Picric acid and other densensitised explosives

Picric acid (2,4,6-trinitrophenol), and related compounds 2,4-dinitophenol and 2,4-dinitrophenylhydrazine, are normally sold with >30% water content: in this state they are stable and classified under explosives legislation as desensitised explosives. If they are allowed to dry out completely they become Class 1 explosives and are highly sensitive to shock, heat and friction.  If supplied ‘wetted’ they must be disposed of as ‘wetted’.

Where such chemical stocks are old or little used, dried crystals may be present on the threads of bottle lids, presenting a risk of detonation when opening the container. Merely unscrewing the cap on one of these bottles can cause an explosion if there is dried material embedded in the screw threads of the cap.

On several occasions the Safety Office has been asked to dispose of significant quantities of shock sensitive chemicals that have been neglected and allowed to dry out, becoming potentially dangerous. Such explosive material cannot be disposed of via the Safety Office hazardous waste disposal system, and in these cases special disposal arrangements were put in place, incurring significant costs.

Safety Office memos M11/08 and M11/14 have already alerted departments to this risk and required them to produce an inventory of picric acid stocks, to dispose of all unwanted picric acid stocks promptly and, to help avoid the deterioration of any new stocks, all bottles were to be dated and used or discarded (using the Safety Office’s waste disposal system) within two years.

Importantly, frequent checks were required to ensure that in-use chemicals did not dry out during storage and deteriorate to the point where they had become dangerous.

In spite of these warnings discoveries of explosive material continue to be made. In such cases the following applies:

  1. If a dry bottle of picric acid, or related chemical, is found it must be handled with extreme care.

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

  1. 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. The bottle should be placed in a suitable container with a good head of water and held down with a heavy object, such as a brick, and left for at least two weeks. After this time it is possible that water may have penetrated into the threads and may even have seeped into the bottle itself, and may be safe to open. If in any doubt the Safety Office should be contacted for advice.
  3. If after careful examination there is no evidence that any water has penetrated the threads or bottle then further advice should be sought from the Safety Office.
  4. The Safety Office will contact the University’s explosive disposal contractors (who will pack, transport and dispose of the material by detonation).
  5. Payment for these costly services will have to be met by the department or the relevant research group.

Note that this information is not limited to picric acid but also applies to certain organic peroxides, like benzoyl peroxide which can be ~30% to 70% water stabilised. Great care must be taken to ensure that these chemicals do not dry out.

In certain cases also material that was once supplied ‘dry’ cannot now be sent for disposal in its dry form and must be wetted for disposal. Material Safety Data Sheets, which comply with the Globally Harmonised System (GHS), should be consulted for important disposal information.

In all cases the arrangements for final disposal of chemicals must be detailed in the relevant COSHH assessment.

23 May 2017                                                                                                                          J Black