Kinetics of the reaction of aqueous iron(VI) ((FeO42-)-O-VI) with ethylenediaminetetraacetic acid

The reaction of aqueous iron( VI) ((FeO42-)-O-VI, Fe( VI)) with ethylenediaminetetraacetic acid ( EDTA) was studied kinetically as a function of pH (1.98-12.40) and temperature (15-45 degrees C) using a stopped flow kinetic technique. The rate law for the reaction of Fe( VI) with EDTA was found to be first-order with respect to each reactant over the entire studied pH range. The observed rate constants, k, decrease with an increase in pH, varying from 4.19 x 10(4) to 8.60 x 10(-2) M-1 s(-1) over the pH range. The speciation of Fe( VI) (H3FeO4+, H2FeO4, HFeO4-, and FeO42-) and EDTA(H4Y, H3Y-, H2Y2-, HY3-, and Y4-, Y = EDTA) species was used to explain the pH dependence of the k values. From the temperature effect on k at pH 5.4, 7.1, and 9.2, activation parameters, Delta S-double dagger and Delta H-double dagger, were obtained for the reactions of Fe( VI) with EDTA. The values of DS for the reactions were found to be negative, implying a highly ordered transition state in the reaction. The DH for the reaction at pH 7.1 and 9.2 showed similar values within experimental error. Using the observed enthalpy parameters and the enthalpy of deprotonation of HFeO4- and EDTA species (HEDTA(3-) and H(2)EDTA(2-)), the enthalpy of deprotonation of H2FeO4 (Delta H-H2FeO4(0)) was determined as 5.7 +/- 3.0 kJ mol(-1). The reactivity of Fe( VI) with aminopolycarboxylates (APCs) was also studied in alkaline medium. The order of reactivity was determined as primary > secondary > tertiary, which suggests that (FeO42-)-O-VI attacks at the nitrogen atom sites of APCs.