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Picric acid is the common term for the chemical compound 2,4,6-trinitrophenol; the material is a yellow crystalline solid. Like other highly nitrated compounds (eg. trinitrotoluene), picric acid is an explosive. When picric acid is dry, it is extremely sensitive to shock and friction, so laboratories that use it store it in bottles under a layer of water, rendering it safe. Glass bottles are required, as picric acid can form metal picrate salts that are even more sensitive and hazardous than the acid.
Chemistry Picric acid can be made from phenol, benzene, or acetylsalicylic acid (aspirin). The most common route is through aspirin, since it is much easier to synthesize picric acid from aspirin.
History Picric acid was first mentioned in the alchemical writings of Glauber in 1742. The old timers made it from nitrating animal horn, silk, indigo, natural resin, and the like. Its synthesis from phenol, and the correct determination of its formula, was in 1841. It was not until 1830 did anybody think to use picric acid as an explosive. Before then it was assumed that only the salts of picric acid were explosive, not the acid itself. In 1873 H. Sprengel proved it could be detonated and by 1894 the Russians had worked out a method of manufacture for artillery shells. Soon after, every military power used picric acid as their primary high explosive material. The 20th century saw the decline of picric acid, the replacement being TNT. Shells filled with picric acid become highly unstable as the compound reacts with the metal bomb shell, forming extremely sensitive metal picrates, making them unusable. Today picric acid is more suited to detonators or booster charges. It is also used in the analytical chemistry of metals, ores, and minerals.
Uses By far the largest use has been in munitions and explosives; it was known in World War I as Lyddite or Melinite.
The principal laboratory use is in microscopy, where it is used as a reagent for staining samples, e.g. Gram staining.
Bouin's picro-formol is a preservative solution used for biological specimens.
Picric acid was one of the agents in the Halifax explosion.
Much less commonly, wet picric acid has been used as a skin dye or temporary branding agent. It is not acidic enough to cause chemical burns, and reacts with proteins in the skin to give a dark brown color that may last as long as a month.
Other Picric acid is well known as another easy 'homemade explosive,' since its synthesis requires chemicals that can be bought at the store. Many people have injured or killed themselves during the synthesis of picric acid, since its synthesis with acetylsalicylic acid produces the deadly nitrogen dioxide gas.
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