Kinetics and mechanism of the complex formation of ethylenediaminetetraacetatoferrate(III) with sulfur(IV) oxides in aqueous solution

Abstract

A detailed kinetic study has been undertaken of the complex formation of [Fe(edta)]^–(edta = ethylenediaminetetraacetate) with sulfite as a function of pH, [Fe(edta)] and [sulfite], during which stopped-flow and temperature-jump techniques were employed. The complex formation kinetics is controlled by the equilibria [Fe(edta)(H₂O)]^–⇌[Fe(edta)(OH)]^2–+ H⁺ and 2[Fe(edta)(OH)]^2–⇌[(edta)Fe–O–Fe(edta)]^4–+ H₂O, in which the aqua complex is extremely labile and the hydroxo and oxo complexes are substitution inert. The rate constant for the formation of [Fe(edta)(SO₃)]^– from [Fe(edta)(H₂O)]^– and SO₃^2– was found to be 4 × 10 dm³ mol^–1 s^–1 at 25 °C and 0.5 mol dm^–3 ionic strength. A limiting rate constant of 2 × 10⁵ s^–1 was reached at high [SO₃^2–] under which conditions the dissociation of the co-ordinated water molecule becomes the rate-determining step. This observation is in agreement with the known solvent-exchange rate constant for [Fe(edta)(H₂O)]^– and supports the operation of a limiting D mechanism. Both kinetic techniques revealed evidence for the participation of a slow, sulfite-independent reaction step, which could be related to the rate-determining dissociation of [(edta)Fe–O–Fe(edta)]^4– at higher complex concentrations. The results are discussed with reference to the available literature data.