Oxidation of hexcacyanoferrate(II) ion by hydrogen peroxide: inhibition by polyalcohols and related compounds

Abstract

The oxidation of hexacyanoferrate(ll) ion by a large excess of hydrogen peroxide, in slightly acidic aqueous media containing potassium dihydrogen phosphate (pH 5.10 ± 0.05), was followed by monitoring the increase of absorbance at 420 nm as the colorless Fe(ll) complex gradually evolved into the yellow Fe(lll) complex. The reaction was inhibited by OH-containing organic compounds, either alcohols or carbohydrates, and two different inhibition pathways were observed, an iron(III)-independent pathway (rate constant k1) and an iron(III)-mediated pathway (rate constant k2). A BASIC-language computer program was developed in order to use the fourth-order Runge-Kutta integration method to obtain the concentrations of the Fe(ll)-inhibitor complex and the Fe(lll) reaction product. Rate constant k1, whose value is determined by that of the initial rate, decreased slightly as the concentration of alcohol / carbohydrate increased, and a mechanism involving the formation of hydroxyl radicals in a Fenton-like reaction and its posterior scavenging by the organic antioxidant additive has been proposed. Of the 8 inhibiting agents that were tried, the most potent antioxidant under the experimental conditions of this study was D-mannitol. Rate constant k2, whose value is a measurement of the deviation from a pseudo-first order behavior provoked by the inhibiting agent, increased notably as the concentration of the latter increased, and a mechanism involving the complexation of the Fe(lll) product by the organic inhibitor and its posterior outer-sphere one electron reduction from hexacyanoferrate(ll) ion has also been proposed. This might result in a blockage of the regeneration of pentacyanoaquaferrate(ll) ion, an intermediate believed to be essential for the redox reaction to take place.