Extractive Properties of Tri-n-octylphosphine Oxide Towards Thorium (IV) and Europium (III) Ions in Aqueous Nitrate Media

Phosphoryl containing ligand tri-n-octylphosphine oxide (TOPO) in dichloromethane has been used as an extracting agent for tetravalent thorium and trivalent europium ions from their 1 M sodium nitrates. The equilibrium data reveal that both ions are extracted as the complexes at 1:3 metal/ligand ratio. The values of conditional extraction equilibrium constant (Kex) show the superior extractive properties of the ligand towards thorium than europium ions. The effects of solvent, type and concentration of the  salting-out agent on the extraction abilities of the ligand are investigated. The influence of the temperature on the extraction of these ions from the sodium nitrate medium into 1,2-dichloroethane by TOPO is studied as a function of temperature in the range 20-40 ºC. The results show that the distribution ratios of thorium and europium ions decrease with the increase in temperature. Free energy (Gº), enthalpy (º) and entropy (Sº) changes associated with the extraction process are evaluated from the distribution measurements.