High extraction power:

The high extraction power of neutral organophosphorus esters has been demonstrated for a large number of metal salts. The nitrates and chlorides have received the most attention. The extraction from sulphates and phosphate solution is generally poor. The extraction of metals from halide solutions other than chlorides follows closely that of the latter. The extraction by these reagents involves the solvation of the electrically neutral metal salts formed by their depressed ionization in the aqueous solution. The reagent functions satisfactorily in the presence of high concentration of salting out electrolytes in the aqueous phase. Thus, the extent of extraction will depend on the degree of formation of the extractable species and the solvation number of the metal. There is a strong competition between the extractants molecules and water to occupy the coordination sites of the metal ion. This will depend upon the type and the structure of the neutral organophosphorus extractants. Nearly all metal extractions are increased as the number of direct C - P linkages in the series increase, i.e., in phosphate < phosphonate < phosphinate < phosphine oxide. The salting out effect observed in the case of these extractants is more or less on similar lines as with solvating molecule with carbon oxygen bond. A number of salting out agents has been investigated for their capacity to draw the metal to the organic phase. The extraction increases as the radius of the cation of the salting out agent decreases in the alkali and alkaline earth series. The extraction increases with the increasing charge on the cation.