SPECIATION OF THORIUM IN PHOSPHATE-CONTAINING SOLUTIONS

The equilibrium between thorium orthophosphate (Th-3(PO4)(4) labelled with the radioactive thorium isotope Th-227, and phosphate-containing solutions has been investigated as a function of pH and total concentration of phosphate groups (5<[pH] <9; 0<c(PO4) <1.5 mol 1(-1)). Since Th-3(PO4)(4) is a very insoluble compound, even in relatively concentrated phosphoric media, Th-227 has been detected in solutions near neutrality only for c(PO4)>0.1 mol 1(-1). Under these conditions, the thorium concentration has been measured after ultrafiltration (cutting size of the species at 5 nm) by gamma-ray spectroscopy in the range 10(-9)-10(-5) mol 1(-1). The solubility curves, c(Th) vs. pH value at constant C-PO4 and O-2 vs. c(PO4) at constant pH, lead to the determination of the prevailing species in the solutions: at a pH of 6-7, [ThO(HPO4)(3)(H2PO4)](5-) and [ThO(HPO4)(3)(H2PO4)(2)](6-) are the presumed complex forms of thorium for 0.3 <c(PO4) <0.8 mol 1(-1) and 0.8 <C-PO4 <1.5 mol 1(-1) respectively.