Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

ISOLATION OF CEYSTALS 9
The steam-heated funnel (as in Fig. 5) is likewise very useful. When
small amounts of liquid are to be filtered, the empty glass funnel may
he heated over a naked flame before use, or the paper may be fixed
FIG. 3 FIG. 4 FIG. 5
in the funnel, moistened with alcohol, ignited, and allowed to burn
till it begins to char, while the funnel is held in a horizontal position
and rotated.
It is often advisable, especially in the case of aqueous solutions
which are difficult to filter, to use a porcelain Biichner funnel and
apply suction. A well-fitting filter paper is required and the filter
flask must be cautiously warmed before use, preferably by standing
it in an enamelled pail of warm water and heating to boiling.
If the filter paper becomes choked by crystallisation of the substance,
it is best not to push, a hole through it. The paper should rather be
held upright in a small beaker in which fresh solvent is kept boiling,
and the more dilute solution thus obtained is poured through the same
paper. In such cases the whole solution must generally be concentrated
by evaporation.
If it is desired to produce well-developed crystals when recrystallising,
the filtrate, in which separation of crystals often occurs
even during filtration, must be reheated till a clear solution is
obtained and then allowed to cool slowly without being disturbed.
The isolation of the crystals is never accomplished by ordinary
filtration, but always by collecting them at the pump on a filter
paper, or, in the case of concentrated acids and alkalis, on glass
wool, asbestos, or, best of all, on Schott filters of sintered glass.
Large amounts of substance are collected on Biichner funnels (Fig.
6) of size appropriate to the quantity of the material to be separated.