We have had several interesting and informative comments about formalin
storage.
I apologize if I repeat anything that has already been said.

I think that one has to be specific when discussing formalin as there are
several formulations and several degrees of purity of the formalin. Most
commercial samples of formalin have contaminants and antioxidants as mentioned
by other netters.
Formaldehyde in solution can polymerize to paraformaldehyde  present as a white
precipitate or can be converted to formic acid. The original rationale for
initially using marble chips and later phosphate buffers for storage of
formalin solutions was to prevent acid formaldehyde hematein pigment being
produced in these acidic solutions.
The amount of paraformaldehyde that is found in formalin is relatively
insignificant unless this is in a bottle donated by the Smithsonian several
years ago. While any paraformaldehyde can be reconstituted by gentle heating in
slightly alkaline solutions, the reduction in concentration of formalin by its
formation is probably unimportant. It can simply be filtered off. Most
fixatives use concentrations of the fixing agent that are grossly in excess of
what is required for adequate fixation and cross linking. It should be noted
that when glutaraldehyde is used in the leather industry to "fix" leathers, the
minimum amount of glutaraldehyde is used. This is based on the principle that
if excess glutaraldehyde is used, there will be some potential bonds that are
available on the glutaraldehyde but no sites for them left on the tissue for
them  to bind to. As I keep my leather billfold in my back or side pocket I am
grateful that there will be no excess glutaraldehyde to fix adjacent parts of
my anatomy!
The purity of formalin as regards methanol. The concentration of alcohol in
most solutions is around 5%.  Upon dilution for most formalin solutions, it
becomes 0.5%. I don't believe that this could has any significant effect at all
on in the fixing process.
Highly purified formalin solutions can be obtained from EM companies and are in
sealed glass vials. While these cannot be used for all specimens due  to the
expense, I would recommend that they be used for special cases and for electron
microscopy. I am in the process of moving my office so do not have all papers
available, but I do remember sending to the histonet reference to an article in
which storage of fixatives at 4C was compared for solutions made fresh, or
stored for varying periods up to 6 months. No difference was found between the
EM appearance when using these solutions. Hopefully this has been archived.
Thank you
Barry


rkline@emindustries.com wrote: