fl-Diketones are among the most versatile and commonly employed chelating agents in solvent extraction of lanthanides and actinides. In connection with our studies of uranyl ion employing fl-diketones ~-3, it was desirable to know the formation constants of their complexes to assess their relative ease of solvent extraction. Thermodynamic data pertaining to UO 2 + fi-diketone systems are rather rare except for a few instances 4 s.
The present paper deals with the potentiometric determination of the stepwise thermodynamic formation constants of UO 2+ ions with six fi-diketones and two monothio/3-diketones in a 50Yo (v/v) dioxane-water mixture using the Bjerrum-Calvin pH titration technique. The thermodynamic acid dissociation constants (rpKa) of the fl-diketones employed in the present investigation have been determined under similar experimental conditions 9. The low water-solubilities of these reagents and their complexes necessitated the use of a mixed dioxane-water solvent. Some of the factors affecting the metal-ligand stability constants are briefly discussed. EXPERIMENTAL Laboratory reagent grade 2-thenoyltrifluoroacetone (TTA), benzoylacetone (BA) from BDH and trifluoroacetylacetone (TFAA) from Columbia Organic Chemicals were used. Benzoyltrifluoroacetone (BTFA) from Peninsular Chemical Research Inc., USA, dibenzoylmethane (DBM) from Koch Light Laboratories Inc., UK and acetylacetone (AcAc) from Riedel-De Haen, AG, Hanover were used as supplied. Monothio derivatives of TTA (STTA) and of DBM (SDBM) were prepared by the method of Livingstone et al. 1°. Purity of the reagents was checked by elemental analyses, m.p., u.v. and i.r. spectra. BDH p-dioxane was purified by the procedure recommended by Weissberger and Proskauer 1 ~. Pure distilled water, redistilled over alkaline permanganate and freed from carbon dioxide, was used throughout this investigation. An aqueous solution (10Yo) of tetraethylammonium hydroxide titrant was procured from Hopkin and Williams Ltd. It was standardised against analytical grade potassium hydrogen phthalate and diluted to 0.1 M, the proportions of dioxane and water being 1 : 1.
Uranyl perchlorate was prepared by the reaction of perchloric acid with orange uranium trioxide. After crystallisation from aqueous acidic solutions,