An Insight into the Kinetics and Mechanism of Oxidation of Tris(4,4'-dimethyl-2,2'-bipyridine)iron(II) by Bromate

Kinetic investigation of oxidation of tris(4,4'-dimethyl, 2,2'-bipyridine)iron(II) by bromate has been undertaken. The redox reaction between [Fe(dmbpy) 3 ] 2+ and bromate ion (BrO ) was monitored spectrophotometrically under the pseudo-first order condition i.e.[BrO ] ≫ [Fe(dmbpy) 3 ] 2+ . Kinetic data revealed that the pseudo first order rate constant ( k obs ) is independent of the concentration of [Fe(dmbpy) 3 ] 2+ . Though, when concentration of bromate ion is increased in reaction mixture at fixed pH, the rate also increases up to the saturation point at higher concentrations, indicating a precursor complex formation and an outer-sphere mechanism. The influence of accelerating [H + ] and ionic strength on reaction rate were also investigated. The increasing values of the rate constant at low pH mention the involvement of protonated bromate species (HBrO 3 and H 2 BrO ) in the rate-determining step. However, a rise in the value of the rate constant with increasing ionic strength implies the diprotonated species, H 2 BrO , as the reactive species. On the basis of these conclusions, we were able to postulate the appropriate mechanism and the rate law of this redox reaction.