The active ingredient in both products is formaldehyde.  Commercial
formaldehyde solution contains at least 10% methanol as a stabilizer.
Paraformaldehyde, which is simply polymerized formaldehyde, is a pure
compound - nothing but formaldehyde molecules.  When you dissolve it in
distilled water you get a formaldehyde solution free of methanol and other
compounds that might be present in commercial formaldehyde solution.  In my
opinion, either "10% formalin" solution (which contains about 4%
formaldehyde) and 4% paraformaldehyde solution (which also contains 4%
formaldehyde) provide identical results for most histological applications,
certainly for purely morphological studies.  However, some biochemical
studies could be compromised by the presence of methanol, and in such cases
the addditional trouble and expense of using paraformaldehyde are
justifiable.  I do projects for many different researchers, and quite a few
of them routinely use para when formalin would work just as well for what
they are doing.



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