Picric acid and picrate salts

Authors: CANUTEC Emergency Response Advisors

CANUTEC receives many calls involving old forgotten bottles of picric acid. Picric acid is stable when dissolved in solvents, like water. But when the mixture dries out, picric acid crystals are highly explosive. Understanding the dangers of picric acid and storing it correctly are crucial to protecting any environment where picric acid is present.

This article summarizes information on this topic gathered by CANUTEC Emergency Response Advisors.

On this page

• Introduction
• Potential hazards
• Handling
• Storage
• First aid
• Fire response
• Spill response
• Disposal
• Conclusion

Introduction

Picric acid, or 2,4,6-trinitrophenol is one of the most dangerous chemicals being used today in chemistry labs.

Discovered in 1771 by British chemist Peter Woulfe, the chemical got its name from the Greek word “pikros”, which means bitter due to its bitter taste. Classified as a flammable solid when wetted with more than 30% water (UN1344 / Class 4.1) and an explosive with less than 30% water (UN0154 / Class 1.1D), picric acid has some very interesting properties and is used in many fields like chemistry, medicine, and manufacturing. During World War I, picric acid was used in bombs and grenades, due to its similarities to TNT.

There have been several catastrophic events that involve picric acid. The substance played a major role in the Halifax Explosion. In 1917, an ammunition ship in Halifax, Nova Scotia carrying tons of picric acid and thousands of pounds of TNT caught fire and exploded. Over 1,900 people were killed, and 9,000 were injured.

Due to its intense bright yellow colour, picric acid is mostly used to make explosives and dyes. It's present in many chemistry labs where it's used as a chemical reagent. Water is added to picric acid to act as a desensitizer. The wetted product is significantly less shock sensitive than the dry acid. Picric acid is highly reactive with a wide variety of chemicals and forms explosive picrate salts easily. Many of these salts are even more reactive and shock sensitive than the acid itself.

While it's useful in many contexts, the substance also poses major hazards if mishandled or stored improperly.

Potential hazards

Picric acid is a highly shock, heat, and friction sensitive explosive. High temperatures or sudden impacts can lead spontaneously explode. It's also considered a strong acid that reacts with common metals (except tin and aluminum), bases, ammonia, and concrete to produce picrate salts.

Pay particular attention if picrate salts forms during normal use of picric acid. Picric acid must never be allowed to dry out but even more importantly, it should never be allowed to dry out on metal or concrete surfaces. Metal picrates are particularly sensitive and can be formed with metals like copper, nickel, lead, iron and zinc. Calcium picrate is formed when picric acid interacts with concrete.

Old bottles of picric acid pose 2 main hazards:

• the picric acid, which usually exists as a wetted paste, may dry out and form shock-sensitive acid crystals, and
• impurities can be introduced into bottles that have been previously opened and are no longer airtight

Both these situations could form picrate salts inside the bottle or within the threads of the lid. Either of these situations are extremely dangerous and must be handled correctly.

Picric acid is toxic by all routes of entry (ingestion, inhalation, and absorption). It's also a skin irritant and allergen and will produce toxic products on decomposition. Picric acid is also corrosive to skin and eyes.

Handling

Before handling and working with picric acid:

• make sure the tools used to handle the chemical are cleaned and dried before handling a different chemical
• never use metal tools to handle picric acid or its derivatives
• double-check the expiry date and labels on the bottle before handling. If you don't see water, see any dried crystals or the material appears dry inside the bottle, do not open or handle the container

Storage

Properly storing picric acid is key to preventing incidents. Chemical manufacturing companies recommend that picric acid be disposed 2 years after opening a bottle. They also propose the following storage guidelines:

• store in a cool dry place away from heat sources, open flame, direct sunlight, and moisture. High temperatures can increase the risk that the product breaks down and forms crystals, which will make it unstable
• inspect the bottles and add water every 6 months (as needed)
• gently rotate bottles to distribute water every 3 months

Chemical inventory should include full identification of the product, the quantity on hand and expiration dates where applicable. The shelf life of picric acid can be significantly extended by regularly adding water to the container. This prevents the product from drying out and inhibits the formation of explosive picrate crystals.

Picric acid must be stored with at least 10% moisture content and must be inspected regularly to make sure its minimum moisture content is maintained. If the picric acid is allowed to dry out to less than 10% water by volume, it will become unstable and could explode. Never let picric acid dry out.

First aid

For moderate to serious exposures, call 911 or the Poison Control Centre in your area and get medical treatment.

