Electrochemical characteristics and dyeing properties of selected 9,10-anthraquinones as mediators for the indirect cathodic reduction of dyes

Substituted 9,10-anthraquinones were characterised using cyclic voltammetry for their potential to serve as mediators for the indirect cathodic reduction of dispersed sulphur dyes, vat dyes and indigo. Besides characterisation of the electrochemical behaviour, the compounds were assessed in model dyeing experiments for chemical stability and staining of cotton fabric. 1,2-, 1,4- and 1,8-dihydroxy-9,10-anthraquinones exhibited negative mid-point potential ((E_p)_d + (E_p)_a)/2 of −713 to −820 mV (vs. Ag/AgCl, 3 M KCl). Under model dyebath conditions staining of cotton fabric occurred. 9,10-anthraquinone-1,5-disulfonate and 9,10-anthraquinone-2,6-disulfonate both imparted low staining to cotton; their mid-point potentials (−698 mV and −475 mV, respectively) limit their applicability to the reduction of sulphur dyes. Dihydroxy substituted 9,10-anthraquinone-sulfonates exhibited low staining owing to their good solubility. The mid-point potentials of 1,4-dihydroxy-9,10-anthraquinone-2-sulfonate and 5,8-dihydroxy-9,10-anthraquinone-2-sulfonate were only −618 mV and −675 mV respectively. Of the 9,10-anthraquinoids studied, 1,2-dihydroxy-9,10-anthraquinone-3-sulfonate (C.I. Mordant Red 3) could be identified as the most promising reversible redox couple, displaying a mid-point potential of −848 mV, low staining of cotton fabric and sufficient chemical stability under dyebath conditions.