Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

16 DISTILLATION
Distillation, whether at atmospheric pressure or in vacua, serves
not only to separate the product from non-volatile impurities but
also to fractionate mixtures of volatile substances having different
boiling points (fractional distillation).
Distillation at Atmospheric Pressure.—The simple distilling flask
with side tube sloping downward (Kg. 11) serves exclusively as the
distilling vessel. In general the side tube should be attached
high in the case of low-boiling liquids and nearer
the bulb in that of less volatile liquids.
The thermometer is held in the flask by means of a
clean bored cork ; the bulb of the thermometer must be
completely immersed in the vapour and hence must be
below the junction of neck and side tube.
Since the ordinary laboratory thermometers are often
inaccurate, they must be compared
with, a standard thermometer before
use. The most accurate method of
standardisation is to hang the two
thermometers side by side with the
FIG. 11 bulbs dipping in concentrated sulphuric
acid or paraffin at 250°, and then to
observe the temperatures during cooling
at intervals of 10° and record the deviations. Thermometers for
distillations should have small bulbs so that they record rapidly.
Distilling flasks should be chosen of such a size that the bulb is
half or two-thirds full of liquid. In order to avoid bumping and
superheating a few pieces of porous plate (pot) half as large as peas
are dropped into the flask before each distillation. If boiling is
again delayed fresh pieces of pot must be added, not to the superheated
liquid, however, but after brief cooling.
The flask is fixed above the side tube in a clamp lined with cork.
Sources of Heat.—Liquids which do not boil above 80° are
heated in the water bath (enamelled jar or beaker); the temperature
of the bath should be about 20° above the boiling point of the substance.
The maintenance of the correct heating temperature is of
the greatest importance, since if it is raised too much the boiling
point of the distillate will be found too high in consequence of
superheating.
In the case of substances of high boiling point where, for preparative
purposes, a margin of a few degrees in the boiling point