Kinetics of the chromium(III)/L-glutamic acid complexation reaction: formation, decay, and uv-vis spectrum of a long-lived intermediate

Abstract

The kinetics of the aqueous reaction of Cr(III) with either L-glutamic acid or sodium hydrogen L-glutamate at pH 2.46-5.87 have been followed by means of absorbance readings. The rate of formation of the reaction products showed acceleration-deceleration periods, caused by the accumulation and posterior decay of an intermediate in nonnegligible concentration. A double-exponential integrated rate law allowed obtaining two rate constants for each absorbance-time experimental series, associated with the appearance (k1) and decay (k2) of the long-lived intermediate. An increase of the initial concentrations of either hydrogen L-glutamate (apparent kinetic orders < 1) or hydroxide (kinetic orders = 1) ions resulted in an increase of both k1 and k2, but addition of an inert electrolyte (KNO3) resulted in opposite effects on k1 (decrease) and k2 (increase). The experimental activation energies were 83 ± 10 (for k1) and 95 ± 5 (for k2) kJ mol−1. The electronic spectrum of the low reactivity detected intermediate resembled more closely to that of the blue/green reactant than that of the violet reaction product. The low number of protons set free by the complexating hydrogen L-glutamate ligand seems to suggest that some polymerization of the coordinated amino acid (to form a di- or tripeptide) might take place. The available experimental data indicate that the coordination of the organic ligand must be preceded by the breakdown of a strong Cr(III)-H2O chemical bond in the slow steps of the mechanism.