KINETIC INVESTIGATIONS INTO THE MECHANISM OF PEROXOSULFATE OXIDATION OF CALMAGITE DYE CATALYZED BY MANGANESE(II) IONS

Manganese(II) ions were found to be capable of catalysing the peroxosu lfate (KHSO5) oxidation of Calmagite, a dye containing the o,o'-dihydr oxy azo structural motif at pH 10 but not the more common monohydroxy azo dyes or those that contain no o-hydroxy group. The catalysed decom position of Calmagite results in decomposition profiles that resemble an autocatalytic process, i.e. the reaction rate increases as dye conc entration decreases. Spectroscopic investigations indicated that two c omplexes are formed between Mn-II and Calmagite (D), having 1:1 and 1: 2 stoichiometries, i.e. [MnD] and [MnD2]. Under experimental condition s [KHSO5] much greater than [D] much greater than [Mn-II] so the catal yst exists in the form [MnD2] and it is shown that the unusual kinetic s are due to a reaction of order -1 in [D]. This negative order is exp lained by attributing [MnD] as the active catalyst, formed from the in ert [MnD2], with which it is in equilibrium, by the loss of one dye mo lecule. The experimental data were found to conform to the rate expres sion -d[D]/dt = k(0)[Mn-II][KHSO5]/[D] and the rate constant k(0) can be determined from plots of [D](2) against time. It is concluded that a key factor for promoting catalysis is specific complexation of Mn-II to the dye substrate, and that the o,o'-dihydroxy azo structural unit is specifically required for efficient manganese catalysis. Some insi ght into the mechanism of catalysis was obtained by investigating Mn-I II as catalyst and from studies where H2O2 was employed as the oxidant . It is proposed that the mechanism involves nucleophilic attack by ox idant on the metal centre followed by peroxide-bond scission, leading to formation of manganese-(III) or -(IV) species which subsequently in itiate dye oxidation via an inner-sphere reaction mechanism.