Electrochemical And Spectroelectrochemical Studies Of Vat Dye Indigo

Indigo is a well known vat dye involving a reduction/oxidation mechanism. The water-insoluble indigo is reduced by reducing agents such as sodium dithionite and iron(II) triethanolamine complex to soluble leucoindigo which, having penetrated into the material to be dyed, is oxidized back to the original indigo by oxygen. The present work resolves the issue, that indigo is insoluble, with two different strategies, then studies the electrochemical properties and redox mechanism of indigo:(1) Transform the indigo to water-soluble indigo carmine (IC) by sulfonation with fuming sulphuric acid. Cyclic voltammetry and double potential step UV-Vis spectroelectrochemical methods were used to study both reductive and oxidative decolorization of indigo sulfonate (IC) in a long-path-length thin-layer electrochemical cell. The obtained results help improve the understanding of the decolorization mechanisms in the two cases. The leuco of IC that was formed via a 2e–/2H+ electro-reduction reaction can be reoxidized to IC both electrochemically and chemically by dissolved oxygen. The electrochemical re-oxidation had rate constant an order of magnitude more than the electro-reduction, as well as the chemical re-oxidation by dissolved oxygen greatly reduced the current efficiency for the formation of leuco-IC. On the other hand, a complex electrochemical- chemical-electrochemical (ECE) mechanism has been proposed for the electro-oxidation of IC. The oxidized intermediate formed in the first step can decompose to isatin sulfonic acid (ISA) upon cleavage of the center C=C double bond of IC, and the ISA may be further electro-degraded to small molecules at more positive potentials.(2) Prepare an indigo-dispersed carbon paste electrode (InCPE) with mixing solid indigo particles and graphite powders. Cyclic voltammetry, square wave voltammetry and double potential step UV-Vis spectroelectrochemistry were used to study the redox processes of indigo microparticles dispersed in a solid carbon paste electrode. Both the indigo/leucoindigo and indigo/dehydroindigo redox couples underwent reversible 2e–/2H+ surface-confined reactions. Alkaline electrolytes showed more negative effect on the reduction of indigo to leucoindigo than on its oxidation to dehydroindigo. A new species (probably indolone) was monitored in the re-oxidation of leucoindigo, while isatin was found in the oxidation of indigo at enough positive potential. More detailed electrochemical mechanisms were proposed for the two redox systems, respectively. The present work shows that the microparticle-dispersed carbon paste is an attractive electrode material not only for solid state voltammetry but also for stripping spectroelectrochemistry. The absence of the starting reactant in the solution simplified the spectral analysis and thus the product identification.