Kinetic investigations of azo dye oxidation in aqueous media
A detailed kinetic and spectroscopic investigation has been made of the oxidation of 4-(4-sulfophenylazo)-1-naphthol (Orange I, 4) and 1-(4-sulfophenylazo)-2-naphthol (Orange II, 5) and a series of specially synthesised, substituted 1-arylazo-2-naphthol dyes (6a, 6b) by hypochlorite, peracids and hydrogen peroxide in aqueous media. In all cases, observed second-order rate constants exhibit maxima in alkaline media at a pH equal to the midpoint of the pKA values of dye and oxidant. It is found that substituents upon the aryl ring ortho to the azo group suppress dye reactivity towards hypochlorite and, in addition, it is shown that peracids exhibit similar trends. In contrast, oxidation by hydrogen peroxide is enhanced by ortho substituents, suggesting that it functions via a different mechanism. Surprisingly, the dye common anion has been identified as the form of the dye most susceptible to oxidation by peracids and hypochlorite, with the undissociated oxidant molecules acting as electrophiles. In contrast, dye oxidation by hydrogen peroxide proceeds via the perhydroxyl anion and the hydrazone tautomeric form of the dye. The main function of ortho substituents is to increase the dye pKA, thereby influencing observed rates (k2obs) by changing the equilibrium concentration of reactive species. Substituents also exert an electronic effect on absolute rate constants, k2, giving good Hammett plots for both ortho and para substituents. Reactive oxygen species, e.g. singlet oxygen, superoxide or hydroxyl radicals, are not implicated in the rate-controlling step of the reaction.