Kinetics of the complexation of UO2(NO3)2 with tributyl phosphate (TBP) in xylene

The kinetics of ligand exchange between bulk solution TBP and uranyl nitrate bound TBP in o-xylene has been investigated by P-31 dynamic NMR spectroscopy. First order ligand exchange rate constant, k, and the activation energy for the uranyl nitrate-TBP system were determined from plots of transverse relaxation rates (In (1/tau)) versus reciprocal temperature (1/T). Using the Eyring equation, the average values of activation enthalpy, Delta H* (30.8+/-5.1 kJ.mole(-1)), and activation entropy, Delta S* (-109+/-19 J/mol.K) have been calculated. The negative entropy of activation value indicates an increase in order on proceeding from the reactants to the activated complex. Therefore, an associative mechanism may be proposed for the ligand exchange reaction between UO2(NO3)(2) TBP2 and TBP. Equilibration of the extractant phase with 1.0 M HCl did not change the ligand exchange rate constants or the activation parameters appreciably. These results are discussed in comparison with previous reports on TBP exchange rates for this system.