Complexation of protactinium(V) with nitrilotriacetic acid: a study at the tracer scale

Complexation of Pa(V) with nitrilotriacetic acid (NTA) in aqueous solution (1 M (Na, H)ClO4) was studied by solvent extraction at different acidities (pcH = 0.6; 1.0; 2.0 and 2.5) with the element at the tracer scale (C-Pa < 10(-10) M). Two types of metal-ligand complexes were identified in solution: (1 : 1) and (1 : 2) complexes. The mean charge of the two successive complexes formed in aqueous phase was deduced from the variations of Pa distribution coefficient as a function of acidity. The apparent formation constants of PaO(NTA) and PaO(NTA)(2)(3-) were also determined from the solvent extraction experiments. Capillary electrophoresis coupled with inductively coupled plasma mass spectrometry (CE-ICP-MS) experiments confirmed the charge -3 of the complex of maximum stoichiometry.