DRYING 15mention the liquid which adheres to the crystal surfaces. By
dissolving the crystals, the small amount of impurity likewise
passes into the solution, but only a small fraction of this impurity
is later entrapped by the crystals when they separate from this
mother liquor. By several recrystallizations, then, a substance
can be obtained in a very high state of purity.
- DRYING
(a) A preparation that is not affected by the atmosphere can
be dried by being spread in a thin layer, allowing the liquid adher-
ing to the grains or crystals to evaporate. Paper towels are
extremely useful in drying preparations because a great deal of the
moisture is absorbed into the pores. When a corrosive liquid, for
example nitric acid, clings to the product, the latter is best spread
on an unglazed earthenware dish, which absorbs the liquid without
being attacked by it. During the drying the material should
occasionally be turned over with a spatula.
If the material is not decomposed by heat it can be dried much
more rapidly in a warm place, as on a steam-heated iron plate
(steam table); but a product containing water of crystallization
should never be dried at an elevated temperature. During the drying
the preparation must, of course, be carefully protected from
dust.
(b) Efflorescent crystals and crystals which absorb carbon di-
oxide should be quickly pressed between paper towels until as
much as possible of the liquid is soaked up, and then they should
be wrapped in a tight package in several layers of fresh paper
towels and left in the cupboard for not more than 24 hours. The
liquid is drawn by capillarity into the paper and evaporates from
the outer surface, but the paper so impedes the circulation of air
that water vapor does not escape, and the crystals will not effloresce
unless left for more than a day. With preparations that react
with carbon dioxide (such as barium hydroxide) the solution which
soaks into the paper retains all the carbon dioxide which might
otherwise contaminate the product.
(c) Substances which decompose on standing exposed to the air
may be quickly dried if they are first rinsed with alcohol, or with
alcohol and then ether. Rinsing with alcohol removes nearly all
the adhering water, and a further rinsing with ether removes
the alcohol. Alcohol evaporates more rapidly than water, but