Multiple Extractions with Successive Portions:
We have seen the importance of batch extraction because it is most widely used for analytical separations. In this procedure, a given volume of solution is brought in contact with a solvent until equilibrium is attained. Even if we suppose that none of the interfering substances is extracted, the completeness of the extraction is a very important factor. It has already been pointed out that the distribution ratio is a concentration ratio and, therefore, the actual fraction of the solute extracted will depend upon the ratio of volumes of the two solvents competing for the solute.
Let v1 mL of solution (phase 1) containing w g of solute is contacted to equilibrium with v2 mL of another solvent (phase 2) immiscible with the first. Let w1 is weight of the solute remaining in the phase 1 after equilibrium is attained.
Now concentration in phase 1 (C1) = w1/v1 g mL-1
And concentration in phase 2 (C2) = w-w1/v2 g mL-1
Therefore, D = (w - w1 /v2)/w1 /v1
And, w1= w ( v1 / Dv2 + v1 )
If we again extract phase 1 with another portion v2 mL of solvent, w2 g will be the weight of the solute remaining in the phase 1. After this extraction,
W2= w1 (v1 / Dv2 + v1)
Therefore, W2= w ( v1 / Dv2 + v1 )2
If the same volume of the extracting solvent is used for each successive extraction and wn g is the weight of the solute remaining in phase 1, after n extractions wn will be given by the expression
Wn= w ( v1 / Dv2 + v1 )n
The fraction remaining in phase 1after n extractions = f1 = Wn/ w (v1/ Dv2 + v1)n
And, the fraction transferred to phase 2 = f2 =1 - f1
If phase 1 is aqueous and phase 2 is organic, then
3. Extraction by salvation.
4. Extraction by ion pair formation.
A special mention is being made of crown ethers as extractants.
This involves the distribution of simple covalent molecules like GeCl4, HgBr2 and AsBr3 between aqueous phase and inert organic solvent.