Oil of peppermint is obtained by distillation from the fresh, flowering peppermint, Mentha piperita, Sm. (N.O. Labiatae), a plant found growing wild throughout Europe, and cultivated in England, France, Italy, Germany, and America. It is also official in the U.S.P. Both the black and white varieties are used, the former yielding more oil than the latter, but of a less delicate aroma. Oil of peppermint occurs as a colourless, pale yellow or greenish-yellow liquid when recently distilled, but becomes darker and thicker on keeping, and has the characteristic odour of the herb, and a strong aromatic taste followed by a sensation of coldness in the mouth, caused by the menthol it contains. Specific gravity, 0.900 to 0.920 (0.894 to 0.94 at 25°); rotation, -18° to -35°. The oil of Mentha piperita may be distinguished as a rule from that of Mentha arvensis by the following colour reaction:—Two decimils (0.2 milliliters) of American or English peppermint oil mixed with 1 mil of glacial acetic acid will produce a blue colouration after a few hours, reaching its maximum intensity in about twenty-four hours; by reflected light a reddish or copper-coloured fluorescence will be noticed. Japanese oil does not give this reaction, the mixture remaining colourless. It is due to a nitrogen-free volatile body in the oil, and is considered an oxidation phenomenon. Oil which has been much exposed to sunlight will not give the reaction. Besides the specific gravity and optical rotation, the only really useful methods of analysis are the determination of the menthol, both free and as esters, and of the menthone, the latter by reducing it to menthol by means of sodium in a solution of the oil in alcohol. Adulteration with dementholised oil, known as menthene, is frequently practised. The odour and taste afford a good indication of the quality of the oil, and by this means it is quite possible to distinguish between English, American, and Japanese oils.
Soluble in alcohol (2 in 1), the solution sometimes becoming turbid on adding more of the solvent; soluble in all proportions of absolute alcohol. The rectified oil is soluble in 4 volumes of alcohol (70 per cent).
Constituents.—The chief constituent of the oil is menthol, but it also contains menthyl acetate, C12H22O2, and isovalerate, together with menthone, C10H18O, cineol, inactive pinene, l-limonene, cadinene, phellandrene, acetic aldehyde and acid, isovaleric aldehyde and acid, amyl alcohol, and dimethyl sulphide. On cooling to a low temperature, separation of menthol occurs, especially if a few crystals of that substance be added to start crystallisation. The English oil contains 60 to 70 per cent. of menthol, the American oil less (U.S.P., not less than 6 per cent. of ester calculated as menthyl acetate, and not less than 50 per cent. of total menthol), while the Japanese and Chinese oils (specific gravity, 0.895 to 0.905) obtained from Mentha arvensis, DC., var. piperascens et glabrata, Holmes, are the richest of all in menthol, sometimes containing 85 per cent.
Action and Uses.—Oil of peppermint is an aromatic stimulant, and carminative to the gastro-intestinal tract. It relieves gastric and intestinal flatulence and colic, and is employed with purgatives to prevent griping. The oil may be given on sugar, or as peppermint water or spirit of peppermint in mixtures. It is also added to pills, 1/4 to 1 minim in each, a little soap being used to facilitate massing, if necessary. The oil acts as a local anaesthetic; Japanese oil (Po-ho-yo) is employed to relieve toothache and to paint over neuralgic areas. Oil of peppermint has mildly antiseptic properties, and is used to flavour dentifrice pastes, powders, and washes.
Dose.—1/4 to 2 decimils (0.025 to 0.2 milliliters) (1./2 to 3 minims).
- Aqua Menthae Piperitae, B.P.—PEPPERMINT WATER.
- Oil of peppermint, 0.1; water, 150. Add the oil to the water and distil 100. Peppermint water forms a convenient vehicle for mixtures. Its carminative properties are useful in association with stomachics and purgatives such as rhubarb and magnesium sulphate, Dose.—30 to 60 mils (1 to 2 fluid ounces).
- Aqua Menthae Piperitae, U.S.P.—PEPPERMINT WATER, U.S.P.
- Oil of peppermint, 0.2; purified talc, 1.5; distilled water, to 100. Average dose.—16 mils (4 fluid drachms).
- Aqua Menthae Piperitae Concentrata, B.P.C.—CONCENTRATED PEPPERMINT WATER.
- One part of this solution corresponds to 40 parts of peppermint water.
- Emulsio Menthae Piperitae, B.P.C.—EMULSION OF PEPPERMINT.
- Oil of peppermint, 10; tincture of quillaia, 3.75; distilled water, to 100. Contains the same proportion of oil of peppermint as Spiritus Menthae Piperitae. Used for stock mixtures where it is desirable to effect an economy in the consumption of alcohol. Dose.—3 to 12 decimils (0.3 to 1.2 milliliters) (5 to 20 minims).
- Spiritus Menthae Piperitae, B.P.—SPIRIT OF PEPPERMINT. Syn.—Essence of Peppermint.
- Oil of peppermint, 10; alcohol, sufficient to produce 100. Spirit of peppermint is used as a flavouring agent and carminative. Dose.—3 to 12 decimils (0.3 to 1.2 milliliters) (5 to 20 minims).
- Spiritus Menthae Piperitae, U.S.P.—SPIRIT OF PEPPERMINT, U.S.P.
- Oil of peppermint, 10; peppermint, bruised, 1; alcohol (95 per cent.), sufficient to produce 100. the oil is dissolved in 90 of the alcohol, and in this solution the peppermint is macerated for twenty-four hours, the product filtered, and made up to 100 with alcohol (95 per cent.). Average dose.— 2 mils (30 minims).
- Syrupus Menthae Piperitae, B.P.C.—SYRUP OF PEPPERMINT. 1 (spirit) in 8.
- Dose.—2 to 8 mils (1/2 to 2 fluid drachms).
The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.