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
John A. Kiernan
Department of Anatomy
Univ. of Western Ontario
LONDON, Canada N6A 5C1
e-mail: kiernan at .uwo.ca