Page 127 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

DESlCCATORS I D DRY BOXES 3.22
in the dry atmosphere of a desiccator, allied to minimum exposure to the
atmosphere during subsequent operations, will be sufficient to prevent appreciable
absorption of water vapour. Some substances, however, are so sensitive to
atmospheric moisture that al1 handling must be carried out in a 'dry box'.
A desiccator is a covered glass container designed for the storage of objecis
in a dry atmosphere; it is charged with some drying agent, such as anhydrous
calcium chloride (largely used in elementary work), silica gel, activated alumina,
or anhydrous calcium sulphate ('Drierite'). Silica gel, alumina and calcium
sulphate can be obtained which have been impregnated with a cobalt Salt so
that they are self-indicating: the colour changes from blue to pink when the
desiccant is exhausted. The spent material can be regenerated by heating in an
electric oven at 150-180 OC (silica gel); 200-300 OC (activated alumina)
230-250 OC (Drierite); and it is therefore convenient to place these drying agents
in a shallow dish which is situated at the bottom of the desiccator, and which
can be easily removed for baking as required.
The action of desiccants can be considered from two points of view. The
amount of moisture that remains in a closed space, containing incompletely
exhausted desiccant, is related to the vapour pressure of the latter, i.e. the vapour
pressure is a measure of the extent to which the desiccant can remove moisture,
and therefore of its efficiency. A second factor is the weight of water that can
be removed per unit weight of desiccant, i.e. the drying capacity. In general,
substances that form hydrates have higher vapour pressures but also have greater
drying capacities. It must be remembered that a substance cannot be dried by
a desiccant of which the vapour pressure is greater than that of the substance
itself.
The relative efficiencies of various drying agents will be evident from the data
presented in Table 3.6. These were determined by aspirating properly conditioned
air through U-tubes charged with the desiccants; they are applicable, strictly,
to the use of these desiccants in absorption tubes, but the figures may reasonably
be applied as a guide for the selection of desiccants for desiccators. It would
appear from the table that a hygroscopic material such as ignited alumina
should not be allowed to cool in a covered vesse1 over 'anhydrous' calcium
chloride; anhydrous magnesium perchlorate or phosphorus pentoxide is
satisfactory.

Table 3.6 Comparative efficiency of drying agents

Drying agent Residual water Drying agent Residual water
(mg per L of air) (mg per L of air)
CaCI, (gran. 'anhyd.' tech.) 1.5 0.005
NaOH (sticks) 0.8 CaSO, 0.005
H,SO, (95%) 0.3 Molecular sieve 0.004
Silica gel 0.03 H2SO4 0.003
KOH (sticks) 0.0 14 Mg((JO4)2 0.002
0.00002

The normal (or Scheibler) desiccator is provided with a porcelain plate having
apertures to support crucibles, etc.: this is supported on a constriction situated
roughly halfway up the wall of the desiccator. For small desiccators, a silica
triangle, with the wire ends suitably bent, may be used. The ground edge of the