- ''"TNP" redirects here. For the airport with the IATA airport code TNP, see Twentynine Palms Airport.
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Picric acid is the chemical compound more formally called 2,4,6-trinitrophenol (TNP). This, a yellow crystalline solid, is one of the most acidic phenols. Like other highly nitrated compounds such as TNT, picric acid is an explosive. Its name comes from Greek πικρος—"bitter", reflecting the bitter taste of picric acid.
Picric acid was probably first mentioned in the alchemical
writings of Johann Rudolf Glauber
in 1742. Initially, it was made by nitrating
substances such as animal horn, silk
, and natural resin
. Its synthesis from phenol
, and the correct determination of its formula, were successfully accomplished 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 Hermann Sprengel
proved it could be detonated and by 1894 the Russian workers had worked out a method of manufacture for artillery shells. Soon after, most military
powers used picric acid as their primary high explosive material
. However, shells filled with picric acid become highly unstable as the compound corrodes bomb casings to form metal picrates
which are more sensitive than the parent phenol. The sensitivity of picric acid was demonstrated in the Halifax Explosion
. Picric was used in World War I , but the 20th century saw picric acid largely replaced by TNT
. Picric acid is also used in the analytical chemistry of metals, ores, and minerals.
In 1885, based on research of Hermann Sprengel, French chemist Eugène Turpin patented the use of pressed and cast picric acid in blasting charges and artillery shells. In 1887 the French government adopted it under the name melinite, with addition of gun cotton. Since 1888, Britain started manufacturing a very similar mixture in Lydd, Kent, under the name lyddite. Japan followed with an improved formula known as schimose. In 1889, a similar material, a mixture of ammonium cresylate with trinitrocresol, or an ammonium salt of trinitrocresol, started to be manufactured under the name ecrasite.
The aromatic ring of phenol
is highly activated towards electrophilic reactions, and attempted nitration of phenol, even with dilute nitric acid, results in the formation of high molecular weight tars. In order to minimize these side reactions, anhydrous phenol is sulphonated with oleum
, and the resulting p-phenolsulfonic acid is then nitrated with concentrated nitric acid
. During this reaction, nitro groups are introduced, and sulfonic acid groups are displaced. The reaction is highly exothermic, and careful temperature control is required.
By far the largest use has been in munitions
and explosives, as discussed above.
In microscopy, picric acid is a reagent for staining samples, e.g. Gram staining. It has found some use in organic chemistry for the preparation of crystalline salts of organic bases (picrates) for the purpose of identification and characterization.
Bouin's picro-formol is a preservative solution used for biological specimens.
Workplace drug testing utilizes picric acid for the Jaffe Reaction to test for creatinine. It forms a colored complex that can be measured using spectroscopy.
Much less commonly, wet picric acid has been used as a skin dye or temporary branding agent. It reacts with proteins in the skin to give a dark brown color that may last as long as a month.
In the early 20th century, picric acid was stocked in pharmacies as an antiseptic and as a treatment for burns, malaria, herpes, and smallpox.
Modern safety precautions recommend storing picric acid wet. Dry picric acid is relatively sensitive to shock
, so laboratories that use it store it in bottles under a layer of water
, rendering it safe. Glass or plastic bottles are required, as picric acid can easily form metal picrate salts
that are even more sensitive and hazardous than the acid itself. Industrially, picric acid is especially hazardous because it is volatile and slowly sublimes even at room temperature. Over time, the buildup of picrates on exposed metal surfaces can constitute a grave hazard.
In 2007, a bomb squad removed a World War I-era bottle containing two ounces of picric acid in Columbus, Wisconsin. In another 2007 incident, a bomb squad detonated a sample in Hayward, California. In 2008, bomb disposal experts removed and later detonated a 200 g (7 oz) sample found at the Marling School (Stroud, Gloucestershire) science department. In the same year, a 1-lb. bottle of the acid was removed from an old pharmacy in Jerseyville, Illinois, and was later detonated by a bomb squad.
In the summer of 2008, German police blasted at least 50 picric acid bottles they had found in schools (34 bottles), pharmacies and industry.
The official justification given in news media was that the dry picric acid would be too dangerous to handle in order to add water for desensitization, and the picric acid might detonate when the bottles were opened.
- Cooper, Paul W., Explosives Engineering, New York: Wiley-VCH, 1996. ISBN 0-471-18636-8