pH Dependent redox behaviour of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) – Cyclic voltammetry in presence of dispersed vat dye

The redox chemistry of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) was studied as function of pH by photometry, potentiometric titration and cyclic voltammetry. Dependent upon solution pH three species of the oxidised form of Alizarin Red S are present. Electrochemical reduction of the anthraquinone group leads to 1,2,9,10-tetrahydroxy-anthracene-3-sulfonate. The four phenolic hydroxyl groups dissociate with increasing pH value, above pH 12 a tetra anion is the main species in solution. Cyclic voltammetry experiments in unbuffered solution indicate that the overall cathodic reduction contains electron transfer and pH-dependent protonation reactions. Above pH 12 dispersed Vat Yellow 1 could be reduced by indirect cathodic electron transfer using the fully deprotonated system as reversible redox couple. In cyclic voltammetry a reduction potential of (Ep)d = −900 mV (vs. Ag/AgCl/3 M KCl) was observed, which is sufficiently negative to reduce Vat Yellow 1. No indication for an indirect cathodic reduction of Vat Yellow 46 could be registered.

► By photometry three different species of Alizarin Red S could be observed in solution as function of pH.
► Cathodic reduction of Alizarin Red S can be observed in cyclic voltammetry experiments.
► Dependent upon solutions pH five different species of reduced Alizarin Red S can be observed by photometry.
► In alkaline solution indirect cathodic reduction of dispersed Vat Yellow 1 can be achieved by Alizarin Red S.