The effect of pH on the kinetics of the reaction of iron(II) with hydrogen peroxide in perchlorate media

Abstract

The rate of the reaction of iron(II) with hydrogen peroxide increases in perchlorate media at high pH until a high limiting rate is attained. This has been interpreted analogously to the effect of other anions on the reaction, and it is concluded that the formation of a species containing two OH^– ions for each iron atom, Fe(OH)_2aq, formed rapidly after raising the pH, is responsible for this increase in rate. This is comparable with the effect of fluoride ions on this reaction. pH-Difference measurements in the absence of H₂O₂ show that FeOH⁺_aq and Fe²⁺_aq are the predominant species of iron at pH =ca. 3, and suggest that at pH > 4 a hydrated dimer (Fe²⁺H₂OFe²⁺)_aq is formed slowly via Fe(OH)_2,aq. Thermodynamic data for the formation of FeOH⁺_aq and the Arrhenius parameters for the reaction of Fe(OH)_2,aq with H₂O₂ are given. The mechanism of the autoxidation of iron(II) in perchlorate media is discussed.