Dhristi JEE 2022-24

An Introduction to Picric Acid

You might have seen different kinds of acids and might have used diluted forms of some of these acids in your school laboratory. But have you heard about an acid called Picric Acid? If not, then you will get to learn everything about Picric Acid in this article. You will study its chemical formula, chemical name, structure, its preparation and its different uses. This will help you in the exam preparation as you can read all the concepts related to Picric Acid from this article. Picric acid is an important topic and so many questions are asked about this topic. So, you need to prepare thoroughly about the topic of Picric Acid. 


Picric acid can be described as an organic compound whose formula is (O2N)3C6H2OH and the IUPAC name of this acid is 3C6 2,4,6-trinitrophenol (TNP). Due to its bitter taste, the name "picric" comes from the Greek word (pikros), which means "bitter."


Among all the Phenols, Picric Acid is known to have the highest acidity. When percussion or rapid heating is applied to it (or its salts containing heavy metals like copper, silver, or lead), it can explode. 


In 1771, a British scientist named Peter Woulfe created picric acid by mixing indigo with nitric acid. Beginning in 1849, it was employed as a yellow dye, initially for silk. Picric acid was once highly valuable as an explosive and was first used as a bursting charge for shells by the French in 1886, under the name melinite. Picric acid was the most extensively utilized military explosive by the time of the Russo-Japanese War. It's very corrosive action on the metal surfaces of shells, on the other hand, was a drawback, and its use dropped after World War I.


Modern armor-piercing shells use ammonium picrate, one of the salts of picric acid, because it is insensitive enough to tolerate the strong shock of penetration before detonating. Picric acid is both antibacterial and astringent. It's used in medical applications as a surface anesthetic ointment or solution, as well as burn ointments. Picric acid is a significantly stronger acid than phenol, capable of decomposing carbonates and titrating with bases. Lead acetate creates a vivid yellow precipitate, lead picrate, in basic media.


Picric acid, also known as 2,4,6-trinitrophenol (chemical name of picric acid), is a crystalline solid that is pale yellow and odorless 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 color 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 color results. The red color 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 color 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.


Picric Acid Preparation

  • You can prepare Picric Acid if you sulfonate phenol and then treat it with nitric acid or you can treat benzene with nitric acid and mercuric nitrate by nitration of 2-tert-butyl-4,6-dinitrophenol. 

  • Another form of preparation of Picric Acid is the nitration of phenolsulfonic acid obtained by heating phenolsulfonic acid. 


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. 


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FAQs on Picric Acid

1. Is picric acid dangerous?

If ingested, inhaled, or absorbed through the skin, picric acid is poisonous. Dust inhalation can cause lung damage. Chronic exposure can damage the liver and kidneys.

2. Is phenol or picric acid more acidic?

Picric acid has a higher acid strength than phenol. Picric acid, also known as 2,4,6-trinitrophenol, comprises three heavy electron-withdrawing NO2 groups. As a consequence, the oxygen atom attracts O – H bound electrons to it. As a result, the O–H bond becomes more polar and weak.

3. Which is more acidic picric acid or benzoic acid?

Since picric acid contains three NO2 groups, which are electron-withdrawing groups, it is more acidic than benzoic acid. The presence of an electron-withdrawing group on a ring raises the acidic character, while the presence of an electron-donating group on the ring reduces it.

4. What are the different uses of Picric Acid?

There are a lot of uses of Picric Acid. It can be used in the preparation of various types of explosives. In metallurgy, it can be used to chisel alloys like AZ31. Red isopurpurate can be prepared from Picric Acid as well. It can also be used as a temporary marker for the skin. In the past, Picric Acid was used to treat burns and as an antiseptic and was sometimes used to measure glucose levels in the blood. 

5. How can Picric Acid be prepared?

2,4,6-trinitrophenol or Picric acid is prepared by reacting concentrated nitric acid and concentrated sulphuric acid with phenol. When phenol is made to react with these two acids, the ortho and para positions on phenol get attacked by the nitro group of nitric acid. The addition of nitro groups on the ortho and para positions of phenol results in Picric Acid and it becomes more stable because of these two positions. 

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