Picric Acid (Gr. , bitter; called also carbazotic, trinitrophenic, and nitrophenisic acid), a frequent product of the action of nitric acid upon complex organic substances. Carbolic acid, salicine, coumarine, phloridzine, silk, indigo, and a variety of resins yield it when treated with fuming nitric acid. The most economical raw material is the tar oil which distils over between 302° and 392° F. It can also be conveniently made from carbolic acid. In preparing it on a large scale, Grace Calvert allows 7 lbs. of carbolic acid to fall drop by drop into 42 lbs. of nitric acid of specific gravity 6.52. After the lapse of about 36 hours, the whole of the carbolic acid having been introduced, heat is applied, and the acid liquor is concentrated to one fourth of its bulk; on cooling it becomes solid. It is dissolved in water and allowed to crystallize. Picric acid crystallizes in long, pale, yellow, brilliant, rectangular plates, soluble with difficulty in cold, readily in hot water, and also soluble in alcohol, ether, and benzole. The acid has an intensely bitter taste, which has led some persons fraudulently to substitute it in beer for a portion of the hops.
It is employed for dyeing wool and silk yellow; and with aniline green, indigo, and Berlin blue, it is used for dyeing silk and wool green. In France extensive use is made of the acid for the manufacture of the picrate gunpowder.