Selective Stripping from the Organic Phase:
The major difference is that out of the different metal ions extracted in the organic phase, the stripping agent selectively strips the metal ion of interest there. The desired metal ion can be stripped in a subsequent step. The stripping agents are the same as already cited in the sub-Sec. 3.35. Again, the examples are too many and only a few can be cited here.
Fe (III) and Ga (III) are extracted in diisopropyl ether from 6 M HCl. Iron can be selectively stripped from the organic layer by washing it with hydroxylamine hydrochloride dissolved in 6 M HCl. This is because Fe (II) formed shows negligible extraction in ether from HCl medium. Ga (III) remaining in the organic layer is back washed by using < 1 M HCl.
Ce (IV) and Th (IV) are extracted in TBP from nitrate medium. Ce (IV) can be back extracted from the organic phase by washing it with a reducing agent like hydroxylamine because Ce (III) is not extracted under those conditions.
If we refer to the three examples of separations, viz. Ti (IV) - V (IV), Fe (III) - Zn (II) and Fe (III) - Cd (II) given in Tables 3.1 (a) and (b), these separations are based on selective stripping. In each case, both the metal ions are extracted by Cyanex 923 and this is followed by selective stripping. Before deciding upon the reagents for selective stripping, complete information on the extraction behaviour of the metal ions under consideration from different aqueous media is required. In the Ti (IV) - V (IV) case, the latter is selectively back extracted by 10 M H2SO4. In the other two cases, Zn (II) or Cd (II) are selectively back extracted with water followed by stripping of Fe (III) by 0.1 M H2SO4.