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formaldehyde vs paraformaldehyde

J. A. Kiernan jkiernan at julian.uwo.ca
Wed Jun 28 15:11:12 EST 1995

In article <3sr26m$ag3 at neuro.usc.edu> william at neuro.usc.edu (William Sun) writes:

>I am wondering about the difference between the above mentioned two compounds.
>Our lab uses formulin (3.7% formaldehyde) to fix brains for histology.  
>However . . . most protocols call for 4% paraformaldehyde.  Is one preferable 
>over the other?  Please reply if you know the details.

Paraformaldehyde is a polymer. It decomposes to formaldehyde when dissolved
in acid, alkali or buffer.  It won't dissolve in plain water.

Formalin (37% formaldehyde) contains some methanol. Most of it is put there
by the makers because it slows down the spontaneous formation and 
precipitation of paraformaldehyde in the solution. A little methanol, and 
also some formic acid, are slowly formed in solution when formaldehyde
molecules react with one another.

It is doubtful whether traces of methanol and formate in diluted formalin
have any harmful effects on antigens in the tissue.  Formaldehyde
certainly does!  However, if you're worried about the impurities make
your fixative from paraformaldehyde.  Occasionally you get a bad batch
of paraformaldehyde that just won't depolymerize (dissolve).  This is
no good.
                                     John A. Kiernan
                                     Department of Anatomy
                                     Univ. of Western Ontario
                                     LONDON, Canada  N6A 5C1
                                     e-mail: kiernan at .uwo.ca

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