1911 Encyclopædia Britannica/Picric Acid

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PICRIC ACID, or Trinitrophenol, C6H2·OH·(NO2)3 [1·2·4·6], an explosive and dyestuff formed by the action of concentrated nitric acid on indigo, aniline, resins, silk, wool, leather, &c. It is the final product of the direct nitration of phenol, and is usually prepared by the nitration of the mixture of phenol sulphonic acids obtained by heating phenol with concentrated sulphuric acid (E. Eisenmann and A. Arche, Eng. pat., 4539 (1889). It may also be obtained by oxidizing the symmetrical trinitrobenzene with potassium ferricyanide in alkaline solution (P. Hepp, Ann. 1882, 215, p. 352). It Crystallizes from water in yellow plates melting at 122·5° C., which sublime on careful heating, but explode when rapidly heated. It is poisonous and possesses a bitter taste, hence its name from the Greek πικρός, bitter. It has a strongly acid reaction, being almost comparable with the carboxylic acids. By the action of bleaching powder it is converted into chlorpicrin, CCl3·NO2. Phosphorus pentachloride converts it into picryl chloride, C6H2Cl(NO2)3, which is a true acid chloride, being decomposed by water with the regeneration of picric acid and the formation of hydrochloric acid, with ammonia it yields picramide, C5H2NH2(NO2)3. Silver picrate and methyl iodide yield the methyl ester, which gives with ammonia picramide. Picric acid forms many well-defined salts, of a yellow or red-brown colour. It also yields crystalline compounds with many aromatic hydrocarbons and bases. It imparts a yellow colour to wool and silk. The chief application of picric acid and its salts is in the manufacture of explosives. When ignited, picric acid burns quietly with a smoky flame, and it is very difficult to detonate by percussion, its salts, however, are more readily detonated. The more important picric powders are melinite, believed to be a mixture of fused picric acid and gun-cotton; lyddite, the British service explosive, and shimose, the Japanese powder, both supposed to be identical with the original melinite; Brugère's powder, a mixture of 54 parts of ammonium picrate and 45 parts of saltpetre, Designolle’s powder, composed of potassium picrate, saltpetre and charcoal, and emmensite, invented by Stephen Emmens, of the United States.

It may be detected by the addition of an aqueous solution of potassium cyanide, with which it gives a violet-red coloration, due to the formation of isopurpuric acid R. Anschutz (Ber., 1884, 17, p. 439) estimates picric acid by precipitation with acridine.