Iron-complexes of bis(2-hydroxyethyl)-amino-compounds as mediators for the indirect reduction of dispersed vat dyes – Cyclic voltammetry and spectroelectrochemical experiments

The indirect cathodic reduction of dispersed vat dyes CI Vat Yellow 1 and CI Vat Blue 5 was investigated by cyclic voltammetry and spectroelectrochemistry. N,N-bis(2-hydroxyethyl)-3-amino-2-hydroxy-propane-sulfonic acid and 2,2-bis-(hydroxyethyl)-(iminotris)-(hydroxymethyl)-methane were used as ligands to form electrochemically active Fe-complexes in alkaline solution. At a concentration of c(Fe3+) = 5 × 10−4 mol dm−3 and ligand concentrations of 5 × 10−3 mol dm−3 in 0.1 mol dm−3 NaOH cathodic peak potential (Ep)d of the complexes was observed between −1000 mV and −1020 mV. Reversible behaviour was found at a HMDE electrode. Suitability of the complexes for indirect cathodic dyestuff reduction was described with the so-called enhancement factors, e.f., which are defined as the quotient of the cathodic peak current (Ip)c in the presence of dispersed dye and the diffusion controlled cathodic peak current of the mediator system (Ip)d, thus e.f. = (Ip)c/(Ip)d. Spectroelectrochemical experiments were used to prove indirect cathodic dyestuff reduction in case of very low enhancement factors of ⩽1.