Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

6 CKYSTALLISATION
be extracted by aqueous solutions of alkali, and organic bases by
aqueous solutions of acid, from a mixture of neutral substances in
a solvent like ether.
When a substance has not the necessary moderate solubility in
any solvent but is either too readily or too sparingly soluble, the
combination of different solvents is a useful expedient. The solvents
which are used together must be miscible with each other. The
following are most frequently used :
Alcohol, glacial acetic acid, acetone with water.
Ether, acetone, benzene, chloroform with petrol ether.
Pyridine with water, ether, or alcohol.
The method of procedure is to add the solvent used as diluent
drop by drop to the cold or hot concentrated solution until turbidity
is just produced ; crystallisation is then induced by leaving
the liquid to stand or by scratching with a sharp-edged glass rod.
When crystallisation has begun the solution is cautiously diluted
further. It is a mistake to precipitate the dissolved substance at
one stroke with large amounts of the diluent.
In the case of all operations which are not yet under control, 'preliminary
test-tube experiments should be carried out. The student
should acquire the habit of doing this from the very beginning.
Aqueous nitrates should be collected in beakers, but organic
solvents in conical flasks, which prevent evaporation and so check the
formation of crusts. Already in order to obtain some idea of the
degree of purity from the appearance of the crystals, the crystallisation
should be left to go on undisturbed so that crystals may separate
in the best possible form. It is an error to assume that fine crystals
produced by immediate strong cooling of a solution constitute an
especially pure substance ; on the contrary, the large surface of the
deposit favours the adsorption of by-products. Moreover, with wellformed
crystals it is much easier for the organic chemist to meet
the imperative requirement that he should check the homogeneity
of a substance. The examination of the preparation with a lens
or under the microscope should not be neglected ; 50- to 100-fold
magnification is sufficient.
If a solution has become saturated at room temperature the
yield of crystals can be increased by placing the vessel in ice-water
or in a freezing mixture.
Substances of low melting point occasionally separate as oils
when their hot saturated solutions are cooled. The solution must