Extraction by Compound Formation:

The reagents which fall under this category are called liquid cation exchangers. These extractants operate, at least, formally by the exchange of the H⁺ ions of the acidic organic compound with the cation present in the aqueous phase

M ^{n +} _{( a )}  + n HA_(o)     ↔MA_n(o) + n H ^{n +} _{( a )}

The process is similar to solid-liquid ion exchange in which HA is equivalent to an ion exchange resin. Although there is some similarity between the reagents of this class and solid ion exchangers, very important differences exist as regards their interaction with the metal ions. These cation exchange extraction systems can be broadly classified under the following three categories.

i)          Extraction by chelating agents.

ii)        Extraction by acidic organophosphorus esters.

iii)       Extraction by carboxylic and sulphonic acids.

There are some important differences between extraction processes involving chelates on one side and phosphorus and carboxylic acids on the other. The acids forming chelates are usually weak and more soluble in organic than in inorganic solvents. The composition of the chelates is usually unaffected by the nature of the diluents. The extraction mechanism in these cases is well described by simple equation of Law of Mass Action. In several classification systems, the chelates forming system, forms a separate class by itself.