Picric acid

Overview
Picric acid is the chemical compound more formally called 2,4,6-trinitrophenol. 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.

History
Picric acid was first 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, indigo, 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 power 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. 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 Eugene 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.

Synthesis
Picric acid can be made by nitration of phenol, benzene (in the Wolfenstein-Boters reaction), or even acetylsalicylic acid (aspirin).

Uses
By far the largest use has been in munitions and explosives, as discussed above.

In microscopy, picric is used as 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.

Other
Picric acid is well known as an easy 'homemade explosive', since its synthesis requires chemicals that can be bought at some convenience store or pharmacy&mdash;but many people have injured or killed themselves in the process. As well as the obvious explosive danger, highly toxic nitrogen dioxide gas is produced during its synthesis.

Safety
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 or plastic bottles are required, as picric acid can form metal picrate salts that are even more sensitive and hazardous than the acid. In a 2007 incident, pharmacy employees in Columbus, Wisconsin found a World War I-era bottle containing two ounces of the liquid, which required a visit from the bomb squad for safe disposal. Improper lab handling can also turn picric acid into a dangerous chemical as illustrated in another 2007 incident, where a bomb squad dispatched to a forensic crime laboratory in Hayward, California caused numerous civilians to be evacuated while the chemical was intentionally detonated.