Sublimation apparatus is equipment, commonly laboratory glassware, for purification of compounds by selective sublimation. In principle, the operation resembles purification by distillation, except that the products do not pass through a liquid phase.
A typical sublimation apparatus separates a mix of appropriate solid materials in a vessel in which it applies heat under reduced pressure or vacuum. If the material is not at first solid, then it freezes under the reduced pressure. Conditions are so chosen that the solid volatilizes and condenses as a purified compound on a cooled surface, leaving the non-volatile residual impurities or solid products behind.
The form of the cooled surface often is a so-called cold finger. If the operation is a batch process, then the sublimed material can be collected from the cooled surface once heating ceases and the vacuum is released.
Among the advantages of applying the principle to certain materials, are the comparatively low working temperatures and reduced exposure to gases such as oxygen that otherwise might harm certain classes of desired products.
More sophisticated variants of sublimation apparatus include those that apply a temperature gradient so as to allow for controlled recrystallization of different fractions along the cold surface. Thermodynamic processes follow a statistical distribution, and suitably designed apparatus exploit this principle with a gradient that will yield different purities in particular temperature zones along the collection surface. Such techniques are especially helpful when the requirement is to refine or separate multiple products or impurities from the same mix of raw materials. It is necessary in particular when some of the required products have similar sublimation points or pressure curves.
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