Department of Chemistry, Faculty of Science, Zanjan University, P. O. Box 45195-313 Zanjan, I.R. IRAN
The extraction-separation of lanthanum and europium ions with bis(2-ethylhexyl) phosphoric acid (DEHPA) in cyclohexane was improved by adding 12-crown-4 (12C4) to the aqueous phase as a masking agent. In the presence of this crown ether the extraction curve of lanthanum ions versus pH shifts to the higher pH region, while the curve is not influenced for the europium ions. Consequently the separation of these ions is enhanced. The treatment of the extraction data in the presence of 12C4 allows to determine the stability constant of the lanthanum-crown ether complex in the aqueous phase. The results have been compared with those reported using 18-crown-6 (18C6). This comparison shows that the improvement in the separation of La/Eu in the presence of 12C4 is the same order as in the presence of 18C6. The influence of temperature on the extraction of the studied cations in the absence and the presence of 12C4 by DEHPA in cyclohexane is studied in the range 293-305 K. This study leads to assess the thermodynamic parameters i.e. the free energy (DGº), enthalpy (DHº) and entropy (DSº) changes associated with the extractionprocess as wellas those of the complexation oflanthanum ions by12C4 in aqueous phase.
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