Memo 11/08: Picric Acid

During the past two months the Safety Office has been asked to dispose of eight bottles of picric acid, all of which had been neglected and allowed to dry out so that they became potentially dangerous.

Picric acid (2,4,6-trinitrophenol) is normally sold with >30% water content: in this state it is stable and classified as a desensitised explosive. If allowed to dry out completely, picric acid is a Class 1 explosive and is highly sensitive to shock, heat and friction. Where stocks are old or little used, dried crystals may be present on the threads of bottles, presenting a detonation hazard when opening the container. Older containers sometimes had metal lids and shock-sensitive metal picrates are likely to have formed in the threads and the container.

Storage of picric acid is subject to the requirements of the Manufacture and Storage of Explosives Regulations 2005 and registration with Thames Valley Police will be required.

It would be most prudent to ensure that all stocks of unwanted or unstable picric acid are disposed of, and that any material still in use is not allowed to deteriorate. Heads of departments are asked to delegate the following actions to a responsible person:

1.  Ensure that all chemical stores and laboratories are examined and an inventory of picric acid stocks is produced. Wherever possible picric acid containers whose contents have dried out should not be handled (or where this is impossible handle them gently). Old, unknown, dried-out yellow powders, especially in bottles with metal lids, should be treated with great caution and suspected as being picric acid.

2.  Note whether the material

(a)  is wetted, with only water visible above the solid

(b)  has visible crystals above the liquid or around the lid

(c)  is dry.

If (b) or (c) apply, then care must be taken to ensure the container cannot be further disturbed until it can be safely dealt with.

No attempt must be made to open any container where there are signs of crystallisation, or where the picric acid has dried out, as detonation may result.

3.  Using the attached form, please notify the Safety Office of the results of the survey by 15 July 2008. Unwanted material that is still wetted will be disposed via the Safety Office’s normal waste disposal route. If crystallised or dried-out picric acid is notified, the material will be examined and rewetting will be attempted by submerging the unopened container in water for some time. If this is unsuccessful, then disposal becomes extremely difficult – waste disposal contractors will not accept it, so it may be necessary to contact bomb disposal services.

4.  Where picric acid stocks are needed, then the department must register with the licensing authority[1]. A fee will be payable and the police may wish to inspect the premises.

5.  To help avoid deterioration of new stocks of picric acid, bottles should be dated and preferably used or discarded (using the Safety Office’s waste disposal system) within two years. This will minimise the chance of it drying out, or the formation of unstable picrates.

[1] Firearms & Explosives Licensing Department, Police Headquarters, Oxford Road, Kidlington, Oxfordshire OX5 2NS.

June 2008