Page 201 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SOME PRACTICAL CONSIDERATIONS 6.7

1. A high distribution ratio for the solute and a low distribution ratio for
undesirable impurities.
2. Low solubility in the aqueous phase.
3. Sufficiently low viscosity and sufficient density difference from the aqueous
phase to avoid the formation of emulsions.
4. Low toxicity and flammability.
5. Ease of recovery of solute from the solvent for subsequent analytical
processing. Thus the b.p. of the solvent and the ease of stripping by chemical
reagents merit attention when a choice is possible.

Sometimes mixed solvents may be used to improve the above properties.
Salting-out agents may also improve extractability.

Extraction. Extraction may be accomplished in either a batch operation or a
continuous operation. Batch extraction, the simplest and most widely used
method, is employed where a large distribution ratio for the desired separation
is readily obtainable. A small number of batch extractions readily remove the
desired component completely and may be carried out in a simple separatory
funnel. The two layers are shaken in the separatory funnel until equilibrium is
attained, after which they are allowed to settle completely before separating. The
extraction and separation should be performed at constant temperature, since
the distribution ratio as well as the volumes of the solvent are influenced by
temperature changes. It must be borne in mind that too violent agitation of the
extraction mixture often serves no useful purpose: simple repeated inversions
of the vesse1 suffice to give equilibrium in a relatively few inversions. If droplets
of aqueous phase are entrained in the organic extract it is possible to remove
them by filtering the extract through a dry filter paper: the filter paper should
be washed several times with fresh organic solvent.
When the distribution ratio is low, continuous methods of extraction are
used. This procedure makes use of a continuous flow of immiscible solvent
through the solution; if the solvent is volatile, it is recycled by distillation and
condensation and is dispersed in the aqueous phase by means of a sintered glass
disc or equivalent device. Apparatus is available for effecting such continuous
extractions with automatic return of the volatilised solvent (see the Bibliography,
Section 9.10).

Stripping. Stripping is the removal of the extracted solute from the organic
phase for further preparation for the detailed analysis. In many analytical
procedures involving an extraction process, however, the concentration of the
desired solute is determined directly in the organic phase.
Where other methods of analysis are to be employed, or where further
separation steps are necessary, the solute must be removed from the organic
phase to a more suitable medium. If the organic solvent is volatile (e.g. diethyl
ether) the simplest procedure is to add a small volume of water and evaporate
the solvent on a water bath; care should be taken to avoid loss of a volatile
solute during the evaporation. Sometimes adjustment of the pH of the solution,
change in valence state, or the use of competitive water-soluble complexing
reagents may be employed to prevent loss of the solute. When the extracting
solvent is non-volatile the solute is removed from the solvent by chemical means,
e.g. by shaking the solvent with a volume of water containing acids or other