Selective Extraction and Separation of Thorium from Monazite Using N-Phenylbenzo-18-crown-6-hydroxamic Acid

Abstract.

 A method for the liquid–liquid extraction, separation, preconcentration and trace determination of thorium with N-phenylbenzo-18-crown-6-hydroxamic acid (PBCHA) is described. Thorium is extracted from the dichloromethane solution of PBCHA at pH 4.5 which gives a colourless extract (λ_max 375 nm, ε=1.5×10⁴ L mol⁻¹ cm⁻¹, linear range following Beer’s law 0.77–18.48 µg mL⁻¹). The extract is directly inserted into the plasma for ICP-AES measurements of thorium. A linear calibration graph was obtained between 10 and 130 ng mL⁻¹ of thorium. The proposed method has been applied for the separation and determination of thorium in the presence of several ions in monazite sand, rare earths, sand and sea water samples. The thorium-PBCHA complexes are very stable, having the formation constant (log β₂) 13.45 compared to lanthanum (log β₂) 2.95, cerium (log β₂) 2.95 and uranium (log β₂) 3.80. Hence, thorium can easily be separated and determined in monazite sand.