Indirect cathodic reduction of dispersed CI Vat Blue 1 (indigo) by dihydroxy-9,10-anthraquinones in cyclic voltammetry experiments

Abstract

The indirect cathodic reduction of dispersed indigo (CI Vat Blue 1) was studied by cyclic voltammetry (CV). Dihydroxy-9,10-anthraquinones were used as organic mediators in 0.2M aqueous NaOH. Reversible behaviour was observed for the anthraquinoids studied. Cathodic peak potentials were observed an −710mV to −870mV (vs. Ag/AgCl, 3M KCl reference electrode). The efficiency for indirect cathodic dyestuff reduction was characterised by calculation of so-called enhancement factors e.f., which are defined as the quotient of the cathodic peak current in presence of dispersed dye (Ip)c and the diffusion controlled cathodic peak current of the mediator system (Ip)d, thus e.f.=(Ip)c/(Ip)d. Representative values calculated for e.f. were between 1.2 and 1.7. In case of indirect dyestuff reduction an anodic current is observed during the reverse scan of the CV at a potential of -620mV, which is due to the reoxidation of soluble leuco-indigo to indigo. The peak current of this anodic wave can be used as a measure for the efficiency of the anthraquinoid system used. Highest efficiency was observed for 1,8-Dihydroxy-9,10-anthraquinone, followed by the 1,2- and the 1,4-substituted dihydroxy-anthraquinones and 1,2-Dihydroxy-9,10-anthraquinone-3-sulfonic acid (Alizarine Red S). With increasing indigo concentration the cathodic peak current (Ip)c of the respective anthraquinone increased. This behaviour is characteristic for the catalytic mechanism in cyclic voltammetry.