Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

4 INTRODUCTORY.

2. Evaporation.

Large quantities of solutions are concentrated in evaporating

dishes, which give a large surface for the escape of vapors from

the liquid. They should be heated, unless otherwise directed,

with a free flame without wire gauze or asbestos; but the flame

should never be allowed to come in contact with the dry upper

part of the dish, as in that case superheating would ensue which

might result in the breaking of the dish or in the decomposition

of the solid material that separates on the sides. Thus as the

concentration progresses the flame must be lowered or the evap-

oration finished on the water bath. Small quantities of liquid can

also be boiled down in beakers; but if this is done it is advisable

to avoid bumping by stirring or by adding to the liquid such

substances as small splinters of wood, bits of unglazed porcelain, or

of pumice, or, best of all, some of Siwoloboff's capillaries.^1

3. Crystallization.

When crystals are to be obtained from a solution, it is often

necessary before the evaporation is finished to filter from impuri-

ties. For this purpose an ordinary plaited filter* is used

which is placed in a short-stemmed funnel. In case the crystals

separate rapidly, a Buchner funnel with suction is employed.

For strongly corrosive liquids, hardened filter paper must be

used or, often better still, a felt of asbestos fibers; the latter is

made by suspending the asbestos in water and pouring the sus-

pension on to the perforated bottom of a Buchner funnel, or

Gooch crucible, and drawing firmly in place by suction. Crys-

tallization will take place on cooling if the solubility of the

material decreases with sinking temperature, otherwise by

evaporation at a moderate heat, or at the room temperature in

a desiccator. Impure crusts tend to form around the sides of

the dish at the surface of the liquid; to prevent this as much as

possible, dishes with perpendicular walls (beakers or glass crystal-

lizing dishes) should be used. If this expedient is not success

ful, the crystals are collected by themselves and the crusts dis-

1 Chem.-Ztg. 19, 304 (1895).

2

Filters of hardened paper are more expensive and allow the filtrate to

flow more slowly than those of common paper, but they retain even the

finest precipitates and are thus at times indispensable.