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The synergistic extraction of thorium(IV) and uranium(VI) from nitric acid solutions has been investigated using mixtures of 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI) and dicyclohexano-18-crown-6 (DC18C6), dibenzo-18-crown-6 (DB18C6) or benzo-15-crown-5 (B15C5). The results demonstrate that these metal ions are extracted into chloroform as Th(FBPI) 4  and UO 2 (FBPI) 2  with HFBPI alone and as  Th(FBPI) 4 ·CE and UO 2 (FBPI) 2 ·CE  in the presence of crown ethers (CE). The equilibrium constants of the above  extracted complexes have been deduced by non-linear regression analysis.  The addition of a crown ether to the metal chelate system enhances  the extraction efficiency and also improves the selectivities significantly  between thorium (IV) and uranium(VI).  The complexation strength of uranium(VI) with various crown ethers follows the order: DC18C6&gt;B15C5&gt;DB18C6, which can be explained on the basis of crown ether basicity sequence and steric effects. On the other hand, the complexation strength of Th-FBPI chelate with crown ether follows the order: B15C5&gt;DC18C6&gt;DB18C6. The high extraction of thorium(IV) with B15C5 can be explained on the basis of ‘size-fitting effect’. Solid complexes of these metals with 4-acyl-3-phenyl-5-isoxazolones and with mixtures of HFBPI and crown ether have been isolated and characterised by elemental analyses, IR and  1 HNMR spectroscopic techniques.

Crown ether as size selective synergist in the solvent extraction of thorium(IV) and uranium(VI) with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone