Kinetics and mechanism of the reaction between plutonium(III) and chlorine in chloride solutions

Abstract

Oxidation of plutonium(III) to (IV) by chlorine in chloride solutions follows an apparently first order rate law with respect to both the reactants, and the rate is independent of acidity. At room temperature, and with 0·01 M initial plutonium concentration and about 0·025 M initial chlorine concentration, the specific reaction rate, k′ increases from 0·04 to 2·8 × 10 −2 min −1, as the hydrochloric acid concentration of the medium changes from 1·5 to 8·0 M. In solutions having constant hydrochloric acid, k′ increases at first linearly with initial chlorine concentration and finally approaches a limiting value for a given initial plutonium concentration. The limiting values in 7·0 M hydrochloric acid with 0·025 M of chlorine concentration are 0·6, 1·2 and more than 2·2 × 10 −2 min −1 respectively with 0·005, 0·01 and 0·02 M initial plutonium concentrations. The rate of oxidation is relatively very slow at room temperature in chloride free perchloric acid solutions, while in solutions containing mixture of hydrochloric and perchloric acids, it is markedly increased by chloride ion concentration for a given total molarity of the acid mixture as well as by the total molarity having fixed concentration of hydrochloric acid, showing thereby that the rate is influenced both by chloride ions as well as by ionic strength of the medium. Thermodynamic quantities of activation at 26°C have been calculated from the experimentally determined activation energy of the reaction, the average value of which is 16·0 ± 1·8 kcal/mole. A mechanism is suggested according to which a transient species is assumed to be formed in steady state concentration by the action of Cl 3 − ions on Pu(III) ions, and the rate is determined by the reaction between these transient species and Pu(III) ions.