vogel_practical_ochem_3.pdf

42] EXPERIMENTAL TECHNIQUE 149
Fig. //, 41, 4 is an apparatus for the continuous steam distillation of
a liquid which is heavier than water. The aqueous mixture is placed
in the flask A of relatively large capacity ; the receiver B consists of a
distilling flask (provided with a stopcock at the lower end), the capacity
of which is less than the volume of the liquid in A, and is placed several
inches higher than the opening in the wide vertical tube C. The lower
end of the funnel D must-be below the side arm of the receiver in order
to prevent bubbles of the heavy hquid from collecting on the surface of
the steam distillate and being carried back to the flask
A. The rubber tube E for the returning aqueous liquor
must at some point be lower than the entrance to the
tube C so that a trap of liquid will be formed and prevent
the vapours from A entering the side tube.
Sometimes an air bubble enters the tube E and prevents
the regular flow of hquid from B; the air bubble is
easily removed by shaking the rubber tube. The flask A
is heated (e.g., by a ring burner) so that distillation
proceeds at a rapid rate ; the process is a continuous
one. If the liquid to be steam distilled is lighter than
water, the receiver must be modified so that the aqueous
liquors are drawn off from the bottom (see Continuous
Fig. //, 41, 5.
Extraction of Liquids, Section 11,44).
The steam distillation of small quantities of material may be conducted
in the apparatus of Fig. //, 41, 5. The substance to be distilled is
placed in the small inner tube (a specially constructed test-tube) and water
is boiled in the outer bolt-head flask. The volume of the hquid in the
inner tube does not increase appreciably since it is immersed in the hot
liquid in the flask.
The theory of, and the apparatus for, distillation with superheated
steam are described in Section 1,7.
11,42, TECHNIQUE OF EXTRACTION WITH SOLVENTS
A common operation in practical organic chemistry is the separation
of an organic compound from a solution or suspension in a liquid by
shaking with a second solvent in which the organic compound is soluble
and which is immiscible (or nearly immiscible) with the liquid containing
the substance. The hquid is generally water, so that the subsequent
discussion will be concerned with extraction from this medium. The
solvents generally employed for extraction are diethyl ether, di-iso-propyl
ether, benzene, chloroform, carbon tetrachloride, and petroleum ether.
The solvent selected will depend upon the solubility of the substance to
be extracted in that solvent (compare Section 1,22) and upon the ease
with which the solvent can be separated from the solute. Diethyl ether,
owing to its powerful solvent properties and its low boiling point (35°)
thus rendering its removal extremely facile, is very widely used ; its chief
disadvantage lies in the great fire hazard attending its use, but this may be
reduced to a minimum by adopting the precautions given in Section
11,14. The fire hazard is reduced also by employing di-iso-propyl ether
(b.p. 67-5°), but this solvent is much more expensive than diethyl ether.