Kinetic study of the complexation of chromium(III) by L-glutamic acid

Abstract

The complexation of chromium(III) by L-glutamic acid / hydrogen L-glutamate ion in aqueous media under slightly acidic conditions (pH 2.46 - 5.87) has been studied by the use of UV-Vis spectroscopy. Although the reaction in the presence of a large excess of organic ligand is considered in the chemical literature to be a pseudo-first order kinetic process, the rate-time plots corresponding to the formation of the reaction product show a distinct bell-shaped profile, caused by the accumulation of a long-lived intermediate. Given that this intermediate is not reactive enough for the steady state approximation to apply, an integrated rate law involving a double-exponential function has been used to obtain two rate constants for each kinetic experiment, corresponding to the formation of the long-lived intermediate from the reactants (k1) and to its decay into the reaction products (k2). Both rate constants increased with the initial concentration of hydrogen L-glutamate ion in a double-reciprocal linear relationship, k1 decreased with the ionic strength whereas k2 increased, both evidenced the existence of base catalysis and the corresponding activation energies were 83 ± 10 and 95 ± 5 kJ mol-1, respectively. The spectroscopic data indicated the formation of several complexes, differing in the number of organic ligands per chromium atom and in their acid-base properties. A mechanism in agreement with the available experimental data has been proposed, involving the breakage of a Cr(III)-aqua chemical bond as a previous step to the coordination of the organic ligand