THE EQUILIBRIA AND KINETICS OF THE COMPLEX FORMATION BETWEEN Fe(III) AND TARTARIC AND CITRIC ACIDS

Abstract

The equilibria and kinetics of formation of 1 : 1 iron(III) complexes with tartaric and citric acids have been studied in the pH range 1.0–2.0 in aqueous alcoholic perchlorate media. The equilibrium constants for the reactions Fe³⁺ +

H₄L^keq Complex + nH⁺ were obtained from spectrophotometric measurements in the wavelength range 360–420 nm. The values of K _eq determined at 20°, μ=1.0 M (water 100%), were 0.21 M with n=2 for tartaric and 0.0186 M² with n=3 for citric acid. The stoichiometry of the complex formation and the reaction sites of the ligands were discussed with reference to previous findings on ligands of related structures. The kinetics of the reactions were carried out by the stopped-flow technique. From the hydrogen ion dependence of the reaction rates it can be shown that complex formation occurs between FeOH²⁺ and differently protonated forms of the ligands. Forward rates for the different paths are consistent with an SN1 IP interaction in agreement with the Eigen mechanism; forward and reverse rate quotients enabled the evaluation of the equilibrium constants which agreed satisfactorily with the spectrophotometric ones. The effect of varying the solvent composition (water-alcohol) was discussed with reference to the reaction mechanism.