Studies on the complexation behavior of thorium(IV). 1. Hydrolysis equilibria

The stability constants of thorium(IV) hydrolysis species have been measured at 15, 25, and 35 degrees C (in 1.0 mol dm(-3) NaClO4) using both potentiometry and solvent extraction. The results indicate the presence of the monomeric species Th(OH)(3+), Th(OH)(2)(2+), Th(OH)(3)(+), and Th(OH)(4), in addition to the polymeric species Th-4(OH)(8)(8+) and Th-6(OH)(15)(9+). The polymeric species were found to be important, although the total thorium concentration was limited to 0.01-0.1 mmol-dm(-3). The solvent extraction measurements required the use of acetylacetone. As such, the stability constants of thorium(IV) with acetylacetone were also measured using both potentiometry and solvent extraction. All logarithms of the stability constants were found to be linear functions of the reciprocal absolute temperature indicating that Delta R-0 and Delta S-0 of reaction are both independent of temperature (over the temperature range examined in the study).