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

在季铵离子单独离子对萃取EDTA的过程中，EDTA中带负电荷最大的物种EDTA^&lt；4-gt；被有效地萃取到有机溶剂中。对于Co(II)、Ni(II)、Cu(II)、Zn(II)、Cd(II)和Pb(II)络合物的萃取，提取了[M(EDTA)]^&lt；2-gt；和[(EDTA)(OH)]^&lt；3-&gt；除铅(II)外，[M(EDTA)]^&lt；2-&gt；的萃取常数随金属离子的变化差异不大，而[(EDTA)(OH)]^&lt；3-&gt；的萃取常数相差一个对数单位。[M(EDTA)]^&lt；2-&gt；与[(EDTA)(OH)]^&lt；3-&gt；的萃取常数相差不大。萃取常数与EDTA络合物的形成常数、金属离子的离子半径和水解常数之间没有显著的相关性。对于Fe(III)体系，二聚体物种[Fe_2(EDTA)_2(OH)_2]^&lt；4-&gt；随着[Fe(EDTA)]-在pH=7左右的水解而被萃取。对于Al(III)，In(III)和稀土(III)，[M(EDTA)]-和[M(EDTA)(OH)]^&lt；2-&gt；被萃取，但二聚体物种没有形成，也没有被萃取。Ga(III)-EDTA络合物通过Ga(III)水离子的水解而解离。Co(III)-EDTA与共存配体形成混配络合物，但没有形成二聚体，也没有被萃取。Fe(III)二聚体的形成和萃取是溶剂萃取的特有行为。这些结果表明，二价和三价金属的萃取分离是可能的。然而，二价金属之间的分离是相当困难的，需要使用色谱技术。对称的四烷基铵离子的萃取率很低，尽管它们具有很高的疏水性。这些结果表明，对于复杂结构的阴离子，在离子对的形成和提取过程中存在隐身效应。

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.