A. Chemicals used for the Extraction of Aromatic Substances.
For the extraction of aromatic substances from plants a number of bodies are used which possess great solvent power for essential oils, and are besides very volatile, or have a low boiling-point. These are particularly ether, chloroform, petroleum ether, and bisulphide of carbon.
Ether.
This liquid, in commerce also called sulphuric ether, is made in large quantities in chemical laboratories by the distillation of alcohol with sulphuric acid, followed by a second distillation or rectification. When pure, ether forms a mobile, thin, strong-smelling, and inflammable liquid which when inhaled produces insensibility, for which reason it is used as an anæsthetic in surgery. Its specific gravity is about 0.720 when anhydrous, and its boiling-point 35° C. (95° F.). It forms an excellent solvent for essential oils, resins, fats, and similar bodies. Owing to its great volatility, its vapors are quickly diffused in the air, and, as they are very inflammable, lights must be kept away from a bottle containing this substance. The same remark applies to most of the substances to be presently described.
Chloroform.
is prepared by the distillation of chlorinated lime, alcohol, and water, acetone being more recently substituted for the alcohol, followed by rectification of the product. When inhaled it produces insensibility like ether. It has a pleasant odor and sweet taste. Its specific gravity is about 1.49 and its boiling-point 61° C. (142° F.). Owing to its great solvent power and low boiling-point, chloroform is largely used for the extraction of aromatic vegetable substances; it does not take fire directly in the air.
Petroleum Ether.
Petroleum, which is brought into commerce in immense quantities, especially from Pennsylvania, for illuminating purposes, cannot be used in its crude state, but requires rectification. Petroleum as it issues from the earth consists of various hydrocarbons mixed together, some of which have very low boiling-points, so that their vapors readily take fire and would make the use of petroleum in lamps dangerous. Petroleum, therefore, is heated in large apparatuses to about 70 or 80° C. (158 to 176° F.), when the more volatile products pass over, and the petroleum for illuminating purposes remains in the stills. A certain fraction of the volatile distillate, the so-called petroleum ether, is largely used in the manufacture of varnishes. Owing to its great solvent power for aromatic vegetable substances and its low price, petroleum ether has become quite an important body for the extraction of perfumes, which will be further discussed hereafter. Good petroleum ether is colorless, has a peculiar, not unpleasant odor and a boiling-point between 50 and 55° C. (112° and 131° F.).
Benzin.
is a common name for another fraction of the volatile distillate from petroleum, viz., that which boils between 50° and 60°C. (122° to 140° F.) and has a spec. grav. of 0.670 to 0.675°.
This liquid, which is also used as a volatile solvent for the extraction of odorous substances, must not be confounded with Benzene or Benzol, a distillate from coal tar, boiling at about 80° C. (176° F.) and having a spec. grav. of 0.878. The latter is not used for the extraction of perfumes.
Bisulphide of Carbon.
This is made by conducting vapors of sulphur over glowing charcoal or coke. The vapors of bisulphide of carbon thus formed are led into vessels filled with ice or ice-cold water, where they condense. Bisulphide of carbon is a colorless liquid, heavier than water and very refractive. It is inflammable, and possesses a peculiar odor which is not disagreeable if the liquid has been thoroughly purified. Its boiling-point is about 45° C. (113° F.) and it has great solvent power. At the present time, the market affords bisulphide of carbon of a high degree of purity.
Some manufacturers who prepare their odors by extraction, may find it advantageous to make also the bisulphide of carbon necessary for it, and this is best done in Gérard’s apparatus (Fig. 1). It consists of a cast-iron cylinder a, two metres high and one metre in diameter. This cylinder is heated on the outer surface in an oven, and two tubes, c and d, are attached to it. Tube d is connected by e with the hemispherical vessel b which is connected by the tube i with the condenser mlk. The condenser is formed of three cylinders made of sheet zinc which are surrounded with cold water. The condensed liquid escapes into the vessel p, while the gaseous products pass through n into the chimney. The cylinder a is filled with about 1,500 pounds of charcoal or coke in small pieces, after which it is closed and all tubes are carefully luted with clay; a is then heated to a strong red heat and at intervals of three minutes 3 pounds of sulphur are thrown in through c. In twenty-four hours, by the use of 478 pounds of sulphur, 568 pounds of crude bisulphide of carbon are obtained; a portion of the sulphur distils over uncombined into the vessel b.
Fig. 1.
The crude bisulphide of carbon contains about twelve per cent of sulphur and other combinations in solution and is redistilled at exactly 48° C. (118.4° F.) in a steam-heated apparatus with a long exit tube cooled with ice below and water above. In order to obtain the bisulphide of carbon absolutely pure, which is essential to render it suitable for extraction, it is again distilled at the same temperature, with the addition of two per cent of palm oil. As the vapors of bisulphide of carbon are injurious to the organism, the vessels containing it must always be kept well closed.