[Histonet] RE: formalin

[Glencross Hedley (RW3) CM&MC Manchester]

Hi everyone

It has always been my understanding that "formalin" like "Hoover" &
"Sellotape" (and countless other words) is just a trade name. This
describes a 37% solution of formaldehyde gas in water, and we should be
talking about 3.7% (4%) formaldehyde and not 10% formalin.

Regards

Hedley Glencross

Manchester Cytology Centre UK

For all my years I have believed that formaldehyde was the 37%
concentrate without buffers, and that formalin was 10% buffered... learn
something new every day!

Thanks John! 

-----Original Message-----
From: John A. Kiernan [mailto:jkiernan <@t> uwo.ca]
Sent: Friday, May 13, 2005 11:31 AM
To: Weems, Joyce
Cc: histonet <@t> lists.utsouthwestern.edu
Subject: Re: [Histonet] quick question about formaldehyde


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