Interaction of Phosphate with Iron(III) in Acidic Medium, Equilibrium and Kinetic Studies

Equilibrium reactions of iron(III) with phosphate were studied spectrophotometrically by UV-Vis in the pH range of ∼ 1.0-2.20. The STAR-94 Program was used to determine the number of absorbing species as well as the stoichiometries and formation constants of the complex species. Some literature values were further confirmed and new values of different stoichiometries were obtained. The kinetics and mechanism of Fe(III) with phosphate were studied in acidic medium. The reactive phosphate species were found to be only H3PO4 and H2PO− 4 and for Fe(III) were only Fe3+, FeOH2+ and Fe(OH)+ 2. The observed rate constants were pH as well as Tphos (total concentration of phosphate) dependent, i.e. Kobs,i = A i + B i Tphos + C i T2 phos (at a given pH).