Picric acid is toxic by all routes of exposure, is a skin irritant and an allergen. Eye contact with picric acid can cause irritation and possible damage to the eye. Exposure to high concentrations can result in yellow staining of the skin and hair and in some cases yellow tinted vision. Inhalation or ingestion of picric acid can cause dizziness, headaches, stomach upset, and ulcers.

• Administer oxygen if breathing is difficult and bring the victim to fresh air
• Remove and isolate contaminated clothing and shoes
• For skin or eye contact, avoid spreading material on unaffected skin. Remove product immediately by flushing skin or eyes with running water for at least 20 minutes

Fire response

Dry picric acid may explode if exposed to heat, flame, friction, or shock, and should be treated as an explosive.

If the product is exposed to fire, it could release carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and other toxic, irritating chemicals. Nitrous oxide gases are toxic if inhaled and will irritate the respiratory tract. They can cause severe irritation, burns, and even organ damage. Prolonged exposure may lead to more serious health effects, including liver and kidney damage. Adequate respiratory protection, such as a self-contained breathing apparatus (SCBA), is required when exposed to these toxic gases. Closed containers may rupture violently due to rapid decomposition if exposed to fire or excessive heat for long enough.

In most cases, contact your local fire department and evacuate the area.

• From a distance, apply a large quantity of water to the fire to reduce the heat
• Do not use dry chemical fire extinguishing agents or halogenated extinguishing agents
• Runoff water should be contained

Spill response

For technical advice, call CANUTEC at 1-888-226-8832 (Canada and US only), 613-996-6666, or *666 (cellular only). For large spills, call 911 immediately.

Cleaning up a picric acid spill requires care due to the chemical's dangerous nature. For small spills, do not allow spilled material to dry. Dampen the product with water to reduce the hazard of cleanup. Do not attempt to sweep up dry material. Spilled product can be contained using a compatible absorption pad. Contaminated pads should be placed in a compatible container and submerged in water. Thoroughly wash the spill site after material pickup is complete.

Disposal

If you discover old or previously unaccounted for bottles of picric acid:

1. first and foremost: do not touch the bottle. Dry picric acid or picrate salts should not be touched or moved under any circumstances
2. immediately secure the area and restrict access
3. hidden crystals may have formed within the threads of the bottle's lid. Any attempt to open the bottle could result in enough friction to produce an explosion large enough to blow up a small lab
4. visually inspect the bottle for product identification and check for an expiration date. If the product is relatively new, there may not be a problem. Nevertheless, treat the situation carefully
5. inspect the contents of the bottle to determine water content and check for signs of crystallization inside the bottle and around the lid. If there is no evidence of crystal formation and the water content is high, there's probably little cause for concern. If there's even the slightest indication of crystallization or low levels of water in the bottle, the situation could be more serious. A local bomb disposal service or an explosive disposal company should be contacted to dispose of the product
6. a measure of safety can be obtained by lightly misting any attainable crystals (such as those that may have formed on the outside of the bottle) with large quantities of water. A water spray bottle is ideal for this

Conclusion

While picric acid offers various benefits in certain applications, its hazards can't be overlooked. Proper storage practices are key to managing the risks related to this chemical compound. By understanding its dangers and implementing appropriate safety measures, people and organizations can minimize the likelihood of accidents and protect everyone involved in handling or storing picric acid.

References

[1] S. Gad, “Picric Acid”, Encyclopedia of Toxicology (Second Edition), 20005, pp. 438-440

[2] “Picric Acid” The MAK-Collection for Occupational Health and Safety,2012, [Online]. Available: https://onlinelibrary.wiley.com/doi/pdf/10.1002/3527600418.mb8889e0017

[3] T. L.  Davis, “The Chemistry of Powder and Explosives”, Angriff Press, 1984, pp-164.

[4] P. Cooper, “Explosives Engineering”, Wiley-VCH, 1996, pp. 33.

[5] R. Phifer, “Picric Acid: When is Panic Justified?”, Speaking of Safety, vol. 9, no. 2, 2000, page 1-3.

[6] Public Health England, “Picric Acid”, [Online]. Available: https://assets.publishing.service.gov.uk/media/5a82b9b940f0b6230269c5f8/Picric_acid_general_information.pdf

[7] EPA, “Picric Acid and Ammonium Picrate”, [Online]. Available: https://cfpub.epa.gov/ncea/pprtv/documents/PicricAcid246Trinitrophenol.pdf

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