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

在通过四烷基季铵离子单独萃取EDTA的离子对中，EDTA的<4->带最多负电荷的物质edta 2被有效地萃取到有机溶剂中。在萃取Co（II）、Ni（II）、Cu（II）、Zn（II）、Cd（II）和Pb（II）配合物的过程中，主要萃取了[M（edta）]^<2->和[（edta）（OH）]^<3->。除Pb（II）外，[M（edta）]^的萃取常数<2->随金属离子浓度的变化无明显差异，而[（edta）（OH）]^的萃取常数随金属离子浓度的变化仅<3->相差一个对数单位。[M（edta）]^<2->和[（edta）（OH）]^之间的萃取常数差异<3->很小。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均无显著相关性。对于Fe（III）体系，在pH约为7的条件下，二聚体[Fe（edta）_2（OH）_2]^<4->与[Fe（edta）]-的水解一起被沿着萃取。对于Al（III）、In（III）和镧系元素（III），[M（edta）]-和[M（edta）（OH）]-<2->被萃取，但二聚物物种未形成且未被萃取。Ga（III）-EDTA络合物是通过Ga（III）水溶液的水解而解离的。Co（III）-EDTA与共存配体形成混合配体络合物，但不形成二聚体物质且不被萃取。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，二价金属离子的分离是相当困难的，并且需要使用色谱技术。对称的四烷基铵离子的萃取率相当低，而与它们的高疏水性无关。这些结果表明，络合结构阴离子在离子对的形成和萃取过程中存在空间效应。

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.