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 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 formation Fe(III) at 420 nm. initial hydrolysis Fe(II)-cyanide complex reactant to yield a pentacyanoaquaferrate(II) intermediate, well decrease solution pH (acid catalysis), had both positive effect on rate, whereas increase concentrations Fe(III), chloride d-mannitol negative (inhibition). A computer program, specifically designed for this purpose, allowed arriving eight-coefficient experimental rate law application method, accounting dependences Fe(II), H2O2 Fe(III). was characterized low value apparent activation energy (28.9 ± 2.7 kJ mol−1). Finally, multi-step mechanism, involving participation penta-coordinated Fe(II) complex, hydroxyl, superoxide other free radicals intermediates, proposed. This mechanism supported available information confirmed numerical simulations performed via fourth order Runge–Kutta integration method.