Electrochemical Reduction of AQ27DS in Aqueous Solution

It is known that anthraquinone derivatives act as aqueous sulphide oxidation catalysts, so the redox chemistry of the compound anthraquinone 2,7-disulphonate (AQ27DS) stimulated our interest, as reported here. AQ27DS was reduced in aqueous solution at pH 9.0 to give a deep red coloured air-sensitive solution. Cyclic voltammetry and exhaustive electrolysis indicated that the anthraquinone was reversibly reduced in a two electron, one proton process at a variety of electrode surfaces. From limiting current results at a rotating disc electrode, the diffusion coefficient of AQ27DS was calculated to be 3.37 x 10-10 m2 s-1. Spectroscopic results confirmed that AQ27DSH- was the major reduced species, but also indicated that the di-anion (AQ27DS2-) and radical species AQ27DS• were also present. ESR spectroscopy showed that the radical was formed via a comproportionation reaction between the di-anion and the AQ27DS starting material. The peak separation from voltammetry enabled the comproportionation constant (Kc) to be estimated, and it was found to be in the range of 0.4 to 4.