Two Rate constant kinetic model for the chromium(III)-EDTA complexation reaction by numerical simulations

Abstract

The complexation reaction of Cr(III) ion in the presence of a large excess of EDTA does not follow a pseudo-first-order kinetics as sometimes suggested. There are two causes for the deviation from this simple behavior: the involvement of a long-lived intermediate, precluding the application of the steady-state approximation, and the autoinhibition provoked by the release of hydrogen ions from the organic ligand to the medium as the final Cr(III)-EDTA violet complex is formed. Numerical simulations have allowed obtaining for each kinetic experiment the values of two rate constants, k1 (corresponding to the formation of the long-lived intermediate from the reactants) and k2 (corresponding to the formation of the final complex product from the long-lived intermediate), as well as the number of hydrogens liberated per molecule of final complex product formed (Hkin). The results indicate that k1 is associated to a fast step (Ea = 87 ± 4 kJ mol-1) and k2 to a slow step (Ea = 120 ± 2 kJ mol-1), whereas the number of hydrogen ions lies within the range 0 < Hkin < 2 in all the kinetic runs. A mechanism in accordance with the experimental data has been proposed.