Page 200 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION

Tri-n-octylphosphine oxide, (n-CsHl7),PO. This compound (TOPO) dissolved
in cyclohexane (0.1M) is an excellent extraction solvent. Thus the distribution
ratio of U(V1) is of the order of IO5 times greater for TOPO than for
tri-n-butyl phosphate. The following elements are completely extracted from
1 M-hydrochloric acid: Cr(V1) as H2Cr207,2TOPO; Zr(1V) as ZrC14,2TOPO;
Ti(1V); U(V1) as U02(N0,)2,2TOPO; Fe(II1); Mo(V1) and Sn(1V). If the
hydrochloric acid concentration is increased to 7M, Sb(III), Ga(II1) and V(1V)
are completely extracted.
Poly (macrocyclic) compounds. The analytical application of compounds such
as crown polyethers and cryptands is based on their ability to function as
ligands and form stable stoichiometric complexes with certain cations. Special
importance is due to their preference for alkali metal ions which do not form
complexes with many other ligands. A number of these compounds are
commercially available and their properties and analytical applications have
been described by Cheng et al."
Cetyltrimethylammonium bromide (CTMB). Quaternary ammonium salts
having one or more long-chain alkyl groups (e.g. CTMB and the corresponding
chloride) have important applications as analytical reagents. These applications
are mainly based on the ability of the quaternary ammonium ion to act (a) as
a cationic reagent for the ion-pair extraction of metals as complex anions and
(b) as a cationic micelle-forming reagent for photometric determination of
metals.

In the latter function, the reagent behaves as a surfactant and forms a cationic
micelle at a concentration above the critical micelle concentration (1 x 10-4M
for CTMB). The complexation reactions occurring on the surface of the micelles
differ from those in simple aqueous solution and result in the formation of a
complex of higher ligand to metal ratio than in the simple aqueous system; this
effect is usually accompanied by a substantial increase in molar absorptivity of
the metal complex.
CTMB is commercially available as a colourless hygroscopic powder, readily
soluble in water, alcohol and acetone. The aqueous solution foams strongly
when shaken.

6.7 SOME PRACTICAL CONSIDERATIONS
Solvent extraction is generally employed in analysis to separate a solute (or
solutes) of interest from substances which interfere in the ultimate quantitative
analysis of the material; sometimes the interfering solutes are extracted
selectively. Solvent extraction is also used to concentrate a species which in
aqueous solution is too dilute to be analysed.
The choice of solvent for extraction is governed by the following considerations.