Hydroxyanthraquinones Carminic Acid and Chrysazin Anodic Oxidation

The anodic oxidation of the hydroxyanthraquinones carminic acid (CA) and chrysazin (CR) was investigated. The oxidation of CA proceeds in a pH-dependent cascade mechanism, concerning the hydroquinone, the catechol and the 3-OH groups in the anthraquinone moiety. The oxidation of the hydroquinone following the catechol electron-donating groups occurs first at low positive potentials, the 3-OH group is oxidized irreversibly at a higher potential. The oxidation of CR is pH-dependent and occurs in successive steps. Oxidation of the hydroquinone tautomer in the CR-ring occurs first, and the symmetrical 1-OH and 8-OH groups are irreversibly oxidized at the same higher potential.