Separation of Metal Ions and Anions:

One of the largest uses of ion exchangers is their capability to separate metal ions. Ion exchangers show natural selectivity among cations but it can be enhanced by proper choice of aqueous medium, type of ion exchanger and eluting agent. The literature is full of examples where separations of topical interest are conveniently achieved.

The choice of different cation exchangers for the separation of different cations looks logical but it may be important to point out that almost equal number of separation of metals are achieved on anion exchangers. The metals on anion exchangers are not separated as cations but as anionic metal complexes. In this context, the best example is the separation of metals as anionic chlorocomplexes. The ease of formation of these complexes and their stability determine the selectivity on the anion exchangers. There is a full periodic table type chart available for the sorption behaviour of different metal ions on Dowex-1(strong anion exchanger) in the complete range of acidity of hydrochloric acid. The data given therein have been very useful in designing metal ion separations. As a typical example Co (II) and Ni(II) are separated from hydrochloric medium on a strongly basic anion exchanger (Dowex-1). The separation is based upon the fact that Co (II) but not Ni (II) forms an anionic chlorocomplex (probably CoCl₃^-) in 9M HCl presumably because of instability of the chlorocomplex. The retained Co (II) is washed from the column by water because the complex is decomposed and cobalt is recovered as cobaltous chloride.