PICRIC ACID, or TRINITROPHENOL, C4H2OH(NO.),(1, 2, 4, 6), was discovered by Woulfe in 1771. It is formed by the action of nitric acid on indigo, wood, silk, leather, aloes, resins, and many derivatives of benzene. On a commercial scale picric acid is manufactured from phenol by first treating it with sulphuric acid. The sulphonic acid produced in this way is diluted with water to 30° Be. and heated with steam. Phenol para-sulphonic acid is pro duced under these conditions which yields picric acid when it is nitrated with about 3.5 times its weight of nitric acid of Be. The crys tals obtained on cooling the reaction mixture are washed, and further purified by recrystalli zation from hot water. A product of high purity may be obtained by converting the com pound into the sodium salt and reprecipitating it with excess of hydrochloric acid. Very pure picric acid has been obtained directly by the nitration and subsequent hydrolysis of chloro benzene, the reagents used being crude sodium nitrate and commercial sulphuric acid. Picric acid may also be prepared by the oxidation of trinitrobenzene, or, the nitration of diazoben zene sulphonic acid.
Picric acid is a yellow, crystalline sub stance with a melting point of C. With many metals it forms salts which are highly explosive. It is not very soluble in cold water, but will readily dissolve in alcohol, ether, ben zene and other solvents. Solutions of picric acid, especially those of its alkaline salts, show an intense yellow coloration. Picric acid is a violent explosive but does not readily explode on mere heating. Under ordinary conditions the application of heat may either cause rapid combustion, or sublimation with the liberation of yellow, irritating vapors. The acid will ex
plode with violence when dropped upon a red hot plate. Disastrous explosions in picric acid factories are on record. These have in all probability been caused by metallic picrates present as impurities in the acid, or, by the combustion of picric acid dust produced during the grinding of the dry compound. The acid can be handled with perfect safety, and can even be transported as an ordinary chemical when it contains 20 per cent of water.
Picric acid is extensively employed as a powerful explosive. Melinite, Lyddite, Shimo site, Emmensite, etc., are mixtures of the acid with small quantities of gun-cotton. On ac count of their great sensitiveness metallic pic rates are sometimes mixed with chlorates or nitrates and employed as detonating charges for shells. Picric acid has been used as a dye for silk and wool. In an acid bath it imparts an intense yellow color to these fibres. Although the acid is not used as a dye at the present time, it is sometimes mixed with other dyes for the production of shades of scarlet, green or yellow. In the laboratory picric acid is a valu able reagent since it forms well crystallized and difficultly soluble compounds with a number of organic bases. Picric acid has also been used by brewers for the purpose of imparting bit terness to beer. In large doses the acid behaves as a poison, causing purging, vomiting and col lapse, although small quantities have been em ployed with success in the treatment of eczema, erysipelas and in diseases of the skin and hair. Solutions of varying strengths have also been highly recommended in the treatment of burns.