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Picric acid is the common term for the chemical compound 2,4,6-trinitrophenol, also known as TNP; the material is a yellow crystalline solid. Like other highly nitrated compounds (eg. trinitrotoluene), picric acid is an explosive.
Modern safety precautions recommend storing picric acid wet. When picric acid is dry, it is relatively 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.
Modern safety precautions for plain picric acid are moderately overstated, as it was used successfully as an explosive filler for most artillery shells and bombs in the First World War. Those applications necessarily subject it to shock and friction during firing of the shells.
Properties
Picric acid can be made from phenol, benzene, or acetylsalicylic acid (aspirin). The most common route is through aspirin, since this is the most widely available of the three.
History
Picric acid was first mentioned in the alchemical writings of Glauber in 1742. Initially, it was made by nitrating substances such as animal horn, silk, indigo, and natural resin. Its synthesis from phenol, and the correct determination of its formula, was in 1841. Not until 1830 did chemists think to use picric acid as an explosive. Before then, chemists 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 (from Lydd, Kent, where UK work was carried out) 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.
See also
External links
Safety Information
References
Cooper, Paul W., Explosives Engineering, New York: Wiley-VCH, 1996. ISBN 0-471-18636-8
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