Page 191 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

FACTORS FAVOURUIG SOLVENT EXTRACTION 6.2

Hence:

The concentration in the aqueous layer after three extractions with 8.33 mL of
carbon tetrachloride is given by:

The extraction may, therefore, be regarded as virtually complete.
If we confine Our attention to the distribution of a solute A between water
and an organic solvent, we may write the percentage extraction E, as:

where K and Vw represent the volumes of the organic and aqueous phases
respectively. Thus, the percentage of extraction varies with the volume ratio of
the two phases and the distribution coefficient.
If the solution contains two solutes A and B it often happens that under the
conditions favouring the complete extraction of A, some B is extracted as well.
The effectiveness of separation increases with the magnitude of the separation
coefficient or factor fi, which is related to the individual distribution ratios as
follows:

If DA = 10 and DB = 0.1, a single extraction will remove 90.9 per cent of A and
9.1 per cent of B (ratio 10: 1); a second extraction of the same aqueous phase
will bring the total amount of A extracted up to 99.2 per cent, but increases
that of B to 17.4 per cent (ratio 5.7: 1). More complete extraction of A thus
involves an increased contamination by B. Clearly, when one of the distribution
ratios is relatively large and the other very small, almost complete separation
can be quickly and easily achieved. If the separation factor is large but the
smaller distribution ratio is of sufficient magnitude that extraction of both
components occurs, it is necessary to resort to special techniques to suppress
the extraction of the unwanted component.

6.2 FACTORS FAVOURING SOLVENT EXTRACTION
It is well known that hydrated inorganic salts tend to be more soluble in water
than in organic solvents such as benzene, chloroform, etc., whereas organic
substances tend to be more soluble in organic solvents than in water unless
they incorporate a sufficient number of hydroxyl, sulphonic, or other hydrophilic
groupings. In solvent extraction analysis of metals we are concerned with
methods by which the water solubility of inorganic cations may be masked by
interaction with appropriate (largely organic) reagents; this will in effect remove
some or al1 of the water molecules associated with the metal ion to which the
water solubility is due.
Ionic compounds would not be expected to extract into organic solvents