Re: 4% para vs 10% bnf

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From:"Tim Morken" <>
Date:Wed, 13 Oct 1999 15:10:32 EDT
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4% Paraformaldehyde is essentially the same as 10% "formalin" except that 
Paraformaldehyde will not have methanol in it (which is used as a stabilizer 
in pre-made formalin).

In a well-buffered solution you would not be able to tell any difference 
between tissues fixed in one or the other. Even EM fixation can be done with 
well-buffered formalin with excellent results.

10 percent formalin is made by diluting 40 percent "formaldehyde," the 
concentrated liquid form of formaldehyde, by 10X and so is actually a 4 
percent solution of formaldehyde.

Since formaldehyde is actually a gas it is made useful for us by dissolving 
it in water. 37 to 40 percent formaldehyde in water is a saturated solution 
and is properly called "formalin" (any solution of formaldehyde in water is 

However, most people in histology labs call 40 percent formalin 
"formaldehyde" or "concentrated formaldehyde" to distinguish it from what we 
use as "formalin" or "10 percent formalin." Is that confusing enough for 

As to the cold, it is simply to prevent autolysis. It is the best way to fix 
but takes longer than fixing at room temperature. It is usually the way a 
research lab will fix specimens since time is usually no object. Clinical 
labs don't bother with those nicities, sometimes fixing in formalin for only 
a few minutes.

Tim Morken, B.A., EMT(MSA), HTL(ASCP)
Infectious Disease Pathology
Centers for Disease Control
1600 Clifton Rd.
Atlanta, GA 30333


Phone: (404) 639-3964
FAX:  (404)639-3043

----Original Message Follows----
From: Colleen Forster <>
Subject: 4% para vs 10% bnf
Date: Tue, 12 Oct 1999 17:00:20 -0600


I have an educational test question I need help with. What is the
difference in how para works compared to BNF ??

I use 4% para for all my research profusion and wonder why that is
recommended over 10% BNF? Also what is the actual chemistry going on
that para needs to be at 4 C when fixing vs room temp like BNF??/

Anyone know the answers to the tivia I would appreciate it.

Colleen Forster
U of MN
Dept. of Neurology
Ph 612 626-2477
Fax 612 625-7950

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