Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

14 DEYING IN A VACUUM
are dried in a vacuum at a higher temperature; they are heated in a
small round flask on the water bath or oil bath until they cease to lose
weight. The so-called " drying pistol" (Fig. 10) is especially convenient
for this purpose.
The vapours from the liquid
boiling in A heat the wide
inner tube B, in which the
substance is exposed in a
porcelain boat. C contains
a drying agent—for water
and alcohol, phosphorus
pentoxide, for other vapours,
paraffin wax. According
to the temperature
desired, the heating liquid
chosen is chloroform (66°),
water (100°), toluene (111°),
xylene (140°). C is connected
to the pump.
FIG. 10
For drying small
amounts of substance the copper drying block shown on p. 51 is
greatly to be recommended.
If a non-volatile solvent, such as glacial acetic acid, xylene, highboiling
petrol ether, or nitrobenzene, has been used for recrystallisation,
it is washed out before drying by means of one which is more
easily removed, e.g. ether, benzene, petrol. In general a substance
which is sparingly soluble in glacial acetic acid or nitrobenzene is so
also in ether.
Very finely divided precipitates and also those which choke the
pores of the filter paper are separated from the liquid phase by
means of a centrifuge.
Chromatographic Adsorption. 1 —Coloured substances which cannot
be separated from each other by crystallisation may be separated
by the process known as chromatographic adsorption, a process
which has been applied with great success during the last few years.
Advantage is taken of the fact that the constituents of the mixture
exhibit different affinities for adsorbents (alumina, talc, silica gel,
sugar, calcium carbonate, calcium oxide), the solution of the mixture
(usually an organic solvent is employed) being drawn through a
1 M. Tswett, Ber. Deut. bot. Ges., 1906, 24, 234, 361, 384. Details of the method
are to be found in the paper by A. Winterstein and G. Stein, Z. physiol. Chem., 1933,
220, 247. See also Cook, J.S.C.I., 1936, 55, 724-726.