Interpretation of the unique complexation behavior of transition metal ions in solid and liquid anion exchangers

The transfer of trace level concentrations of transition metal ions from aqueous solution to anion exchangers, when the anion of the supporting electrolyte serves as both the counterion of the exchanger and the metal ion complexing ligand, is a phenomenon of considerable interest. In such systems the only species extracted by the exchanger, present in gel or liquid form, is the fully coordinated one even when this species is absent from the solution phase. The other potentially transferable species, electroneutral and negatively charged, that are present in the aqueous medium do not appear in the exchanger phase. Examples of such behavior, available in studies of the distribution of trace level concentration of Zn 2+ ion between Dowex 1-x4 and LiCl, HCl and CsCl, are duplicated by the liquid anion exchanger, 10% by weight methyldioctyl amine hydrochloride in trichloroethylene. The fact that such duplication occurs has been attributed to the comparable hydrophobicity projected by their positively charged organic constituencies. This hydrophobicity limits their water uptake similarly and leads to the elevated Cl − ion concentration levels defined by the Donnan potential developed at equilibrium. At these elevated Cl − ion concentration levels, formation of the tetrahedrally coordinated ZnCl 4 2− complex is highly favored while the water coordinated presence of any ZnCl 2 ·2H 2 O and ZnCl 3 − ·H 2 O that may be transferred to the exchanger phase, leads to their complete rejection. The difference between the Zn(II) distribution patterns obtained with LiCl, HCl and CsCl is also examined.