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 SO32– 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 [SO32–] 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.