A Kinetic Study of the Oxidation of Oxalate, Ethylenediamine, and Ammine Complexes of Chromium(III) with Hydrogen Peroxide in Aqueous Ammonia

Abstract

A kinetic study is carried out on the H₂O₂ oxidation of oxalate, ethylenediamine and ammine complexes of Cr (III) in ammonia buffer solutions between pH 9 and 11. In the prese nce of a large excess of H₂O₂, the oxidation reaction obeys the pseudo-first-order rate law with res pect to the concentration of the complex.
The oxidation rate of the oxala t e complexes is of the first-order with respect to the concen trations of the complex and H₂O₂. The rates of complex consumption agree with th ose of CrO₄^2- formation, showing the stoichiometry of the reaction. The step of coordination o f H₂O₂ to the central atom is suggested to be the rate-determining step.
The oxidation rates of the complexes of ethylenediamine and am m onia are little affected by the concentration of H₂O₂, except for the oxidation of Cr(en)₃³⁺ at pH below 10. The rate s of oxidation of the ethylenediamine complexes agree with the rate of ethylenediamine rele ase. The rate-determining step for the ammine complexes is also presumed to be the release of the ligand molecule (s) from the complex. The results are discussed in connection with the wo rk of Knoblowitz et al.