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 aq ueous solution to anion exchangers, when the anion of the supporting electr olyte serves as both the counterion of the exchanger and the metal ion comp lexing ligand, is a phenomenon of considerable interest. In such systems th e only species extracted by the exchanger, present in gel or Liquid form, i s the fully coordinated one even when this species is absent from the solut ion phase. The other potentially transferable species, electroneutral and n egatively 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 Zn2+ ion between Dowex 1-x4 a nd LiCl, KCI and CsCl, are duplicated by the liquid anion exchanger, 10% by weight methyldioctyl amine hydrochloride in trichloroethylene. The fact th at such duplication occurs has been attributed to the comparable hydrophobi city projected by their positively charged organic constituencies. This hyd rophobicity limits their water uptake similarly and leads to the elevated C l- ion concentration levels defined by the Donnan potential developed at eq uilibrium. At these elevated Cl- ion concentration levels, formation of the tetrahedrally coordinated ZnCl42- complex is highly favored while the wate r coordinated presence of any ZnCl2. 2H(2)O and ZnCl3-. H2O that may be tra nsferred to the exchanger phase, leads to their complete rejection. The dif ference between the Zn(II) distribution patterns obtained with LiCl, KCI an d CsCl is also examined. (C) 1998 Elsevier Science B.V. All rights reserved .