Picric Acid

Picric Acid (Trinitrophenol), $C\_6H\_2(NO\_2)\_3OH$. This substance appears in the form of pale yellow crystalline scales. It is obtained by the action of nitric acid on phenolsulphonic acid. Equal parts of phenol and concentrated sulphuric acid are mixed together, and placed in a suitable vessel, which is heated till the mixture reaches $212^\backslash circ\; F.$ ($100^\backslash circ\; C.$). Nitric acid of the specific gravity 1.3 is then added. On cooling, a crystalline mass is produced which is filtered and drained. A washing with cold water follows, and then the picric acid is further purified by recrystallising it from water containing a small proportion (0.1 per cent.) of sulphuric acid. Picric acid is easily soluble in hot, but only slightly in cold water. It is also soluble in alcohol and ether. Its taste is intensely bitter, and its tinctorial power is very great, the solutions of it having a strong yellow colour (see DYEING, Vol. IV. p. 141). It has been much used for dyeing silk, wool, and leather. As it does not adhere by itself to vegetable fibre, it serves for a test to distinguish cotton from wool or silk. The presence of cotton in a mixed fabric can therefore be detected by steeping it in a hot solution of the acid, and afterwards washing it. Then, with the aid of a microscope, the difference between the wool or silk, both of which retain the dye, and the cotton, which does not, will be made clear. The salts of picric acid are a very important ingredient in explosives (see MELINITE, GUN-COTTON, SHELL). It was formerly called Carbazotic Acid.