Desiccator
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Desiccators are sealable enclosures containing desiccants used for preserving
moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Moisture is defined as water in the adsorbed or absorbed phase. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some comme ...
-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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
. 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.


Constituents

The lower compartment of the desiccator contains lumps of silica gel, freshly calcined quicklime, Drierite, molecular sieves, phosphorus pentoxide, (not as effective) anhydrous calcium chloride, or other desiccant to absorb water vapor. The substance needing desiccation is put in the upper compartment, usually on a glazed, perforated ceramic plate. The ground-glass rim of the desiccator lid must be greased with a thin layer of vacuum grease, 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.


Operation

In laboratory use, the most common desiccators are circular and made of heavy
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
. 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 (if the indicator is cobalt(II) chloride) 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
flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of a steel beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer o ...
s.


See also

*
Bell jar A bell jar is a glass jar, similar in shape to a bell (instrument), bell (i.e. in its best-known form it is open at the bottom, while its top and sides together are a single piece), and can be manufactured from a variety of materials (ranging fr ...
*
Desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...
* Dry box


References

{{Laboratory equipment Laboratory glassware Containers