Description of partition equilibria for uranyl nitrate, nitric acid and water extracted by tributyl phosphate in dodecane

Experiments were carried out on the partitioning of uranyl nitrate in nitric acid solution extracted by 30 vol.% of tributyl phosphate (TBP) in dodecane at 25 °C. The model proposed by Naganawa and Tachimori (Bull. Chem. Soc. Jap. 70, 809, (1997)) for the case of aqueous solutions of nitric acid was used to additionally describe the extraction equilibria of uranyl nitrate. The treatment is based on the assumption of thermodynamic ideality for the organic solution composed of the diluent, the free extractant and the complexes. The stoichiometries of the metal complexes were determined one by one by a suitable procedure. The formation of 6 different complexes involving uranyl nitrate was found capable of representing the partition equilibria for the three extracted compounds: nitric acid, water and uranyl nitrate. This approach suggests the formation of 2:1 and 3:1 complexes for TBP and uranyl nitrate, respectively.