Picric acid, also known as 2,4,6-trinitrophenol (chemical name of picric acid), is a crystalline solid that is pale yellow and odourless and has been used as a military explosive, and an antiseptic. Picric acid was named for the highly bitter taste of its yellow aqueous solution, by the 19th-century French chemist Jean-Baptiste-André Dumas. It (or its salts containing heavy metals like copper, silver, or lead) will explode when subjected to percussion or rapid heating.
In 1771, a British chemist named Peter Woulfe developed picric acid by treating indigo with nitric acid. Beginning in 1849, it was used as a yellow dye, initially for silk as the picric acid colour is yellow.
This article will study picric acid, picric acid structure, picric acid formula, and picric acid uses.
Picric Acid Structure
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Synthesis of Picric Acid
Attempts to nitrate phenol, also with dilute nitric acid, resulting in the formation of high molecular weight tars because the aromatic ring of phenol is triggered towards electrophilic substitution reactions. Anhydrous phenol is sulfonated with fuming sulfuric acid, and the resulting p-hydroxyphenyl-sulfonic acid is then nitrated with concentrated nitric acid to reduce side reactions.
Nitro groups are added during this reaction, and the sulfonic acid group is displaced. Since the reaction is highly exothermic, temperature regulation is important. Direct nitration of 2,4-Dinitrophenol with nitric acid is another way to produce picric acid.
Picric Acid Uses
The most common use has been in munitions and explosives. The ammonium salt of picric acid, also known as Dunnite, is explosive D. Dunnite is a more strong, but less stable, explosive than TNT (which is produced in a similar process to picric acid but with toluene as the feedstock). Picramide can be further aminated to create the very stable explosive TATB by aminating picric acid (typically starting with Dunnite).
Picral, a 4 per cent picric acid in ethanol etch typically used in optical metallography to expose prior austenite grain boundaries in ferritic steels, has been widely used in metallurgy. Picric acid has been increasingly substituted by other chemical etchants due to the risks associated with it. It is, however, still used to etch magnesium alloys like AZ31.
It's been used in organic chemistry to make crystalline salts of organic bases (picrates) to identify and characterize them.
Bouin solution is a popular fixative solution for histology specimens that contains picric acid. It enhances the staining of acid dyes, but it can also cause any DNA in the sample to be hydrolyzed.
Picric acid reacts with hydrogen cyanide to produce red isopurpurate (HCN). Picric acid can be used to quantify hydrogen cyanide by measuring the resulting dye photometrically.
Picric acid was used to measure blood glucose levels in the early twentieth century. When glucose, picric acid, and sodium carbonate are heated together, a distinctive red colour results. The red colour of a calibrating glucose solution can be used to quantify the glucose levels added. The Lewis and Benedict system of glucose calculation is what it's called.
Wet picric acid has been used as a skin dye or a temporary marking agent on a much smaller scale. It interacts with skin proteins to create a dark brown colour that can last up to a month.
Picric acid was used in hospitals in the early twentieth century as an antiseptic and treatment for burns, malaria, herpes, and smallpox. Picric acid-soaked gauze was also widely used as a burn remedy in first aid kits during that time span. It was famously used to treat burns suffered by victims of the 1937 Hindenburg disaster. During World War I, picric acid was also used to treat trench foot in soldiers serving on the Western Front.
Fly tyers have used picric acid to dye mole skins and feathers a dark olive green for use as fishing lures for many years. Its reputation has been offset by the fact that it is toxic.
Did You Know?
Picric acid should be stored wet to reduce the risk of explosion, according to modern safety measures. Since dry picric acid is vulnerable to shock and friction, it is stored in bottles under a layer of water in laboratories. Picric acid can easily form metal picrate salts, which are much more susceptible and dangerous than the acid itself, necessitating the use of glass or plastic bottles. Picric acid is extremely harmful in the workplace because it is highly flammable and slowly decomposes even at room temperature. Picrates on exposed metal surfaces will build up over time, posing an explosion danger.
Picric acid gauze, if contained in antique first aid kits, poses a safety risk because picric acid of that vintage (60–90 years old) may have crystallized and become unstable, and long storage in a metal first aid case may have resulted in the formation of metal picrates.
If picric acid has dried out, bomb disposal units are often called in to dispose of it. During the 1980s, an attempt was made in the United States to ban dried picric acid containers from high school laboratories.