Page 189 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 6

SOLVENT EXTRACTION

6.1 GENERAL DISCUSSION
Liquid-liquid extraction is a technique in which a solution (usually aqueous)
is brought into contact with a second solvent (usually organic), essentially
immiscible with the first, in order to bring about a transfer of one or more
solutes into the second solvent. The separations that can be performed are
simple, clean, rapid, and convenient. In many cases separation may be effected
by shaking in a separatory funnel for a few minutes. The technique is equally
applicable to trace level and large amounts of materials.
In the case of inorganic solutes we are concerned largely with samples in
aqueous solution so that it is necessary to produce substances, such as neutral
metal chelates and ion-association complexes, which are capable of extraction
into organic solvents. For organic solutes, however, the extraction system
may sometimes involve two immiscible organic solvents rather than the
aqueous-organic type of extraction.
The technique of liquid-liquid extraction has, of course, been widely used
to separate the components of organic systems; in particular, solvent extraction
may be employed to effect a 'clean-up' and to achieve concentration of the
solutes of interest prior to analysis. This is illustrated by the clean-up steps
which have been used for the analysis of organochlorine pesticides, at the
0.1 pg g-1 level, in animal fats and dairy products. In one such procedure the
solution of animal fat in hexane (25 mL) is extracted with three successive
portions (10 mL) of dimethylformamide (DMF) saturated with hexane. The
combined DMF extracts are washed with hexane (10 mL) saturated with DMF
to remove any remaining traces of fat; after separation of the hexane it is
equilibrated with further DMF (10 mL) to reduce loss of pesticide residue. The
pesticide compounds are finally partitioned back into clean hexane after adding
water to the combined DMF extracts. The hexane layer is used for the
quantitative analysis of the extracted pesticide residues using gas chromatography
with an electron-capture detector [Section 9.2(4)]. It is of interest to note that
the extraction procedure involves two organic phases (i.e. no aqueous phase is
involved) and that miscibility is minimised by saturating each solvent with
the other. Extraction procedures for organic species, however, do not in general
possess the same degree of selectivity as may be achieved for metal-containing
systems, and the chief analytical application of solvent extraction is for the
determination of metals as minor and trace constituents in various inorganic
and organic materials.
Although solvent extraction has been used predominantly for the isolation