Reductive decoloration of CI Acid Red 27 and CI Reactive Black 5 is achieved by cathodic electron transfer. The stoichiometry of the dyestuff reduction is studied with redox titrations using an Fe(II)-TEA complex as the reducing agent. CI Acid Red 27 is reduced to the corresponding amines at room temperature, while full reduction of CI Reactive Black 5 is observed first at elevated temperatures. In potentiostatic reduction experiments, CI Acid Red 27 is decolorized with an uptake of 4e-. CI Reactive Black 5 can be readily decolorized by cathodic electron transfer. The reaction proceeds via relatively stable hydrazo intermediates. Investigations with cyclic voltammetry and galvanostatic batch electrolysis experiments indicate that the rate of decoloration with CI Reactive Black 5 depends somewhat on transport processes in the diffusion layer of the cathode, but is limited by a preceding chemical equilibrium.
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