OXIDATION OF L-ASPARTIC ACID AND L-GLUTAMIC ACID BY MANGANESE(III) IONS IN AQUEOUS SULFURIC-ACID, ACETIC-ACID, AND PYROPHOSPHATE MEDIA - A KINETIC-STUDY

Kinetics of oxidation of L-aspartic acid and L-glutamic acid by manganese(III) ions have been studied in aqueous sulphuric acid, acetic acid, and pyrophosphate media. Manganese(III) solutions were prepared by known electrolytic/chemical methods in the three media. The nature of the oxidizing species present in manganese(III) solutions was determined by spectrophotometric and redox potential measurements. The reaction shows a variable order in [manganese(III)](o): the order changes from two to one as the reactive oxidizing species changes from an aquo ionic form to a complex form. There is a first-order dependence of the rate on [amino acid], in all the three media while the other common features include an inverse dependence each on [H+] and on [manganese(II)]. Effects of varying ionic strength and solvent composition were studied. Added anions such as pyrophosphate, fluoride, or chloride alter the reaction rate and mechanism by changing the formal redox potential of Mn(III)-Mn(II) couple. Activation parameters have been evaluated using the Arrhenius and Eyring plots. Mechanisms consistent with the kinetic data have been proposed and discussed. (C) 1995 John Wiley and Sons, Inc.