Authors:O. Machalický, L. Lichý, M. Tomica, R. Hrdina, and O. Šolcová
The photocatalytic decomposition of indigo-5,5′-disulfonic acid (indigocarmine) and indigo-6,6′-dicarboxylic acid has been studied using TiO2 suspension and the polychromatic irradiation of a high-pressure Hg arc lamp. The dyestuffs were dissolved in aqueous medium at pH = 10 and photolyzed in a back-flow tubular photoreactor. The reaction products were determined by HPLC, NMR and MS spectroscopy. Two main photoproducts obtained from indigocarmine (2-amino-5-sulfobenzoic acid and isatine-5-sulfonic acid) were detected. The photocatalysis of indigo-6,6′-dicarboxylic acid gave aminoterephthalic acid and isatine-6-carboxylic acid as two main products. The quantitative analyses of reaction products were performed with UV/Vis and HPLC using chemical standards. A stoichiometric model and kinetic description of photocatalysis of sulfo and carboxy indigo dyes are suggested. The effect of hydroxide and carbonate anions on the rate of photocatalysis was also studied.