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 fromhumidity.
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.
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.
So to make sure it doesn't absorb appreciable moisture from air increasing its weight, it is cooled down in a desiccator.
The lower compartment of the desiccator contains lumps of freshly calcined quicklime or (not as effective) calcined calcium chloride to absorb water vapors. The substance is put in the upper compartment (on the porcelain plate). The ground-glass rim of the desiccator lid must be thoroughly greased with a thin layer of petroleum jelly melted together with beeswax or paraffin wax.
In order to open the desiccator without damage, remove the lid sideways horizontally not to upwards. Cover the desiccator in the same way.
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.
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.