Electrolytic treatment of azo dyes containing o,o′-dihydroxyazo complexation sites

The impact of structure on decolorization rate was studied for two azo dyes containing the o,o′-dihydroxyazo complexation site, Direct Red 83 and Acid Alizarin Violet N. Both dyes were investigated with and without complexed copper; the methoxylated pre-cursor of Direct Red 83 was also investigated. The dye solutions were electrolytically treated and sample conductivity, pH, and temperature were measured before and after each 60-min run. The UV–VIS absorbance spectra of samples were measured at 10 min intervals with and without quenching of chlorine residuals with sodium thiosulfate. The decolorization rate data calculated from the absorbance spectra were plotted as fraction of dye remaining, C_t/C₀, versus time, t, in seconds. First-order curves were fitted to the data, and the apparent first-order rate constant, k, was calculated for each dye. In general, the Acid Alizarin Violet N forms were decolorized more rapidly than the Direct Red 83 forms, the presence of copper protected the dyes from the continuation of indirect oxidation, and methoxylation reduced the first-order rate constant, but it did not protect the azo bond from continued indirect oxidation as was the case with copper. Key words: azo dye, structure, electrolytic treatment, electrochemical, color, decolorization, rate constant.