Page 198 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVEWT EXTRACTION
organic solvents. The solvent generally used is carbon tetrachloride; the organic
layer is heavier than water.
An interesting application is the separation of cobalt and nickel: neither
Co(I1) nor Ni(I1) forms extractable chelates, but Co(II1) chelate is extractable;
extraction is therefore possible following oxidation.
Thenoyltrifluoroacetone (TTA), C,H3S-CO-CH,-COCF,. This is a crystalline
solid, m.p. 43 OC; it is, of course, a fi-diketone, and the trifluoromethyl group
increases the acidity of the en01 form so that extractions at low pH values are
feasible. The reactivity of TTA is similar to that of acetylacetone: it is generally
used as a 0.1 -0.5 M solution in benzene or toluene. The difference in extraction
behaviour of hafnium and zirconium, and also among lanthanides and actinides,
is especially noteworthy.
Other fluorinated derivatives of acetylacetone are trifluoroacetylacetone
(CF3COCH2COCH3) and hexafluoroacetylacetone (CF3COCH2COCF3),
which form stable volatile chelates with aluminium, beryllium, chromium(111)
and a number of other metal ions. These reagents have consequently been used
for the solvent extraction of such metal ions, with subsequent separation and
analysis by gas chromatography [see Section 9.2(2)].
8-Hydroxyquinoline (oxine). Oxine is a versatile organic reagent and
forms chelates with many metallic ions. The chelates of doubly and triply charged
metal ions have the general formulae M(C,H,ON), and M(CgH ,ON),;
the oxinates of the metals of higher charge may differ somewhat in compo-
sition, e.g. Ce(C,H,ON),; Th(CgH6ON),~(C9H7ON); W02(CgH60N)2;
MoO~(C,H,ON)~; U3O6(CgH6ON),~(C9H7ON). Oxine is generally used
as a 1 per cent (0.07 M) solution in chloroform, but concentrations as high as
10 per cent are advantageous in some cases (e.g. for strontium).
8-Hydroxyquinoline, having both a phenolic hydroxyl group and a basic
nitrogen atom, is amphoteric in aqueous solution; it is completely extracted
from aqueous solution by chloroform at pH < 5 and pH > 9; the distribution
coefficient of the neutral compound between chloroform and water is 720 at
18 OC. The usefulness of this sensitive reagent has been extended by the use of
masking agents (cyanide, EDTA, citrate, tartrate, etc.) and by control of pH.
Dimethylglyoxime. The complexes with nickel and with palladium are soluble
in chloroform. The optimum pH range for extraction of the nickel complex is
4-12 in the presence of tartrate and 7-12 in the presence of citrate (solubility
35-50 pg Ni mL-' at room temperature); if the amount of cobalt exceeds 5 mg
some cobalt may be extracted from alkaline solution. Palladium(I1) may be
extracted out of ca 1M-sulphuric acid solution.
1-Nitroso-2-naphthol. .The reagent forms extractable complexes (chloroform)
with Co(II1) in an acid medium and with Fe(I1) in a basic medium.
Cupferron (ammonium salt of N-nitroso-N-phenylhydroxylamine). The reagent
is used in cold aqueous solution (about 6 percent). Meta1 cupferrates are soluble
in diethyl ether and in chloroform, and so the reagent finds wide application
in solvent-extraction separation schemes. Thus Fe(III), Ti, and Cu may be
extracted from 1.2 M HC1 solution by chloroform: numerous other elements
may be extracted largely in acidic solution.