[Histonet] quick question about formaldehyde

[John A. Kiernan]

Dear Joyce,

Formalin = 37% formaldehyde; so no, it doesn't
make a difference. Formalin is not buffered; it
does contain about 10% methanol, which is put
in to retard polymerization. When diluted to
make a 4% formaldehyde fixative, the methanol
concentration is 1%. Buffering of the dilute
solution offsets pH changes due to the
Cannizzaro reaction. It also inhibits the
formation of blood-derived "formalin pigment"
which forms after fixation in an acidic
formaldehyde solution.

Tim Morken is correct in saying we don't know
the extent of chemical change in 12 year-old
formalin. The fact that there's no expiry date
sugggests that it's not much. For what it's
worth, I've used formalin that's more than 5 
years old and fixation has been OK.

                      John Kiernan
                      london, Canada
----------------------------------------------
"Weems, Joyce" wrote:
> 
> Also, John said "formalin" - was the solution formalin or 37% formaldehyde  - without buffers? Would that make a difference?
> 
> Joyce
> 
> -----Original Message-----
> From: histonet-bounces <@t> lists.utsouthwestern.edu
> [mailto:histonet-bounces <@t> lists.utsouthwestern.edu]On Behalf Of Morken,
> Tim - Labvision
> Sent: Thursday, May 12, 2005 2:08 PM
> To: 'John Kiernan'; Andrea Grantham
> Cc: histonet <@t> lists.utsouthwestern.edu
> Subject: RE: [Histonet] quick question about formaldehyde
> 
> John,  you said "   Small amounts of methanol and formate ions are not going
> to change the fixative properties."
> 
> But after 12 years  will it really be a "small amount?" How do we know what
> percentage of the solution will have been converted?
> 
> Tim Morken
> 
> -----Original Message-----
> From: histonet-bounces <@t> lists.utsouthwestern.edu
> [mailto:histonet-bounces <@t> lists.utsouthwestern.edu] On Behalf Of John Kiernan
> Sent: Thursday, May 12, 2005 10:48 AM
> To: Andrea Grantham
> Cc: histonet <@t> lists.utsouthwestern.edu
> Subject: Re: [Histonet] quick question about formaldehyde
> 
> Two things could have happened to unopened
> formalin in 12 years:
> 1. Polymerization (to paraformaldehyde). This is
>    evident as a white precipitate. It slightly
>    reduces the concentration in the liquid,
>    but that does not matter for fixation.
>    Polymerization is accelerated by low room
>    temperature, and it is claimed that the
>    process can be reversed by autoclaving(paper
>    in Stain Technol about 40 years ago).
> 2. Cannizzaro's reaction, in which 2 molecules
>    of formaldehyde react together, producing
>    one molecule each of methanol and formic
>    acid. This happens in all formaldehyde solutions
>    and causes lowering of the pH. This doesn't matter
>    if you make a neutral buffered fixative solution.
>    Small amounts of methanol and formate ions are not
>    going to change the fixative properties.
> Bottom line: OK to use, but be sure to check the
> pH of the working fixative solution and adjust if
> necessary.
> --
> -------------------------------
> John A. Kiernan
> Department of Anatomy and Cell Biology
> The University of Western Ontario
> London,   Canada   N6A 5C1
>    kiernan[AT]uwo.ca
>    http://publish.uwo.ca/~jkiernan/
>    http://instruct.uwo.ca/anatomy/530/index.htm
> _______________________________
> Andrea Grantham wrote:
> >
> > One of the labs here is closing and they have a case of formaldehyde,
> > 37.5%, that they are trying to give away. They have had it in their
> > lab since 1993. The bottles have not been opened. Is it still good to
> > use? Andi
> > .....................................................................
> > : Andrea Grantham, HT(ASCP)     Dept. of Cell Biology & Anatomy     :
> > : Sr. Research Specialist       University of Arizona               :
> > : (office:  AHSC 4212)          P.O. Box 245044                     :
> > : (voice:  520-626-4415)        Tucson, AZ  85724-5044    USA       :
> > : (FAX:  520-626-2097)          (email:  algranth <@t> u.arizona.edu)       :
> > :...................................................................:
> >            http://www.cba.arizona.edu/histology-lab.html
> >
> > _______________________________________________
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-- 
-------------------------------
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1
   kiernan[AT]uwo.ca
   http://publish.uwo.ca/~jkiernan/
   http://instruct.uwo.ca/anatomy/530/index.htm
_______________________________