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A new calix [4]resorcinarene bearing eight hydroxamic acid groups (C4RAHA) has been synthesized and characterized by FT-IR, 1 H-NMR and elemental analysis. Its analytical properties were investigated, and showed high affinity and selectivity toward thorium(IV) in the presence of large quantities of associated metal ions. Thorium(IV) was extracted from an ethyl acetate solution of C4RAHA at pH 4.5. The λmax and molar absorptivity (ε) for thorium(IV) were 341 nm and 7120 l mol -1 cm -1 . The complexation of thorium(IV) with C4RAHA has a 4:1 metal:ligand stoichiometry, which is relatively rare. The extracts were directly aspirated in the plasma for ICP-AES measurements for thorium(IV) in the presence of a diversified matrix. The system obeyed Beer's law over the range 0.1 -6.5 µg ml -1 of thorium(IV) with a Sandell sensitivity of 0.0325 µg cm -2 . The preconcentration factor and overall stability constant evaluated at 25˚C for thorium(IV) were 133 and 15.86, respectively. The complexation was characterized by a favorable enthalpy change. A liquid-membrane transport study of thorium(IV) was carried out from the source to the receiving phase under controlled conditions, and a mechanism of transport proposed. To check the validity of the proposed method, thorium(IV) was determined in monazite sand, rare earth sand and water samples.

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