DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down)

This article appears in Volume V08, Page 318 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: DIO-DRO

DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down) , an operation consisting in the conversion of a substance or mixture of substances into vapours which are afterwards condensed to the liquid form; it has for its object the separation or purification of substances by taking advantage of differences in volatility. The apparatus consists of three parts:the " retort " or " still," in which the substance is heated; the " condenser," in which the vapours are condensed; and the "receiver," in which the condensed vapours are collected. Generally the components of a mixture will be vaporized in the order of their boiling-points; consequently if the condensates or " fractions " corresponding to definite ranges of temperature be separately collected, it is obvious that a more or less partial separation of the components will be effected. If the substance operated upon be practically pure to start with, or the product of

distillation

DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down)

be nearly of constant composition, the operation is termed "purification by

distillation

DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down)

" or" rectification"; the latter

term

TERM

is particularly used in the spirit industry. If a complex mixture be operated upon, and a separation effected by collecting the distillates in several portions, the operation is termed " fractional distillation." Since many substances decompose either at, or below, their boiling-points under

ordinary

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

atmospheric pressure, it is necessary to lower the boiling-point by reducing the pressure if it be desired to distil them. This variation is termed " distillation under reduced pressure or in a vacuum." The vaporization of a substance below its normal boiling-point can also be effected by blowing in steam or some other vapour; this operation is termed "distillation with steam." "Dry distillation" is the

term

TERM

used when solid substances which do not liquefy on

heating

HEATING

are operated upon; "sublimation" is the term used when a solid distils without the intervention of a liquid phase.
Distillation appears to have been practised at very remote times. The Alexandrians prepared oil of turpentine by distilling pine-resin; Zosimus of Panopolis, a voluminous writer of the 5th century A.D., speaks of the distillation of a " divine water " or " panacea " (probably from the complex mixture of

calcium

CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

polysulphides, thiosulphate, &c., and free

sulphur

SULPHUR

, which is obtained by boiling

sulphur

SULPHUR

with lime and water) and advises " the efficient luting of the apparatus, for otherwise the valuable properties would be lost." The Arabians greatly improved the earlier apparatus, naming one form the alembic (q.v.); they discovered many ethereal oils by distilling plants and plant juices, alcohol by the distillation of wine, and also distilled water. The alchemists gave

great

GREAT

attention to the method, as is shown by the many discoveries made. Nitric, hydrochloric and sulphuric acids, all more or less impure, were better studied; and many ethereal oils were discovered. Prior to about the 18th century three forms of distillation were practised: (I) destillatio per ascensum, in which the retort was heated from the bottom, and the vapours escaped from the top; (2) destillatio per lotus, in which the vapours escaped from the side; (3) destillatio per descensum, in which the retort was heated at the top, and the vapours led off by a pipe passing through the bottom. According to K. B.

Hoffmann

the earliest mention of destillatio per descensum occurs in the writings of Aetius, a Greek physician who flourished at about the end of the 5th century. -
In modern times the laboratory practice of distillation was greatly facilitated by the introduction of the condenser named after Justus von

Liebig

LIEBIG, JUSTUS VON, BARON (1803-1873)

; A. Kolbe and E. Frankland introduced the " reflux condenser," i.e. a condenser so placed that the condensed vapours return to the distilling flask, a device permitting the continued boiling of a substance with little loss; W.
first type; N2O4-22NO2, of the second (see CHEMICAL ACTION). Electrolytic or ionic dissociation is the separation of a substance in solution into ions (see ELECTROLYSIS; SOLUTION).

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