Picric Acid

Picric acid is an organic compound with the chemical formula C₆H₃N₃O₇ or C₆H₂(NO₂)₃OH. It is a type of phenol. It is also called 2,4.6-Trinitrophenol (chemical name). It has a bitter taste, so it is named picric acid. The word picric is derived from the Greek word (pikros), which means "bitter".

It is known to be highly acidic among all the phenols. So, if it is hit or heated, it can explode. Its salts that contain heavy metals can also explode if heated or hit continuously.

Structure of Picric acid

It has a phenyl group to which three electron-withdrawing groups (NO₂) are attached at the ortho and para positions of phenol.

History of Picric Acid

Picric acid was created by British scientist Peter Woulfe in 1771. He created it by reacting indigo with nitric acid. In the mid of 19 century, initially, it was used as a yellow dye for silk.

Later, it was highly valued as an explosive called melinite and was used for the first time to aid the bursting of shells by the French in 1886. During the Russo-Japanese, it was the widely used explosive by the military.

In modern times, shells use ammonium picrate (a salt of picric acid) owing to its ability to tolerate the strong shock of penetration before detonating. Further, it has both antibacterial and astringent properties.

Properties of Picric Acid

• It is a yellow, odourless crystalline substance.
• It is less soluble in water. Its solubility is 14 g/l.
• It has a bitter taste.
• Its acidity is very high.
• Its boiling point is more than 300 degrees Celsius.
• Its melting point is 122.5 degrees Celsius.
• It forms metal picrates when reacts with metals.
• Its molar mass is 229.10 g/mol.
• Its density is 1.763 g/cm³.
• It is a highly flammable crystalline substance. Its explosive power is higher or equal to that of TNT.

Preparation of Picric Acid

It is mainly prepared from phenol. The phenol is reacted with concentrated sulphuric acid and concentrated nitric acid. The nitro groups of nitric acid get transferred to the ortho and para positions of phenol. These positions provide stability to the newly formed picric acid. The reaction is shown below:

The above process is completed in two steps. In this first step, the formation of phenol sulphuric acid takes place which in the second step reacts with conc. Nitric acid to form 2,4,6- trinitrophenol, which is called picric acid.

Uses of Picric Acid

Picric acid offers lots of uses, some of them are as follows:

• It is used in the making of explosives, matches, electric batteries, and more.
• It is used in etching copper and in the production of coloured glass.
• It is a component of analytical reagents used for blood and urine test.
• It is used to make crystalline salts of organic bases called picrates that help identify organic bases.
• It is present in bouin solution, a fixative solution for histology specimens. It improves the staining of acid dyes. However, it can hydrolyse and DNA present in the sample.
• Owing to its ability to form red isopurpurate (HCN) by reacting with hydrogen cyanide, it helps quantify hydrogen cyanide by measuring the dye produced in the reaction photometrically.
• In the early 20^th century, picric acid was used to measure glucose levels in the blood. Picric acid when heated with glucose and sodium carbonate produces characteristic red colour. This red colour can be used to measure glucose levels.
• Wet picric acid also can be used as a skin dye or a temporary marking agent owing to its ability to produce a dark brown color on skin that may last up to a month.
• It is used in antiseptics and disinfectants.
• It is also used in the pharma and leather industries.

Health Hazards

Picric acid is toxic in nature. If it is happened to swallow by someone or absorbed through the skin, it may cause health issues. For example, inhalation of its dust may cause lung damage and long-term exposure may affect the liver or kidney. So, while using picric acid one should wear protective clothing, avoid skin contact and use gloves, face masks and goggles to avoid inhalation of dust.

How to check Picric Acid Containers

The picric acid containers should be checked on a quarterly basis. Any trained person can do this. How one can self-inspect is described below:

• Check the presence of dried crystals outside of the bottle (near the cap). The dried crystals may be present within the threads of the container if the acid inside the bottle is not wet. The presence of these crystals poses a detonation risk while opening the container.
• Even if it appears safe to open the container and you open it rehydrate the contents with water to ensure crystals are wet.
• Check the details of manufacturing and expiry on the container and dispose of it as hazardous waste after 2 years of storage.

Storage Instructions

• The acid container should be labelled with the dates when it is received and when it is opened.
• The container should be stored away from heat or sources of heat and in a cool, dry and ventilated area.
• The solid should be wet: it should be under a layer of water.
• As it is classified as Stanford Chemical Storage Group X', it should be stored separately away from other chemicals.