The Th-Acetate Chemical Equilibria: Is It Really That Simple?

The Th/acetate chemical system is truly unique. While it was generally accepted that complexes with one to five acetate ligands are formed, in this work, the formation of hydrolyzed thorium acetate species has been discovered, leading to the redefinition and revisiting of this system. Using the coupling between capillary electrophoresis and ICP-MS, the first four Th-AcO constants have been re-evaluated. Under the experimental conditions, [Th(AcO)₅]^- was not observed. Instead, [Th(OH)(AcO)_i]^3-i(i = 3,4) species were detected. In CE-ICP-MS, kinetically stable species are studied by evaluating the peak areas, while labile species are studied through variations in electrophoretic mobility. For Th, both types of complexes have been observed simultaneously. Based on the variations in the peak area, we were able to determine the first Th⁴⁺ hydrolysis constant (log*K⁰ = -2.7 ± 0.2), in agreement with the value recommended by the Nuclear Energy Agency (NEA) (log*K⁰ = -2.5 ± 0.5). Variations in electrophoretic mobility enabled us to determine the constants β_i (i = 1 - 4) of [Th(AcO)_i]^4-i complexes at two ionic strengths (0.1 and 0.3 M NaClO₄) and to extrapolate them with other data found in literature to zero ionic strength using the specific ion interaction theory (SIT). By combining the constants for the hydrolysis and the binary Th-AcO complexes, the formation constants for the [Th(OH)(AcO)_i]^3-i (i = 1 - 4) species were calculated.