Modifiers:

One of the very serious problems in solvent extraction is the formation of a third phase i.e., two organic phases. Most of the available evidence on third phase formation suggests that it is essentially a solubility problem. Invariably, it is the organic phase that splits into two phases to yield a metal complex-rich phase at the aqueous interface and a diluent rich phase above. In most of the cases, the problem is overcome by the addition of a third component known as a modifier. This helps to restore the organic phases to one. Such modifiers are invariably solvating agents like isodecanol, 2- ethylhexanol, p-nonyl phenol and TBP.

The third phases are of a density intermediate between that of the solvent phase and the aqueous phase and result from the effects of the diluent on the heterogenous equilibrium associated with the system. Based on several observations, some conclusion can be drawn on the third phase formation.

i) The formation of third phase is temperature dependent. Usually increase in temperature will result in the disappearance of third phase.

ii) The appearance of the third phase is more common with aliphatic diluents.

iii) In the case of high molecular weight amine extractants, the tendency for third phase formation with different anions follows the order NO^-₃ < Cl ^- < SO₄^2-. A similar sequence is observed with DEHPA.