RE: [Histonet] formalin nomenclature

From:"Mark Bromley"

Hi Sharon.

Formaldehyde has the formula H2C=O and is a gas at room temperature. If you
open up one of the double bonds between the carbon and the oxygen you can
get formaldehyde to polymerise forming chains like: -H2C-O-H2C-O-H2C-O- etc,
having the general formula (H2C-O)N and it is this polymerised form of
formaldehyde which is called paraformaldehyde. It exists as a white powder
at room temperature.

Neither gaseous formaldehyde or paraformaldehyde powder is much use to a
histologist on its own. But when you dissolve either of them in water to the
point of saturation, you will find you have a 38% solution of formaldehyde
in water. (Paraformaldehyde just dissociates back into individual
formaldehyde units when dissolved, so from our point of view, formaldehyde
and paraformaldehyde are essentially the same thing. It is this 38% solution
of formaldehyde in water that is called Formalin.

Formalin = 38% formaldehyde solution.

Making up the 10% formalin we routinely use is a matter of a 10 fold
dilution of formalin. This 10 fold dilution of a 38% solution of
formaldehyde produces a 3.8% solution. So in answer to your question, a 4%
(or 3.8% to be exact) paraformaldehyde solution, 4% formaldehyde solution
and 10% Formalin are all exactly the same thing.

Hope that helped.

Mark Bromley,
Histology Department,
Douglass Hanly Moir Pathology,
Sydney, Australia.

-----Original Message-----
[]On Behalf Of Sharon
Sent: Wednesday, 21 July 2004 12:10 PM
Subject: [Histonet] formalin nomenclature

Dear Histoneteers,

Can someone please explain to me why some tissue fixation protocols
call for 4% paraformaldehyde while others call for 10% formaldehyde
and what the difference is?

Sharon Cooperman        	     
NIH, NICHD, CBMB                     301.435-7735
Building 18T, room 101               301.402-0078 fax
Bethesda, MD 20892

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