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Two Rate Constant Kinetic Model for the Chromium(III)–EDTA Complexation Reaction by Numerical Simulations
Author(s) -
PerezBenito Joaquin F.
Publication year - 2017
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.21070
Subject(s) - chemistry , ethylenediaminetetraacetic acid , reaction rate constant , kinetic energy , chromium , kinetics , ligand (biochemistry) , ion , inorganic chemistry , computational chemistry , chelation , organic chemistry , biochemistry , physics , receptor , quantum mechanics
The 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, k 1 (corresponding to the formation of the long‐lived intermediate from the reactants) and k 2 (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 ( H kin ). The results indicate that k 1 is associated with a fast step ( E a = 87 ± 4 kJ mol −1 ) and k 2 to a slow step ( E a = 120 ± 2 kJ mol −1 ), whereas the number of hydrogen ions lies within the range 0 < H kin < 2 in all the kinetic runs. A mechanism in accordance with the experimental data has been proposed.