Sublimation

Sublimation, a process by which a solid body is transferred into a gaseous state by means of heating and then back into a solid by cooling, bypassing the liquid stage. It is possible for so-called volatile substances that possess significant vapor pressure already at temperatures below their melting point. It is usually used for purifying substances from non-volatile impurities, which remain in the sublimation vessel. When heating the material, direct heating is avoided; as heat carriers, sand, oil, superheated air, etc., are used. The temperature at which sublimation is performed can be lowered by applying a vacuum, achieved in special vacuum apparatus designed for this purpose. The sublimation process can be accelerated by passing an indifferent gas through the sublimation vessel. In the simplest case, the apparatus consists of a flask in which the material is heated and a receiver, with the latter, unlike distillation apparatus, not being too far from the former, since the vapors of the solid substance, upon reaching a cooler place, quickly settle and therefore cannot be conducted far. Often, settling occurs already on the upper, less heated half of the sublimation vessel. Sublimation is sometimes performed with the aid of steam, in which case the product is obtained in the form of a fine powder, whereas with ordinary sublimation the same substance is obtained in the form of larger crystals (e.g., calomel). Sublimation is widely used in chemical-pharmaceutical production in obtaining preparations: salicylic and benzoic acids, sublimate, camphor (synthetic and natural), iodine, pippgalol, and others.