Patchouli oil is soluble in equal parts of 90 per cent. alcohol, the contrary indicating that it is not pure. It is chiefly adulterated with cedar oil.
Peppermint oil.—The Mentha family furnishes commerce with three oils differing essentially from each other—oil of curled mint, peppermint oil, and poley oil.
Oil of curled mint (oleum menthæ crispæ) is separated by distillation with water from the leaves of Mentha crispa, Linn. It is limpid, yellowish, sometimes greenish; in time it becomes darker and more thickly-fluid. Its specific gravity varies between 0.890 and 0.965. It has a strong odor of curled mint and a pungent, somewhat cooling, slightly bitterish taste. It is soluble in all proportions in 90 per cent. alcohol. In commerce a distinction is made between German and American oil of curled mint, the latter generally containing oil of turpentine and oil of sassafras. Odor, taste, and the alcohol test suffice for the determination of the quality of the oil.
Oil of curled mint contains a terpene, C10H16, and a body isomeric with carvol, the carvol of curled mint, C10H14O. Its specific gravity, boiling point and chemical properties are the same as those of the carvol of caraway oil, but it differs from it by its odor and turning the plane of polarization to the opposite direction. In conjunction with other volatile oils, oil of curley mint is sometimes used for perfuming soaps.
Peppermint oil (oleum menthæ piperitæ) is obtained from the fresh flowering peppermint, Mentha piperita, natural order Labiatæ. In commerce, German, English, American and Japanese peppermint oils are distinguished. As regards fineness, the German oil is inferior to the English and better kinds of American oils, but superior to the Japanese. The best and most expensive oil is the English so-called "Mitcham oil of peppermint," which dissolves in 50 parts of 50 per cent. spirit of wine and possesses a fine, pure taste, it being for this reason preferred by distillers for the fabrication of liqueurs. For perfuming purposes, however, the American as well as the German oils are very suitable. Of American oils that of H. G. Hotchkiss, L. B. Hotchkiss, Hale & Parshall, and Fritzsche Bros. enjoy a high reputation. The Japanese oil is distinguished from the rest by a peculiar train-oil-like odor and taste, and is in but little demand.
Crude oil of peppermint is yellowish to greenish and contains much mucus; it has therefore to be subjected to another distillation with water. The rectified oil is clear as water, limpid, of a strong but pleasant odor and a specific gravity of 0.900 to 0.902. Old oil thickens and then shows an acid reaction. The greater portion of the oil congeals at the freezing point of water; many kinds, however, requiring but slight cooling in order to become solid. The solid portion of the oil, Menthol, (C10H12,H2O) is a colorless, finely crystallized body with an intense odor of peppermint. It melts at 107.6° F., is quite fluid at an ordinary temperature and boils at 413.6° F. Menthol is found in all oils of peppermint most abundantly in the Japanese oil, which contains from 50 to 55 per cent. of it. Mitcham oil is also very rich in menthol, it containing from 40 to 45 per cent., while the American oil contains only from 20 to 25 per cent.
American, German and English oils of peppermint may be distinguished as follows: By adding to 5 or 6 drops of the oil, 25 to 30 drops of pure white concentrated sulphuric acid, American oil of peppermint becomes heated and emits vapors, the mixture acquiring a dark brownish red coloration. After mixing with 8 to 10 cubic centimeters of 90 per cent. alcohol, the fluid becomes turbid, pale yellowish brown, or reddish brown, and on boiling clear pale brown. When mixed with sulphuric acid, German oil of peppermint becomes heated without emitting vapors, becomes yellowish red, not very dark, and turbid. After diluting with alcohol, the fluid becomes turbid and yellowish red, and on boiling somewhat more transparent and currant-red. English oil of peppermint treated in the same manner as the others with sulphuric acid becomes very slightly heated without any emission of vapor. After diluting with alcohol, it becomes clear and raspberry red.
As adulterants of peppermint oil are mentioned: fat oils, alcohol, oil of turpentine, copaiba oil, mustard oil, and ginger oil. The most frequent adulteration is an admixture of oil of turpentine. It is recognized by the oil not dissolving clear in equal parts of 90 per cent. alcohol, as is the case with the pure product. To detect the presence of copaiba oil, mix 5 drops of the oil to be tested with 15 to 20 drops of fuming nitric acid, shake and allow it to stand for 1 to 2 hours. After this time the oily portion should be neither entirely nor partially congealed, but remain fluid. To detect traces of mustard oil, bring 10 drops of the oil into a wide reagent glass, then add 3 to 4 cubic centimeters of absolute alcohol, 2 to 3 drops of silver nitrate solution and 12 to 15 drops of ammonia. The mixture is clear and colorless, and remains so on heating to boiling. In the presence of mustard oil turbidity and blackening take place in consequence of the formation of silver sulphide. After boiling, allow the fluid to stand quietly for 2 or 3 hours. If it then shows a grayish turbidity, the oil is adulterated with another volatile oil.
Instead of pure American oil of peppermint, a product compounded with camphor oil, and mostly freed from menthol, is said frequently to occur in commerce. According to E. C. Federer, this is recognized by dissolving one volume of oil of peppermint in 2 volumes of 94 per cent. alcohol, adding water and shaking. Pure peppermint oil is then separated with a certain portion of the alcohol. For example, mix 10 cubic centimeters of oil of peppermint in a graduated cylinder of 45 to 50 cubic centimeters' capacity with 20 cubic centimeters of 94 per cent. alcohol, then add 10 cubic centimeters of water of 50° F. and shake. After allowing the mixture to stand quietly, two layers are formed, the upper one of which, if the oil is pure, will amount to 14 cubic centimeters, but only to 12.5 cubic centimeters if the oil is freed from menthol and compounded with camphor oil. A larger or smaller addition of water to the alcoholic mixture is without influence upon the height of the upper layer separated.
In perfumery peppermint oil serves chiefly for aromatizing dentifrices, etc. It should be kept in well-closed bottles in a shady place; an addition of 0.5 per cent. of alcohol helps to preserve the oil for a long time.