Chlorine Dioxide-Facilitated Oxidation of the Azo Dye Amaranth

The oxidation reaction of amaranth (trisodium 2-hydroxy-1-(4-sulfonato-1-naphthylazo)naphthalene-3,6-disulfonate or AM(-)) by chlorine dioxide (ClO2) in aqueous conditions was investigated in detail. The major reaction products immediately after decolorization of AM(-) were 1,2-naphthoquinone disulfonate sodium salt and 1,4-napthalenedione. The reaction had first-order dependence on both AM(-) and ClO2. The rate-limiting step involved the reaction between AM(-) and OH- ions. The role of hydroxide ion as a catalyst was established. The second-order rate constant increased with pH, from (19.8 +/- 0.9) M-1 s(-1) at pH 7.0, (97.1 +/- 2.3) M-1 s(-1) at pH 8.0 to (132.5 +/- 2.8) M-1 s(-1) at pH 9.0. In the pH range of 6.0-7.5, the catalytic constant for OH- ion was 4.0 x 10(9) M-2 s(-1). The energy and entropy of activation values for the reaction were 50.0 kJ mol(-1) and -658.7 J K-1 mol(-1), respectively. A probable reaction mechanism was elucidated and was validated by simulations.