Two Rate Constant Kinetic Model for the Chromium(III)–EDTA Complexation Reaction by Numerical Simulations

Abstract

ABSTRACTThe complexation reaction of Cr(III) ion in the presence of a large excess of ethylenediaminetetraacetic acid (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 with 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.