Fe3+-D-gluconate and Ca2+-Fe3+-D-gluconate complexes as mediators for indirect cathodic reduction of vat dyes -: Cyclic voltammetry and batch electrolysis experiments

The indirect cathodic reduction of dispersed vat dyes CI Vat Yellow 1 and CI Vat Blue 5 was investigated by cyclic voltammetry and with batch electrolysis experiments. 0.01 mol L-1 solutions of the complexes Fe3+-D-gluconate and Ca2+-Fe3+-D-gluconate were studied. The addition of dispersed dyestuff to the mediator solution lead to a catalytic current. While the cathodic peak currents of both complexes is comparable, Fe3+-DGL shows higher enhancement factors, which are defined as quotient of catalytic peak current and cathodic peak current of the mediator system (I-p)(c)/(I-p)(d). In the presence of 0.5 g L-1 of dispersed vat dye enhancement factors of 1.8 were determined at scan rates of 0.005-0.010 V s(-1). Galvanostatic batch reduction experiments with use of a laboratory multi-cathode cell confirmed the favourable properties of the Fe3+-DGL in comparison to the binuclear Ca2+-Fe3+-DGL. With use of the Fe3+-DGL mediator complete dyestuff reduction could be achieved. The batch reduction process was followed experimentally by photometry and redox potential measurement in the catholyte.