All separation techniques involve one or more chemical equilibria, consequently the degree of
separation achieved can vary greatly according to experimental conditions. To a large extent, attainment
of optimum conditions has to be approached empirically rather than by application of rigid theory. In
the following sections, which deal with solvent and solidphase extraction, chromatography and
electrophoresis, the minimum theory necessary for an understanding of the basic principles is
presented.
4.1—
Solvent Extraction
Summary
Principles
Selective transfer of material in microgram to gram quantities between two immiscible liquid phases;
separations based on solubility differences; selectivity achieved by pH control and complexation.
Apparatus and Instrumentation
Separating funnels for batch extraction; special glass apparatus for continuous extraction; automatic
shakers used for discontinuous counter-current distribution.
Applications
Concentration and determination of metals as trace and minor constituents; organic materials separated
or concentrated according to type. Batch methods are rapid, simple and versatile; applicable to very
wide range of samples and concentrations.
Disadvantages
Sometimes requires large quantities of organic solvents; poor resolution of mixtures of organic
materials except by counter-current distribution which is slow.
Solvent extraction, sometimes called liquid-liquid extraction, involves the selective transfer of a
substance from one liquid phase to another. Usually, an aqueous solution of the sample is extracted with
an immiscible organic solvent. For example, if an aqueous solution of iodine and sodium chloride is
shaken with carbon tetrachloride, and the liquids allowed to separate, most of the iodine will be
transferred to the carbon tetrachloride layer, whilst the sodium chloride will remain in the aqueous
layer. The extraction of a solute in this manner is governed by the Nernst partition or distribution law
which states that at equilibrium, a given solute will always be distributed between two essentially
immiscible liquids in the same proportions. Thus, for solute A distributing between an aqueous and an
organic solvent,