The kinetics of the complex formation between iron(III)–ethylenediaminetetraacetate and hydrogen peroxide in aqueous solution

The kinetics of the formation of the purple complex [FeIII(EDTA)O2]3−, between FeIII–EDTA and hydrogen peroxide was studied as a function of pH (8.22–11.44) and temperature (10–40 °C) in aqueous solutions using a stopped-flow method. The reaction was first-order with respect to both reactants. The observed second-order rate constants decrease with an increase in pH and appear to be related to deprotonation of FeIII–EDTA ([Fe(EDTA)H2O]− ⇔ Fe(EDTA)OH]2− + H+). The rate law for the formation of the complex was found to be d[FeIIIEDTAO2]3−/dt=[(k4[H+]/([H+] + K1)][FeIII–EDTA][H2O2], where k4=8.15±0.05×104 M−1 s−1 and pK1=7.3. The steps involved in the formation of [Fe(EDTA)O2]3− are briefly discussed.