Page 197 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

EXTRACTION REACENTS 6.6

functional group in these molecules is the semipolar phosphoryl group,
IP'o-, which has a basic oxygen atom with good steric availability. A
/
typical compound is tri-n-butyl phosphate (TBP), which has been widely used
in solvent extraction on both the laboratory and industrial scale; of particular
note is the use of TBP for the extraction of uranyl nitrate and its separation
from fission products.
The mode of extraction in these 'oxonium' systems may be illustrated by
considering the ether extraction of iron(II1) from strong hydrochloric acid
solution. In the aqueous phase chloride ions replace the water molecules
coordinated to the Fe3+ ion, yielding the tetrahedral FeC14 ion. It is recognised
that the hydrated hydronium ion, H30+ (H20), or H,O;, normally pairs with
the complex halo-anions, but in the presence of the organic solvent, solvent
molecules enter the aqueous phase and compete with water for positions in the
solvation shell of the proton. On this basis the primary species extracted into
the ether (R20) phase is considered to be [H30(R20):, FeCl;] although
aggregation of this species may occur in solvents of low dielectric constant.
The principle of ion-pair formation has long been used for the extraction of
many metal ions, but not the alkali metals, due to the lack of complexing agents
forming stable complexes with them. A significant development of recent years,
however, has been the application of the so-called 'crown ethers' which form
stable complexes with a number of metal ions, particularly the alkali metal ions.
These crown ethers are macrocyclic compounds containing 9-60 atoms,
including 3-20 oxygen atoms, in the ring. Complexation is considered to
result mainly from electrostatic ion-dipole attraction between the metal ion
situated in the cavity of the ring and the oxygen atoms surrounding it. The
ion-pair extraction of Na+, K+ and Ca2+ with some organic counter-ions
and dicyclohexyl-18-crown-6 as complex-forming reagent has been described.1°

6.6 EXTRACTION REAGENTS
This section provides a brief review of a number of chelating and other extraction
reagents, as well as some organic solvents, with special interest as to their
selective extraction properties. The handbook of Cheng et al. should be consulted
for a more detailed account of organic analytical reagents."
Acetylacetone (pentane-2,4dione), CH3CO-CH,-COCH,. Acetylacetone is a
colourless mobile liquid, b.p. 139 OC, which is sparingly soluble in water
(0.17g mL-' at 25 OC) and miscible with many organic solvents. It is useful
both as a solution (in carbon tetrachloride, chloroform, benzene, xylene, etc.)
and as the pure liquid. The compound is a fi-diketone and forms well-defined
chelates with over 60 metals. Many of the chelates (acetylacetonates) are soluble
in organic solvents, and the solubility is of the order of grams per litre, unlike
that of most analytically used chelates, so that macro- as well as micro-scale
separations are possible. The selectivity can be increased by using EDTA as a
masking agent. The use of acetylacetone as both solvent and extractant [e.g.
for Al, Be, Ce, Co(III), Ga, In, Fe, U(VI), etc.] offers several advantages over
its use in solution in carbon tetrachloride, etc.: extraction may be carried out
at a lower pH than otherwise feasible because of the higher reagent concentration;
and often the solubility of the chelate is greater in acetylacetone than in many