Oleum Betulae. Oil of Betula.
Synonyms.—Oleum Betulae Volatile; Oil of Sweet Birch.
Oil of betula is obtained by maceration and distillation from the bark of the sweet, black, or cherry birch, Betula lenta, Linn. (N.O. Betulaceae). It is official in the U.S.P. It does not pre-exist in the bark, but is formed by the interaction of the glucoside gaultherin, and a ferment, betulase. It occurs as a colourless or yellowish liquid, having a characteristic odour and taste, distinctly different from that of the otherwise nearly identical oil of gaultheria. Specific gravity, 1.179 to 1.187 (1.175 to 1.183 at 25°). Boiling-point, 218° to 221°. Optically inactive, thus differing from oil of gaultheria, which is slightly laevorotatory. It agrees generally with the other tests and reactions given under Oleum Gaultheriae and Methylis Salicylas. Oil of betula and oil of gaultheria are the heaviest oils known, and adulteration with foreign oils causes a lowering of the specific gravity, and lessens the solubility. It should be soluble in 6 volumes of alcohol (70 per cent.) at 25°.
Constituents.—The oil may contain 99.8 per cent. of methyl salicylate, which can be determined by saponification with alcoholic potash; a good oil should contain at least 98 per cent. of methyl salicylate.
Action and Uses.—Oil of betula has properties resembling those of oil of gaultheria, and is used for the same purposes.
Dose.—½ to 1 mil (8 to 15 minims).