[Histonet] RE: formalin

[Kemlo Rogerson]

"Formaldehyde, as 4% buffered formaldehyde (10% buffered formalin), is the
most widely employed universal fixative particularly for routine paraffin
embedded sections. It is a gas with a very pungent odour, soluble in water
to a maximum extent of 40% by weight and is sold as such under the name of
formaldehyde (40%) or formalin (a colourless liquid). Formaldehyde is also
obtainable in a stable solid form composed of high molecular weight polymers
known as paraformaldehyde. Heated paraformaldehyde generates pure gaseous
formaldehyde which, when dissolved in water, reverts mostly to the monomeric
form. Aqueous formaldehyde exists principally in the form of its
monohydrate, methylene glycol, CH2(OH)2, and as low molecular weight
polymeric hydrates or polyoxymethylene glycols. It has been suggested that
the hydrated form, methylene glycol, is the reactive component of
formaldehyde but this has been disputed2"

Fixation and fixatives
Anthony S-Y Leong

So as it is a gas it must be held in solution by atmospheric pressure? If
that atmospheric is raised or lowered then lesser or greater amount of gas
can be held in solution; the same with increasing or decreasing
temperatures. So is formalin 40% formaldehyde gas up Everest or is it much
less? Atmospheric pressure is reduced so the Saturated Vapour Pressure of
the solution may exceed barometric pressure and the gas is driven off until
the pressures equilibrate; plus it is very cold. Do you have to make a
stronger dilution up Everest to end up with a 4% buffered formaldehyde? Are
there any Histo Labs up there? Who cares?

Kemlo Rogerson
Cellular Pathology Manager
East Lancashire Hospitals NHS Trust
DD. 01254-294162
Mobile 0774-9754194
 

-----Original Message-----
From: Glencross Hedley (RW3) CM&MC Manchester
[mailto:Hedley.Glencross <@t> CMMC.nhs.uk] 
Sent: 16 May 2005 14:02
To: histonet <@t> lists.utsouthwestern.edu
Subject: [Histonet] RE: formalin

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


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