REDOX CHEMISTRY OF H2S OXIDATION BY THE BRITISH-GAS STRETFORD PROCESS .3. ELECTROCHEMICAL-BEHAVIOR OF ANTHRAQUINONE 2,7 DISULFONATE IN ALKALINE ELECTROLYTES

Electrochemical reduction of aqueous anthraquinone 2,7 disulphonate (AQ27DS) solutions at pH 9.3 were studied at Hg, Au and Pt electrodes. Cyclic voltammograms showed about 40 mV potential separation of the single pair of current peaks, precluding a simple one or two electron process. Charge measurements in controlled potential exhaustive reductions indicated a 2 mol e- per mol AQ27DS process leading to quinolate anions, whereas the partially reduced solution showed an EPR spectrum, indicating the presence of radical species, which, if produced directly, would involve only a 1 mol e-per mol AQ27DS process. U.v.-visible and EPR spectra of the deep red partially reduced AQ27DS solutions showed that both radical anions (AQ27DS-) and fully reduced anthraquinolate were present; AQ27DS- radicals predominated at low conversions, while complete conversion produced AQ27DSH- ions. The AQ27DS diffusion coefficient was determined as 3.73 x 10(-10) m2 s-1 from steady-state voltammetry at a gold rotating disc electrode. The results are congruous with a reduction mechanism involving an initial 2 mol e- per mol AQ27DS process to give anthraquinolate anions, from which electron transfer in solution to AQ27DS species produced AQ27DS- radical anions (a comproportionation). The comproportionation equilibrium constant was estimated as between 0.2 and 4.0 from cyclic voltammograms; together with a value of pK(a2)(AQ27DSH2) = 10.8 from the literature, this enabled the solution composition, and hence the major absorption spectral changes, to be predicted as a function of conversion. From a calculated potential-pH diagram, the AQ27DS reduction mechanism was predicted to involve disproportionation of the radical anions at pH much less than 9.3 and two sequential one electron reductions at pH much greater than 9.3