Picric Acid

Picric acid (2,4,6-trinitrophenol) is normally sold with >30% water content: in this state it is stable and classified under explosives legislation 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 risk of detonation when opening the container. Older containers sometimes had metal lids and shock-sensitive metal picrates are very likely to have formed in the threads and in the container.

The Safety Office has been asked to dispose of significant quantities of picric acid that has been neglected and allowed to dry out, becoming potentially dangerous. Departments are reminded once again that stocks of unwanted or unstable picric acid should be disposed of promptly. Any material still in use should not be allowed to deteriorate to the point where it becomes dangerous. 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. Picric acid containers whose contents have dried out should not be handled (where this is impossible they should be handled gently) and the Safety Office should be contacted for advice. 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.

  1. Arrange for disposal of material that is still wetted via the Safety Office’s normal waste disposal route. If crystallised or dried-out picric acid is found, 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 – hazardous waste disposal contractors will not accept picric acid in this state, so it will be necessary to contact specialist explosives disposal services.
  2. 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.

 

A C Kendall

August 2014