KINETICS AND MECHANISM OF THE COMPLEX-FORMATION OF ETHYLENEDIAMINETETRAACETATOFERRATE(III) WITH SULFUR(IV) OXIDES IN AQUEOUS-SOLUTION

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)]- half-arrow-right-over-half-arrow-left [Fe(edta)(OH)]2- + H+ and 2[Fe(edta)(OH)]2- half-arrow-right-over-half-arrow-left [(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 x 10 dm3 mol-1 s-1 at 25-degrees-C and 0.5 mol dm-3 ionic strength. A limiting rate constant of 2 x 10(5) 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.