction of anthraquinone are equal. The variation of the potential of zero photocurrent with pH determines the electrochemical mechanism of the anthraquinone.