Benzoic Kid (H, C1H5O2), an acid which is abundant in the balsamiferous plants, and is produced artificially from bitter-almond oil, hippuric acid, and naphthaline. Gum benzoin, the product of the styrax benzoin of the Asiatic archipelago, is the principal source of the supply of benzoic acid. Common benzoin occurs in reddish lumps, which sometimes have a la-mellated fracture, and certain whitish opaque masses. When recent it emits an odor of bitter almonds. Gum benzoin appears to be composed of a mixture of three varieties of resin, with benzoic acid and a small quantity of a fragrant essential oil. Only one of the resins is soluble in ether; a second is soluble in alcohol only. The white opaque masses appear to consist of the resin which is soluble in ether; they yield less benzoic acid than the brown portions. - Benzoic acid may be extracted from powdered benzoin by boiling it for some hours with milk of lime, filtering the solution of benzoate of lime from the insoluble compound of resin and lime, and, after concentrating the filtrate, adding hydrochloric acid. Benzoic acid is thus precipitated, and may be purified by sublimation.
The acid is, however, generally extracted by the less economical but simpler process of direct sublimation from gum benzoin, which contains 14 or 15 per cent, of the acid. If the resin be coarsely powdered and exposed to a temperature of about 302° F., the acid which exists ready formed in it is expelled, and may be condensed in suitable receivers. Mohr's plan of conducting the sublimation is the simplest and best. His method is to place the gum in a shallow iron pan, which is covered with a sheet of filtering paper, over which a cone or hat of writing paper is fastened; on applying a regulated sand heat, the acid is decomposed, and the benzoic acid is converted into vapors; it passes through the bibulous paper, and rises into the chamber formed by the paper cone, where it is condensed, and is prevented from falling back into the pan beneath by the interposed sheet of filtering paper. This method of sublimation is applicable in many other cases of a similar kind, as for example in the manufacture of pyrogallic acid. The resins of tolu and benzoin, when treated with boiling nitric acid, yield an amorphous form of benzoic acid, colored yellow with a resinous matter which accompanies it into its salts, and hinders them from crystallizing.
Balsam of tolu often yields nearly half its weight of this acid. This resinous acid is completely soluble in boiling water. When this form of the acid is exposed to the sun's rays, it becomes covered with white crystals of pure benzoic acid; and when sublimed, the ordinary crystalline acid is ob-tained. Benzoic acid is now prepared artificially on a large scale from naphthaline and from hippuric acid, and is employed in the treatment of tobacco, as a mordant in calico printing, and especially in the production of aniline colors. - Benzoic acid assumes the form of white, glistening, extremely light, flexible needles, which usually have an agreeable aromatic odor and a hot bitterish taste. The odor, however, is not due to the acid, but to the presence of a trace of essential oil which accompanies the acid during the sublimation. Benzoic acid melts at 248° F. (120° C.); it sublimes at 293° F. (145° C), and boils at 462° F. (239° C). Its vapors are acrid and irritating; when kindled in the open air, they burn with a smoky flame. The acid requires about 200 parts of cold water, and 25 of boiling water, for its solution; but it is readily dissolved by alcohol and by ether. Benzoic acid yields a series of salts called benzoates, mostly soluble in water.
The benzoate of ammonia is sometimes used as a means of separating iron from nickel and cobalt. - When prepared in the usual way by sublimation, benzoic acid contains a portion of the volatile oil. It is used in a few officinal preparations, especially in camphorated tincture of opium. When given internally, it is excreted by the urine, which it renders acid, in the form of hippuric acid. It has been employed as a local haemostatic, though without proved utility.