Presence of Salting Out Agents:

Spectacular results in the enhancement of the extraction by solvating extractant systems by the addition of metal salts have already been discussed earlier. It was pointed out that the addition of inorganic salts increases the distribution of metal in favour of the organic phase. The salting out effect is explained in part by the effect on the activity of the distributing species and strong ability of these ions to bind water thereby depleting aqueous phase of the water molecules to compete. The magnitude of enhancement in extraction by the added salt depends upon the charge and ionic size of the cation for a given anion.

If you go from this yardstick, polyvalent cations provide a better salting out agents and for a given charge, the smaller the size, the greater the effect on extraction. Some anomalies may be observed due to specific interactions. If you look at the efficiency in enhancing the extraction, aluminium and ferric salts are stronger salting out agents than ammonium salts but analytically, the latter are more convenient because it is easier to remove them in the aqueous phase by repeated evaporation with HNO₃ and HCl. Generally, large amounts of these salts are added. One has to make sure that the added salt does not extract in the organic phase to an appreciable extent. It is obvious that the presence of large concentrations of the salt in the aqueous phase may present problems in further processing unless easily removed or destroyed. Another point that one has to keep in mind is that although the enhancement in the extraction of the metal of interest occurs, it may also cause an enhancement in the extraction of impurities. Therefore, it is necessary to choose an agent that produces favourable separation factor between the element of interest and the impurities.