Oleum Limonis, B.P. Oil of Lemon.
Oil of lemon is obtained by mechanical means from the fresh peel of the fruit of Citrus Medica, Linn., var. β-Limonum, Hook. f. (N.O. Rutaceae), a small tree, probably a native of India, cultivated in all countries bordering on the Mediterranean, especially in Sicily, Southern Italy, Spain, and the Riviera. It is also official in the U.S.P. The oil occurs as a pale yellow liquid, having the pleasant, fragrant odour of fresh lemons, and an aromatic, mild, somewhat bitter after-taste. Specific gravity, 0.857 to 0.862 (0.851 to 0.855 at 25°). Rotation, +58 to +65°, occasionally up to +66°. As the aroma and flavour of oil of lemon deteriorate by keeping, it has been recommended that it should be mixed while new with one-tenth of its volume of absolute alcohol. Adulteration with turpentine oils, French and American, both of which decrease the rotatory power, may be approximately estimated by calculating from the rotation figures, provided it is known which variety of turpentine has been used; turpentine may also be detected in the first fraction of about 10 per cent., since pinene, its chief constituent, boils at 156°, whereas pure lemon oil begins to boil at about 173°. Other forms of adulteration, however, are exceedingly difficult of detection, such as mixtures of turpentine and low-grade orange oils, waste terpene with orange oil, and sometimes with citral obtained from lemon grass oil. Determination of the citral at the best can only give approximate results, and is moreover no proof of the genuineness of the oil. Ethyl alcohol as an adulterant may be detected by shaking with water, when a diminution in volume would indicate alcohol. The oil should evaporate from paper without leaving any stain (absence of fixed oils).
Soluble in alcohol (1 in 12), clear solubility often being prevented by gummy and vegetable wax-like constituents; in all proportions of absolute alcohol, ether, chloroform, benzol, amylic alcohol, or glacial acetic acid; solutions in carbon bisulphide or benzene are usually somewhat cloudy on account of a little water contained in the oil.
Constituents.—The constituents of oil of lemon present in greatest abundance are the terpenes, d-limonene chiefly, and l-limonene, C10H16, together forming about 90 per cent. of the bulk of the oil. Traces of phellandrene, pinene, and a sesquiterpene are present. The valuable portion of the oil is the remaining 10 per cent., consisting of oxygenated bodies, chiefly the aldehyde citral, C10H16O, to which the odour of the oil is largely due, and of which there is from 3.5 to 5 per cent. present in the oil (U.S.P., not less than 4 per cent.). It has a flavouring power nearly fifteen times greater than the oil; specific gravity, 0.895 to 0.899; boiling-point, 224° to 228°; on reduction it yields the alcohol geraniol. Other oxygenated constituents are citronellal, C10H18O, geranyl acetate, C12H20O2, about 1 per cent., and, in Palermo oil, linalyl acetate, C12H20O2. From the stearoptene found in the oil two crystalline substances have been isolated, one a yellow crystalline body C14H14O6, melting-point, 115°; the other a white crystalline powder, C10H10O4, melting-point, 144°. Terpeneless oil of lemon has a specific gravity, 0.895 to 0.900; optical rotation, -5° to -8°; and contains 40 to 50 per cent. of citral.
Action and Uses.—Oil of lemon has the stimulant and carminative properties of the volatile oils generally. It is, however, seldom employed for its medicinal properties, but almost entirely as a flavouring agent. Oil of lemon is also known as Essence of Lemon. For culinary purposes an alcoholic solution (1 in 10) of the oil is sometimes sold as "Essence of Lemon," but this should be distinguished as "Prepared Essence of Lemon." A 1 per cent. solution of the terpeneless oil in alcohol is also used for this purpose.
Dose.—¼ to 2 decimils (0.025 to 0.2 milliliters) (½ to 3 minims).
- Essentia Limonis, C.F.—ESSENCE OF LEMON.
- Oil of lemon, fresh, 2.5; lemon peel, freshly grated, 2.5; alcohol (95 per cent.), 70; distilled water, 30; magnesium carbonate, 1.25. The oil of lemon and lemon peel are well triturated with the magnesium carbonate, and the alcohol and distilled water, previously mixed, are slowly added with continuous trituration; the mixture is allowed to macerate for twenty-four hours, filtered, and the filtrate made up to 100 with a mixture of 7 of the alcohol and 3 of the water.