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 concentr
ation 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 concentr
ated phosphoric media, Th-227 has been detected in solutions near neut
rality only for c(PO4)>0.1 mol 1(-1). Under these conditions, the thor
ium concentration has been measured after ultrafiltration (cutting siz
e 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 const
ant 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 com
plex forms of thorium for 0.3 <c(PO4) <0.8 mol 1(-1) and 0.8 <C-PO4 <1
.5 mol 1(-1) respectively.