The Thermodynamic Study on the Solvent Extraction Behavior of EDTA and its Complexes

在单用季四烷基铵离子对萃取EDTA时，EDTA中负电荷最多的EDTA ^<4->被有效地萃取到有机溶剂中。这种现象是由于大量的四烷基铵离子形成的离子对疏水性增强所致。萃取Co（II）、Ni（II）、Cu（II）、Zn（II）、Cd（II）和Pb（II）配合物时，萃取出[M(edta)]^<2->和[(edta)(OH)]^<3->。除Pb（II）外，[M(edta)]^<2->的萃取常数随金属离子的变化变化不大，而[(edta)(OH)]^<3->的萃取常数随金属离子的变化变化一个对数单位。[M(edta)]^<2->与[(edta)(OH)]^<3->的萃取常数相差不大。萃取常数与EDTA配合物的形成常数、金属离子的离子半径以及水解常数之间没有显著的相关性。对于Fe（III）体系，随着[Fe(edta)]-的水解，[Fe_2(edta)_2(OH)_2]^<4->被萃取。对于Al（III）、In（III）和镧系元素（III），[M(edta)]和[M(edta)(OH)]^<2->被提取，但未形成二聚体，未被提取。Ga(III)-EDTA配合物通过Ga（III）的水离子水解解离。Co(III)-EDTA与共存配体形成混合配体配合物，但没有形成二聚体，也没有被萃取。二聚体Fe（III）的形成和萃取是溶剂萃取的特定行为。这些结果表明，二价和三价金属的萃取分离是可能的。然而，二价金属的相互分离是相当困难的，需要使用色谱技术。对称四烷基铵离子的提取率很低，尽管它们具有很高的疏水性。这些结果表明，络合结构阴离子对离子对的形成和萃取存在张力效应。

In the ion-pair extraction of EDTA alone by quaternary tetraalkylammonium ion, edta^<4->, the most negatively charged species of EDTA, is effectively extracted into organic solvents. This phenomenon is due to the increased hydrophobicity of the ion-pair formed with the large numbers of tetraalkylammonium ion.For the extraction of Co(II),Ni(II),Cu(II), Zn(II),Cd(II),and Pb(II) complexes, the species of [M(edta)]^<2-> and [(edta)(OH)]^<3-> were extracted. The extraction constants for [M(edta)]^<2-> did not differ so much with the change of metal ion except for Pb(II), while of [(edta)(OH)]^<3-> differ with one logarithmic unit. The difference of the extraction constants between [M(edta)]^<2-> and [(edta)(OH)]^<3-> was small. There is no significant correlation between the extraction constants and the formation constants of EDTA complexes, ionic radius of metal ions, nor the hydrolysis constants. For Fe(III) system, the dimeric species, [Fe_2(edta)_2(OH)_2]^<4->, is extracted along with the hydrolysis of [Fe(edta)]-at about pH 7. For Al(III),In(III),and lanthanide(III),[M(edta)]-and [M(edta)(OH)]^<2-> were extracted but the dimeric species was not formed and not extracted. Ga(III)-EDTA complex was dissociated by the hydrolysis of aqua ion of Ga(III). Co(III)-EDTA forms mixed-ligand complex with co-existing ligand, but the dimeric species was not formed and not extracted. The formation and the extraction of dimeric species of Fe(III) is specific behavior for the solvent extraction. These results suggest that the extraction separation of divalent and trivalent metals is possible. The separation of divalent metals each other is, however, quite difficult and the use of chromatographic technique is required.The extractions with symmetric tetraalkylammonium ions are quite low irrespective of their high hydrophobicity. These results suggest the steic effect on the formation and extraction of ion-pair for the anion of complex structure.