Acidic Organophosphorus Compounds:
Alkylphosphoric, phosphonic and phosphinic acids fall under this category. They also extract the metals by a cation exchange reaction. As mentioned earlier, the experimental data of these extractions do not necessarily fit into the mass equation of exchange equilibrium. The organophosphorus acids have pronounced tendency of association into dimers and larger aggregates. The other problem arises due to solvation of the metal extractant complex by one more extractant. The extraction of metal by dimers of the extractant can be represented as
Mm+ ( a )+ m(HX) 2(o) ↔M(HX)m(o ) + mH +(a)
This will mean that the extraction is proportional to the m-th power of dimer concentration and inversely proportional to the m-th power of the proton concentration, where m is the charge on the metal ion.
The type of alkyl or aryl group of the organophosphates will influence the acidic nature of the reagent. Increase in the chain length or branching of the substituent will decrease the acid strength. Although the main mode of action of these extractants is that of cation or hydrogen exchange, there are numerous cases where the coordination of the phosphoryl oxygen occurs. This results in the formation of chelation products mainly polymers. Under these circumstances, the substituents on the phosphorus atom play an important part which frequently, is contrary to their role in hydrogen exchange mechanism.