Extraction by Synergism:

The word synergism literally means working together. It is a matter of fact that the two extractants work together to increase the extraction by a huge factor. This word in solvent extraction was introduced by Blake et al in 1958. They observed that when a dialkylhydrogen phosphate (RO)₂PO.OH is used in conjunction with certain organophosphorus solvating molecule like TBP; the extracting power of the mixture exceeds the sum of the extracting power of the individual extracting reagents. In synergism, one of the two extractants is a ligand or an organic acid (HA) which neutralizes the charge on the metal ion and the other a solvating molecule (S)  which either replaces the water molecule from the coordination sphere or forms an adduct that is less hydrophilic. One of the main thoroughly investigated synergistic mixtures of extractants is the theonlytrifluroacetone (HTTA) and tri-n-butylphosphate (TBP). In its simplest form, the equilibria can be written as

Mⁿ⁺_a  + n HA_o  ↔ MA_no + n H⁺

MA_no   + m Bo ↔ MA_n B_mo                                                                                   

Here, HA = HTTA, B = TBP

HTTA has been used with different solvating molecules like TBP, TOPO and TPP (triphenyl phosphate). That was observed in which the synergistic effect increases with the increasing basicity of the ester TOPO > TBP > TPP. The extracting species of some actinides with the above systems were reported as UO₂(TTA)₂S, Am(TTA)₃S₂, UO₂(TTA)₂S₂, Th(TTA)₄S, Cm(TTA)₃S₂.