Mechanistic studies for modelling the metal ioncatalyzed tiron-hydrogen peroxide indicator reaction

Model building of kinetic-catalytic methods of determination is attempted for the tironhydrogen peroxide indicator reaction catalyzed by cobalt(II) and manganese(II) in alkaline medium. As a first approximation, rate equations for the metal ion-catalyzed reactions are derived from kinetic dependences on reagent concentrations and pH. More general models are obtained by considering the metal ion equilibria in the reaction mixture. For Co(II), a ternary tironhydrogen peroxideCo(II) complex was found to be responsible for the catalytic activity. As stability constants of peroxo (Co(II) complexes are unknown, only a qualitative approach can be given. Manganese catalyzes the indicator reaction in the presence of 1,10-phenanthroline or 2,2′-bipyridine. The initial rates under pseudo zero-order conditions in hydrogen peroxide and tiron correlate directly with the fraction of ternary activatortironMn(II) complex present. For bipyridine as activator, the rate constant is 1.31 × 10⁶ M^?1 min^?1 with respect to the ternary complex.