Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

28 SUPBEHBATBD STEAM
vertical safety tube. As a rule distillation is continued until the
distillate passes over clear. If the substance crystallises in the
condenser the jacket is partly emptied for a short time and the
steam then melts the crystals, so that the substance flows away.
Care must be taken, however, when this procedure is adopted,
that there is no loss of uncondensed vapour carried away with the
steam. The readmission of the cooling water to the hot tube must
be done cautiously.
When distillation is complete, the connection between the steam
tube and the flask is broken before shutting off the steam, because
otherwise the residue in the flask might be sucked back through the
inlet tube. Special attention must be paid to this point when the
steam is taken from the laboratory supply.
Smaller amounts of substance can also be steam distilled from a
distilling flask of sufficient size having the side tube attached high
up on the neck, and specially volatile substances can also be distilled
by simple heating with water, without a current of steam.
Substances which volatilise only with very great difficulty are distilled
with superheated steam. The superheating takes place in a
copper tube (Fig. 22) wound in a conical spiral, interposed between
FIG. 22
the steam generator and the flask and heated from below with a
burner. The flask with the substance stands in an oil bath heated
to a high temperature (about 150°).
Sometimes the distillation can be performed without the superheater
if the steam, dried as completely as possible, is passed, not
too rapidly, into the distilling flask, containing the dry substance,
which is kept hot in the bath. Volatile substances, which tend to
decompose, are occasionally distilled with steam under reduced
pressure and consequently at reduced temperature.