The oxidation of uranium(IV) ions by nitrous acid in 30% tri-butyl phosphate

The kinetics of the oxidation of U(IV) ions by nitrous acid in a 30% TBP solution have been determined. The rate equation was found to be: -d[U(IV)]/dt = k(2a)[U(IV)][HNO2][HNO3][H2O]/[HNO3](2) + beta(3)[HNO3][H2O] + beta(4)[H2O](2), where, k(2a) = 0.405 +/- 0.055 M-1 min(-1) at 55degreesC (beta(3) approximate to 0.08; beta(4) approximate to 0.007) and the activation energy was E = 112 +/- 17 kJ mol(-1). The reaction mechanism appeared to involve interaction with the 1(st) hydrolysis product of U(IV)-UOH3+. The data is compared with a previous study of the nitric acid oxidation of U(IV) in 30% TBP. This reaction is autocatalytic due to the formation of nitrous acid during the reaction. The kinetics of the decomposition of HNO2 in 30% TBP (in the absence of U(IV)) have also been reported.