Oxidation of hexacyanoferrate(II) ion by hydrogen peroxide: evidence of free radical intermediacy

The redox reaction between hexacyanoferrate(II) ion as the reducing agent and hydrogen peroxide as the oxidizing one, in slightly acid (pH 4.36-6.65) aqueous solutions containing phosphate ions, has been studied by means of an UV-Vis spectrophotometer monitoring the formation of Fe(III) at 420 nm. The initial hydrolysis of the Fe(II)-cyanide complex reactant to yield a pentacyanoaquaferrate(II) intermediate, as well as a decrease in the solution pH (acid catalysis), had both a positive effect on the reaction rate, whereas an increase in the concentrations of Fe(III), phosphate and chloride ions, and d-mannitol had a negative effect (inhibition). A computer program, specifically designed for this purpose, allowed arriving at a complex eight-coefficient experimental rate law by application of the initial rate method, accounting for the dependences of the reaction rate on the concentrations of Fe(II), H2O2 and Fe(III). The reaction was characterized by a low value of the apparent activation energy (28.9 ± 2.7 kJ mol−1). Finally, a multi-step reaction mechanism, involving the participation of a penta-coordinated Fe(II) complex, and hydroxyl, superoxide and other free radicals as intermediates, has been proposed. This mechanism was supported by the available experimental information and confirmed by numerical simulations performed via the fourth order Runge-Kutta integration method