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Formaldehyde is not stable forever. It can be purchased as EM
grade in small, sealed aliquots. When exposed to air it begins to
oxidize. The stock solution is probably good for 6 months, although
I don't have exact data on that. The working dilution is only good
for 1-2 weeks, although for some purposes, like standard surface
markers that are bright, it will work for a month.
Paraformaldehyde is a solid form that is stable as a solid and can
be made up fresh as formaldehyde to use. Paraformaldehyde is
considerably cheaper, but requires the hassle of working with a
chemical hazard when making the solution. As a solution it has the
same stability as formaldehyde.
Tony Bakke
(bakkea@ohsu.edu)
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