SOLUBILITY OF THORIUM ORTHOPHOSPHATE

The high temperature form (1400-degrees-C) of Th3(PO4)4, labelled with the radioactive thorium isotope Th-227, has been prepared by the wet method from the correct stoichiometric mixtures of Th(NO3)4 and H3PO4 solutions. This labelled compound was added to aqueous solutions in or der to study its solubility. The conservation of its structure after a few months of immersion was verified by X-ray diffraction and zeta-po tential measurements. The total concentration of the dissolved thorium species (the cutting size of the species was 5 nm) in equilibrium wit h the solid phase was measured in water and in different acidic soluti ons prepared using a range of perchloric acid concentrations, which ma inly correspond to the existence of the Th4+ aqua ion. Additional data in phosphoric acid media were obtained to check the conditions of Th4 + complexation. The solubility of Th3(PO4)4 in water is found to be ar ound (7.6 +/- 1.9) x 10(-11) mol L-1. The solubility product of the co mpound, which appears to be much lower than previously reported values , is K(so)=[Th4+]3 [PO43-]4 almost-equal-to 1 X 10(-112+/-2.1) at zero ionic strength.