IMVS Division of Pathology
The Queen Elizabeth Hospital
Woodville Road, Woodville, South Australia 5011
Did you know that picric acid is highly explosive in the dry state?
Because of this it must always be kept under water and never allowed to dry. Its solubility in water at room temperature is approximately 1.5%. So it is not very soluble.
It wise to use cork to seal any receptacle containing picric acid. WHY?
Because glass stoppers cause more friction and greater compression than cork so should there be any dried deposit around the neck of the container, which can and does occur, a glass stopper dropped into the neck of the container could cause an explosion, albeit a small one. But I for one wouldn't fancy having bits of broken glass flying towards my face from even after a small explosion.
While on the subject of Picric acid - what about other hazards associated with the use of Picric acid?
is a strong oxidising agent which reacts violently with combustible materials and reducing agents when dry - whilst wet it is quite safe.
It is toxic and can be absorbed into the body through inhalation of the vapour or ingestion of the liquid.
It is an eye, skin and respiratory tract irritant, a neurotoxin and can cause convulsions.
If you do use picric acid as a fixative it is best used in compound fixatives such as Bouin’s. WHY?
Picric acid used on its own will excessively harden tissue even after reasonably short exposures. It makes cutting sections tough. But there is a plus - delicate structures in tissues such as testis and bone marrow stain really well after fixation in picric acid with better differentiation of the specialised cells. Especiallly when using trichrome stains.
The removal of Picric acid is important because residual picric acid continues to alter tissue characteristics so long as it is present in the tissue and so it is recommended that it be removed before processing, although Lillie did recommend removal after processing.
The recommended procedure for Picric Acid removal is:
Place the tissue in 70% ethyl alcohol saturated with lithium carbonate until all of the yellow colour disappears.
Usually about 4 hours for average sized blocks.
It is best then to go straight into alcohols.
This method is recommended because lithium carbonate has a neutralising effect on the tissue after fixation in picric acid. Ethanol alone can be used but can take days instead of hours for complete removal of picric acid. Also you can wash the fixed tissue in running water, preferably overnight before processing or if you want to be like Lillie simply washing the section in water for 2 to 3 hours after removal of wax also works quite well. BUT - and it is a big but - tissue disposal is also important because if the specimen is allowed to dry out with residual picric acid in the tissue it too can explode under the right conditions. Likewise Picric acid should not be disposed of to sewer because it can dry out or react with other substances in the sewers and cause an explosion.
As a saturated solution it can be disposed of by burning in a high temperature incinerator equipped with an afterburner and scrubber. The method of entry into this type of incinerator is as a fine spray so the solution does not present a real danger. If it has dried out it should be exploded in a deep sand pit out of harms way.
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© Roy C. Ellis 2002