RE: formalin pigment and ammonia alcohol

From:Gary Gill

Posted previously by me on a cytological listserve in response to a question
about formalin-pigment:

Formalin pigment is a brown deposit that forms when tissues are fixed in any
formalin fixative with an acid pH.  It is a reaction product of formalin and
hemoglobin and appears over blood-rich tissues such as spleen or over areas
of hemorrhage.

To prevent its occurrence, fix tissues in neutral buffered formalin:

*	sodium phosphate, monobasic			4 gm
*	sodium phosphate, dibasic			6.5 gm
*	formaldehyde, 37% (w/v)				100 mL
*	distilled water					900 mL

Formalin pigment can be removed by immersing tissue sections in saturated
(8.3%) alcoholic picric acid solution for 1/2-1 hr.  Remove the picric acid
by immersing the sections in saturated (1 %) aqueous lithium carbonate
solution for 5-10 minutes.  Perhaps this procedure will eliminate your using
ammonia alcohol.  But again, maybe it's impractical, given the time

Picric acid is trinitrophenol, which is related to trinitrotoluene (TNT).
When dry, TNP can be explosive, which is why it is shipped with 10% water.
Discovered in 1771, picric acid was responsible in 1915 for the largest
man-made non-nuclear explosion when a French freighter, the Mont Blanc,
loaded with old picric acid caught fire in Halifax Harbor -- killing many
people and demolishing nearby buildings.  A practical perspective on the
perceived dangers of TNP is available at:

Gary Gill

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