Extraction by Ion Pair Formation:
This primarily involves the extraction of a species formed due to an interaction between an anionic metal species in the aqueous phase and the cation furnished by an organic base. The strongly basic high molecular weight amines (HMWA) are able to extract the metals by forming the ion pairs with anionic metal complexes. The other extractable pairs formed by quaternary phosphonium, arsonium, phenylonium bases are also included under this category. Some of the examples of different types are R3NH+FeCl4-, (R4N+)2.ZnCl 42-. (R3NH+)2. (Cd(C2O4)2) and (C6H5)4As+.MnO4 - .
Under this class of extractants, high molecular weight amines have received maximum attention.
R3N(o) + H+(a) + A -(a) ↔ R3NH+A-(o) (salt formation)
R3NH+A -(o)+ B... ↔ R3NH+B(o)-+A-(a ) (anion exchange).
The amine salt can undergo anion exchange with anion (B-) in the aqueous phase, B- can be simple or complex anion like Cl-, CrO42- or an anionic metal complex like FeCl4- , HgCl42- and ZnCl42- . The order of preference in the organic amine solution is similar to that anion-exchange resins, Cl- > NO3 - > HSO4- >F-.
The amine salt on treatment with sodium or ammonium hydroxide can be reverted to amine. The popularity of these extractants probably stems from the fact that the extraction is mainly through ion exchange and thus, they combine some of the advantages of both solvent extraction and ion exchange. They are popularly known as liquid anion exchangers.