Oleum Rusci. Birch Tar Oil.

Related entries: Methyl salicylate - Oil of Gaultheria - Oil of Betula

Synonyms.—Birch Tar; Oleum Betulae Affix; Oil of White Birch; Oleum Rusci Pyroligneum; Oleum Betulae Pyroligneum.

Birch tar oil is prepared in Russia by the destructive distillation of the wood and bark of Betula alba, Linn. (N.O. Betulaceae). Russian leather owes its characteristic odour to the above oil, which is used in its preparation. It occurs as a thick liquid, resembling wood tar in colour, and having a peculiar, agreeable, penetrating odour. Specific gravity, 0.926 to 0.955 at 20°. The oil does not harden on exposure to the air. It can be distinguished from other varieties of tar, which may be substituted for it, by the following test:—Shake a few drops of the tar with water, and filter; the solution thus obtained will give a pink colouration with potassium cyanide solution, intensified on the addition of ammonia. No such reaction is obtained in the case of the Dutch and German varieties. The aqueous solution, obtained by shaking 1 part with 10 parts of water, is almost colourless, and has an acid reaction. With this solution a trace of ferric chloride produces a green colour. Five mils of the aqueous solution with 2 or 3 drops of aniline, and about 5 drops of hydrochloric acid gives a yellow mixture. If adulterated with fir tar or other kinds a red mixture is obtained. Shaken with twenty volumes of benzol, the latter assumes only a pale yellow colour. This benzol solution, on being shaken with a weak aqueous solution (about 1 in 1000) of copper acetate, should not assume a greenish colour (absence of fir tar). A rectified birch tar oil is in commerce, obtained from the crude tar by steam distillation, and is official in the Dutch Pharmacopoeia, where the specific gravity is given as 0.900 to 0.920. It is a light brown oil, consisting chiefly of guaiacol and cresol, with smaller quantities of creosol and xylenol, and probably traces of phenol.

Soluble in oils, fats, or chloroform; partially soluble in alcohol.

Constituents.—Birch tar oil contains, amongst other constituents, guaiacol, cresol, and pyrocatechol (C₆H₆O₂).

Action and Uses.—Birch tar oil resembles oil of cade in its properties, and is used for external application in the form of ointment (10 per cent.) or soap (10 per cent.) for eczema, psoriasis, and other skin affections. Mixed with essential oils it is used to keep away mosquitoes.

PREPARATIONS.

Liquor Rusci, B.P.C.—SOLUTION OF BIRCH TAR OIL.

Birch tar oil, 10; quillaia bark in No. 20 powder, 10; alcohol to 100.

Unguentum Rusci Compositum, B.P.C.—COMPOUND BIRCH TAR OINTMENT.

Birch tar oil, by weight, 8; resorcin, 2; zinc oxide, 24; starch, 24; hydrous wool fat, 20; soft paraffin, white, to 100.

The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.