Page 134 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

material which is usually present. If the substance crystallises out in the funnel,
it should be filtered through a hot-water funnel. The clear hot filtrate is cooled
rapidly by immersion in a dish of cold water or in a mixture of ice and water,
according to the solubility of the solid; the solution is constantly stirred in order
to promote the formation of small crystals, which occlude less mother liquor
than larger crystals. The solid is then separated from the mother liquor by
filtration, using one of the Buchner-type funnels shown in Fig. 3.7 (Section 3.24).
When al1 the liquid has been filtered, the solid is pressed down on the funnel
with a wide glass stopper, sucked as dry as possible, and then washed with small
portions of the original solvent to remove the adhering mother liquor. The
recrystallised solid is dried upon clock glasses at or above the laboratory
temperature according to the nature of the material; care must of course be
taken to exclude dust. The dried solid is preserved in glass-stoppered bottles.
It should be noted that unless great care is taken when the solid is removed
from the funnel, there is danger of introducing fibres from the filter paper, or
small particles of glass from the glass filter disc: scraping of the filter paper or
of the filter disc must be avoided.
Some solids are either too soluble, or the solubility does not Vary sufficiently
with temperature, in a given solvent for direct crystaiiisation to be practicable.
In many cases, the solid can be precipitated from, Say, a concentrated aqueous
solution by the addition of a liquid, miscible with water, in which it is less
soluble. Ethanol, in which many inorganic compounds are almost insoluble, is
generally used. Care must be taken that the amount of ethanol or other solvent
added is not so large that the impurities are also precipitated. Potassium
hydrogencarbonate and antimony potassium tartrate may be purified by this
method.
Many organic compounds can be purified by recrystallisation from suitable
organic solvents, and here again, precipitation by the addition of another solvent
in which the required compound is insoluble, may be effective; while liquids
can be purified by fractional distillation.
Sublimation. This process is employed to separate volatile substances from
non-volatile impurities. Iodine, arsenic(II1) oxide, ammonium chloride and a
number of organic compounds can be purified in this way. The material to be
purified is gently heated in a porcelain dish, and the vapour produced is
condensed on a flask which is kept cool by circulating cold water inside it.

Zone refining. This is a purification technique originally developed for the
refinement of certain metals, and is applicable to al1 substances of reasonably
low melting point which are stable at the melting temperature. In a zone refining
apparatus, the substance to be purified is packed into a column of glass or
stainless steel, which may Vary in length from 15 cm (semimicro apparatus) to
1 metre. An electric ring heater which heats a narrow band of the column is
allowed to fa11 slowly by a motor-controlled drive, from the top to the bottom
of the column. The heater is set to produce a molten zone of material at a
temperature 2-3 OC above the melting point of the substance, which travels
slowly down the tube with the heater. Since impurities normally lower the
melting point of a substance, it follows that the impurities tend to flow down
the column in step with the heater, and thus to become concentrated in the
lower part of the tube. The process may be repeated a number of times (the