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Picric acid

Picric acid is an organic compound with the formula (O2N)3C6H2OH. Its IUPAC name is 2,4,6-trinitrophenol (TNP). The name "picric" comes from Greek: πικρός (pikros), meaning "bitter", due to its bitter taste. It is one of the most acidic phenols. Like other strongly nitrated organic compounds, picric acid is an explosive, which is its primary use. It has also been used as medicine (antiseptic, burn treatments) and as a dye.

Picric acid was probably first mentioned in the 17th-century alchemical writings of Johann Rudolf Glauber. Initially, it was made by nitrating substances such as animal horn, silk, indigo, and natural resin, the synthesis from indigo first being performed by Peter Woulfe in 1771.

The German chemist Justus von Liebig had named picric acid Kohlenstickstoffsäure (rendered in French as acide carboazotique).[citation needed] Picric acid was given that name by the French chemist Jean-Baptiste Dumas in 1841. Its synthesis from phenol, and the correct determination of its formula, were accomplished during 1841. In 1799, French chemist Jean-Joseph Welter (1763–1852) produced picric acid by treating silk with nitric acid; he found that potassium picrate could explode. 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.[citation needed]

A theory to explain why picrate salts detonated whereas picric acid itself didn't, was proposed by the French chemists Antoine Fourcroy and Louis Vauquelin in 1806 and reiterated by the French chemist Michel Chevreul in 1809. Picric acid evidently contained enough oxygen within itself — i.e. it was "super-oxygenated" (suroxigéné) — to combust completely even in the absence of air (because even in the absence of air, heat could transform it completely into gases, leaving no carbon). However, when picric acid was burned, the heat that was generated caused some of the acid to evaporate, dissipating so much heat that only burning, not detonation, occurred. In contrast, picrate salts were solids that did not sublimate, thus did not dissipate heat; hence, they did detonate.

In 1871 Hermann Sprengel proved it could be detonated at the gunpowder works of John Hall & Sons in Faversham in Kent, England. Sprengel filed patents in Britain for "safety explosives" (i.e., stable explosives) on April 6, 1871 (no. 921),[citation needed] and on October 5, 1871 (no. 2642); in the latter patent, Sprengel proposed using picric acid dissolved in nitric acid as an explosive.[citation needed][page needed] Afterwards most military powers used picric acid as their main high explosive material.[citation needed] A full synthesis was later found by Leonid Valerieovich Kozakov.[citation needed]

Picric acid was the first strongly explosive nitrated organic compound widely considered suitable to withstand the shock of firing in conventional artillery. Nitroglycerine and nitrocellulose (guncotton) were available earlier, but shock sensitivity sometimes caused detonation in an artillery barrel at the time of firing. 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. [citation needed]

In 1887 the French government adopted a mixture of picric acid and guncotton with the name Melinite. In 1888, Britain started manufacturing a very similar mixture in Lydd, Kent, with the name Lyddite. Japan followed with an alternative stabilization approach known as Shimose powder which, instead of attempting to stabilize the material itself, removed its contact with metal by coating the inside of the shells with layer(s) of resin and wax.

By 1894 Russia was manufacturing artillery shells filled with picric acid. However, shells filled with picric acid become unstable if the compound reacts with the metal shell or fuze casings to form metal picrates which are more sensitive than the parent phenol. The sensitivity of picric acid was demonstrated by the Halifax Explosion.[citation needed]

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