Effects of pH and phosphate on the oxidation of iron in aqueous solution

Abstract

Studies have been initiated to evaluate the catalytic effect of monohydrogen phosphate ions on the oxidation of ferrous (Fe 2+) to ferric (Fe 3+) ions in an aqueous solution under atmospheric oxygen conditions. The reactions were performed with an initial concentration of 1 × 10 −4 M ferrous sulfate in solutions containing varying concentrations of phosphate buffer (0.005–0.0175 M) over the pH range of 6.6–7.1. The final ionic strength of the solutions were adjusted to 0.1 M with sodium chloride and the temperature was kept constant at 25 ± 0.5 °C. The rates of oxidation reactions were measured by following the increase in UV absorbance due to the formation of ferric ion in solution. The reactions appeared to follow pseudo-first-order kinetics and were very prone to catalysis by monohydrogen phosphate at any given pH. H 2PO 4 − seemed to have no effect on the reaction. HPO s 2− was the sole catalytic species with a second-order rate constant of 116.74 M −1 · min −1. The buffer independent pH-rate profile showed a sigmoidal behavior with the pseudo-first-order rate constant increasing with increasing pH. The sigmoidal nature of the experimental pH-rate profile could possibly suggest a change in the reactivity of the oxidizing species which might follow complex kinetics. The effects of ionic strength and temperature on the reaction rates were also evaluated.