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[[File:Desiccator.jpg|thumb|right|A '''vacuum desiccator''' (left - note the stopcock which allows a vacuum to be applied), and a '''desiccator''' (right). The blue [[silica gel]] in the space below the platform is used as the [[desiccant]]. |
[[File:Desiccator.jpg|thumb|right|A '''vacuum desiccator''' (left - note the stopcock which allows a vacuum to be applied), and a '''desiccator''' (right). The blue [[silica gel]] in the space below the platform is used as the [[desiccant]]. |
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'''Desiccators''' are sealable enclosures containing [[desiccant]]s 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]]. |
'''Desiccators''' are sealable enclosures containing [[desiccant]]s 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]]. |
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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 [[wikt:pan|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]]. |
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 [[wikt:pan|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]]. |
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To prevent adsorption of moisture from the surrounding air, glassware is cooled in a desiccator.<ref>{{cite book|last=Alexeyev|first=V.|title=Quantitative Analysis|publisher=Lightning source Inc.|isbn=0-89875-034-2}}</ref> |
To prevent adsorption of moisture from the surrounding air, glassware is cooled in a desiccator.<ref name=qa>{{cite book|last=Alexeyev|first=V.|title=Quantitative Analysis|publisher=Lightning source Inc.|isbn=0-89875-034-2}}</ref> |
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==Constituents== |
==Constituents== |
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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 |
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 |
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In order to open the desiccator without damage, remove the lid sideways horizontally not to upwards. Cover the desiccator in the same way.<ref name=qa |
In order to open the desiccator without damage, remove the lid sideways horizontally not to upwards. Cover the desiccator in the same way.<ref name=qa/> |
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==Operation== |
==Operation== |
Revision as of 12:44, 18 October 2013
[[File:Desiccator.jpg|thumb|right|A vacuum desiccator (left - note the stopcock which allows a vacuum to be applied), and a desiccator (right). The blue silica gel in the space below the platform is used as the desiccant. 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 open the desiccator without damage, remove the lid sideways horizontally not to upwards. Cover the desiccator in the same way.[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.
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.