Wed, 15 Jan 1997 09:47:38 -0500

Formaldehyde (CHOH) is a gas. A solution of approx. 37 % (by weight)
formaldehyde (gas) in water, generally with 10-15 % methanol added to
prevent polymerization is called formalin, also called "full strength
formalin", "100 % formalin" or Formalin 40 (to denote that there is
about 40 g formalin in 100 ml of solution). This is the widely
available solution which after appropriate dilution with water or PBS
is commonly used as a crosslinking fixative.
Paraforaldehyde (polyoxymethylene) is a polymerized formaldehyde and
as such cannot be used as a fixative.
The confusion regarding the names stems from the fact that in some
fixative solutions the presence of methanol is undesired. It was
common, then, in the past to prepare oneself the formaldehyde
solution from paraformaldehyde, by boiling it. Erronously, the name
"paraformaldehyde" was stuck to denote formaldehyde obtained by this
way. It really defined a solution of formaldehyde without methanol.
Nowdays many vendors (e.g. Polysciences) sell a solution of
formaldehyde (37 - 40 %) without methanol. I presume that they add
some other stabilizer (in lieu of methanol) which prevents
formaldehyde polymerization. They do not specify the stabilizer,
although the solution being sold in ampules has no contact with
oxygen (the solution in ampulles, I was told by the vendor, is packed
in under nitrogen rather than in air).
To avoid a confusion I would suggest that describing the fixative one
should specify whether it is formaldehyde with methanol (e.g. 1 %
formaldehyde and 0.2 % methanol) or formaldehyde without methanol. In
most applications the presence of methanol is acceptable. The name
"paraformaldehyde" to define a fixative is, of course, erroneus,
since by its chemical nature it is a polymer with poor crosslinking
Zbigniew Darzynkiewicz

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