I agree with the temperature changes however, just age sitting on a shelf at
room temperature can also cause the change to paraformaldehyde.  Our old
research lab had problems when a PI left and we inherited the stuff.  I just
disposed of it.  It really wasn't worth trying to reclaim.  We always made
fresh paraformadehyde for EM.
Pam

-----Original Message-----
From: J. A. Kiernan [mailto:jkiernan@julian.uwo.ca]
Sent: Thursday, February 17, 2000 1:39 AM
To: Pam Marcum
Cc: R.Wadley; histonet@pathology.swmed.edu
Subject: Old cloudy formalin (was RE: FLT-1/FLK-1 ISH)


On Wed, 16 Feb 2000, Pam Marcum wrote:

> Methanol is considered a stabilizer to slow the progression of
formaldehyde
> back to paraformaldehyde by most companies.  However, even stabilized you
> should watch 37 to 40% formaldehyde and if a cloud or precipitate appears
in
> the solution it has polymerized back to paraformaldhdye.  It is now
> primarily is simple terms formic acid.  Do not use it.

  The amount of formic acid in old cloudy formalin will not
  significantly lower the concentration of formaldehyde and
  its polymers, and the acidity is easily neutralized.

  Formation of polymers is not associated with the Cannizzaro
  reaction that generates methanol and formic acid in stored
  formaldehyde solutions. Polymers can be depolymerized by
  heating the suspension after addition of the buffer salts.
  For more info consult the HistoNet Archives (www.histosearch.com)
  which contain many messages and refs to books & papers. The
  book "Formaldehyde" by Walker is excellent.

  It is written in the literature (Stain Technol. 1940s; I don't
  have the reference to hand) that heavily precipitated formalin
  can be rejuvenated to a clear liquid by autoclaving it in small,
  strong, tightly capped bottles. I have never tried this because
  I didn't see the paper until about 1980, and since coming to
  Canada in 1972 my lab has never been cold enough to polymerize
  formalin to a solid. Back in Blighty (1960s), where they
  turned off the heating at night, precipitates in the formalin
  were part of the winter scene, and nobody needed to ask why
  conc. acetic acid was called "glacial."

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1