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In pre-modern
In pre-modern chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
and alchemy
Alchemy (from the Arabic word , ) is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practised in China, India, the Muslim world, and Europe. In its Western form, alchemy is first ...
, cohobation was the process of repeated distillation
Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
of the same matter, with the liquid drawn from it (successive redistillation); that liquid being poured again and again upon the matter left at the bottom of the vessel. Cohobation is a kind of circulation, only differing from it in this, that the liquid is drawn off in cohobation, as in common distillation, and thrown back again; whereas in circulation, it rises and falls in the same vessel, without ever being drawn out.
Cohobation is not recognized as a useful process in modern chemistry. Indeed, it is equivalent to performing the same distillation a number of times and does not increase the purity of the distillate or alter the residue any more than would be done by maintaining it at elevated temperature for the same period of time. The Dean-Stark trap does involve returning some distillate to the reaction flask: a solution is distilled and the condensed liquid is collected in a tube wherein water settles to the bottom and is drained out, while an organic solvent returns to the boiling solution. However, the process is not manual, most of the solvent does not leave the reaction flask, and the apparatus achieves a useful purpose (removing water from the reaction mixture). Circulation, on the other hand, is approximately the same as reflux
Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to Chemical ...
, where a solution is maintained at its boiling point by condensing the distilling vapors and returning them directly to the reaction mixture.
References
Alchemical processes Distillation Chemical processes {{chem-hist-stub az:Rektifikasiya ba:Ректификация bg:Ректификация cs:Rektifikace (chemie) de:Rektifikation (Verfahrenstechnik) eo:Rektifikilo et:Rektifikatsioon hr:Rektifikacija (kemija) hy:Ռեկտիֆիկացում kk:Ректификаттау ky:Ректификация lv:Rektificēšana ru:Ректификация sl:Rektifikacija (kemija) sv:Rektifikation uk:Ректифікація uz:Rektifikatsiya