Dessicator 1

Desiccators are sealable enclosures
containing desiccants used for preserving
moisture -sensitive items such as cobalt
chloride paper for another use. A common use
for desiccators is to protect chemicals which
are hygroscopic or which react with water
from humidity .
The contents of desiccators are exposed to
atmospheric moisture whenever the
desiccators are opened. It also requires some
time to achieve a low humidity. Hence they
are not appropriate for storing chemicals
which react quickly or violently with
atmospheric moisture such as the alkali
metals; a glovebox or Schlenk -type apparatus
may be more suitable for these purposes.
Desiccators are sometimes used to remove
traces of water from an almost-dry sample.
Where a desiccator alone is unsatisfactory,
the sample may be dried at elevated
temperature using Abderhalden's drying pistol .
Use
In order to weigh a substance, watch glass or
weighing bottles or crucibles are used. But to
be accurate, the weighed object must be of
the same temperature of the analytical
balance . If a hotter (or colder) object is
placed on a balance pan this has the effect of
lengthening (or shortening) the corresponding
arm of the beam resulting in incorrect
reading. Moreover, a hot object warms the air
in contact with it and makes it rise. The
moving air pushes the corresponding balance
pan upwards and therefore the error is
increased further. Conversely, if a cold object
is weighed, a current of air flows downwards
and this gives rise to an error of the opposite
sign. Thus, the object must be left 20 minutes
to reach room temperature .
To prevent adsorption of moisture from the
surrounding air, glassware is cooled in a
desiccator. [1]
Constituents
The lower compartment of the desiccator
contains lumps of silica , freshly calcined
quicklime or (not as effective) anhydrous
calcium chloride to absorb water. The
substance is put in the upper compartment.
The ground-glass rim of the desiccator lid
must be greased with a thin layer of
petroleum jelly or other lubricant to ensure an
airtight seal
In order to prevent damage to a desiccator
the lid should be carefully slid on and off
instead of being directly placed onto the base.
[1]
Operation
In laboratory use, the most common
desiccators are circular and made of heavy
glass . There is usually a removable platform
on which the items to be stored are placed.
The desiccant, usually an otherwise-inert solid
such as silica gel, fills the space under the
platform. Colour changing silica may be used
to indicate when it should be refreshed.
Indication gels typically change from blue to
pink as they absorb moisture but other
colours may be used.
A stopcock may be included to permit the
desiccator to be evacuated. Such models are
usually known as vacuum desiccators. When a
vacuum is to be applied, it is a common
practice to criss-cross the vacuum desiccator
with tape, or to place it behind a screen to
minimize damage or injury caused by an
implosion . To maintain a good seal, vacuum
grease is usually applied to the flanges .