Paraformaldehyde is a polymerized form formaldehyde. It is hardly soluble and it cannot be used as a fixative. Only formaldehyde is used as a fixative. Howeve, formaldehyde in aqueous solutions spontaneously polymerizes. Therefore, methanol is often added to slowdown the polymerization reaction. Solutions of formaldehyde (usually ~ 37%) in water, containig 10-15 % methanol as a preservative are generally called "formaldehyde"; such solutions are being sold by most reagent companies. Solutions further diluted (4-10 %) received name "formalin". Methanol-free formaldehyde, which sometimes is preferred (e.g. for fixing cells for some some histochemical reactions or in immunocytochemistry), can be obtained by hydrolysis of paraformaldehyde. This is usually done by extensive heating of paraformaldehyde solutions. Because of this procedure the methanol-free formaldehyde received (incorrrectly) the name "paraformaldehyde". In the past, this was the most common way to obtain methanol-free formaldehyde. Unfortunately, this incorrect name is still often used in the literature, generating the confusion. The methanol-free formaldehyde solutions can now be purchased. Some are called "ultrapure". We purchase such solutions (10%) from Polysciences, Inc. (800-523-2575); they can be stored at room temperature. I would not recommend, however, to store them longer than one year, since formaldehyde in these solutions still has tendency to polymerize. It should be noted that all formaldehyde solutions are highly toxic and carcinogenic. Zbigniew Darzynkiewicz
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