Picric Acid and Picrate Salts

by Patricia Charlebois, Chemist

Picric acid

Picric acid or Trinitrophenol is, by far, one of the more dangerous chemicals being used today. Classified as a flammable solid when wetted with more than 30% water (UN1344, class 4.1) and a class A high explosive with less than 30% water (UN0154, class 1.1D), it has some very interesting properties. It is explosive but also highly shock, heat and friction sensitive. In fact, detonation with a speed and power superior to that of TNTcan occur by a 2 kg weight falling onto solid picric acid from a height of 36 cm. Picric acid is toxic by all routes of entry, it's also a skin irritant and allergen and will produce toxic pro-ducts on decomposition.

Picric acid is used primarily in the manufacture of explosives and as an intermediate in dye manufacturing. It is also present in many laboratories, for use 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 extremely susceptible to the formation of picrate salts. Many of these salts are even more reactive and shock sensitive than the acid itself.

Picrate Salts

Picrate salts are formed by the reaction of picric acid with any of the following: metals, metal salts, bases, ammonia and concrete. Particular attention must be paid in order to prevent the formation of picrate salts 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 such as copper, nickel, lead, iron and zinc. Calcium picrate is formed by the reaction of picric acid with concrete. In the last several years, CANUTEC has assisted many individuals in dealing with incidents involving picric acid. The scenario is usually the same, involving bottles of product that have been left on a laboratory shelf over a period of years. The dangers of this kind of situation are twofold. First, the acid, which usually exists as a wetted paste, may dry out causing the formation of the shock sensitive acid crystals mentioned above. The second hazard possibility arises from the introduction of impurities into bottles that have been previously opened and are no longer airtight. These could bring about the formation of picrate salts inside the bottle or within the threads of the lid. Either of these situations should be considered extremely serious and handled accordingly.

Intervention

If old or previously unaccounted for bottles of picric acid are discovered, the following steps should be taken.

  1. First and foremost: DO NOT TOUCH THE BOTTLE! Depending on the length of time the bottle has been left and the state of the product inside, even a slight movement could be critical. 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 laboratory.
  2. 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.
  3. 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 fairly high, there is probably little cause for concern. If there is even the slightest indication of crystallization or low levels of water in the bottle, the situation is more serious. A local bomb disposal service or an explosives disposal company should be contacted to dispose of the product.
  4. Immediately secure the area and restrict access. A measure of security 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 purpose.
  5. Dry picric acid or picrate salts should not be touched or moved under any circumstances.

Prevention

These situations are easily prevented by establishing an inventory of all laboratory chemicals. The inventory should include full identification of the product, the quantity on hand, expiration dates where applicable and notes on the particular concerns for each product. This list should be reviewed regularly and expired chemicals disposed of according to appropriate environmental le-gislation. The shelf life of picric acid can be extended significantly by adding water to the container on a regular basis. This prevents the product from drying out and inhibits the formation of picrates.

Some companies recommend that picric acid be disposed of after 2 years and propose the following storage guidelines:

  • Store in a cool dry place away from heat sources or open flame.
  • Inspect the bottles and add water as needed every 6 months.
  • Gently rotate bottles to distribute water every 3 months.

Disposal

There are several acceptable ways of disposing of old picric acid. The safest, however, is to let explosives' experts such as a local bomb disposal unit, handle the situation.

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Publication: TDG Dangerous Goods Newsletter, Vol. 16, No. 3, Winter 1996.


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