P-31-NMR study of the organophosphorous complexes of uranium(VI) .2. The system uranyl nitrate-tri-N-butylphosphate-tri-N-octylphosphine oxide

The formation of the mixed complex UO2(TOPO)(TBP)(NO3)(2) is reported by P-31-NMR spectroscopy for the first time. The extraction of the uranyl nitrate by a TBP and TOPO mixture was simulated in a NMR vial and monitored ''in situ''. The equilibrium constant values of the processes in which the mixed complex is involved, i.e.: K-R, for the redistribution of TBP and TOPO ligands between their uranyl complexes, K-M, for the formation from constitutive molecules, K-S, for the substitution of the uranyl coordinated TBP by TOPO are determined. The redistribution constant, K-r at 25 degrees C (4.43 +/- 0.5) and the changes of the thermodynamic variables Delta H (0.59 +/- 0.14 kcal/mol), Delta S (5.04 +/- 0.51 cal/mol . degree) have values close to the statistical values, suggesting a random nature of this process. The K-S value (4.5 . 10(3)) reveals the much greater affinity for uranium(VI) of TOPO relative to TBP.