[Histonet] (para)formaldehyde (was: in situ question)

[Rene J Buesa]

Paraformaldehyde is the 80 to 100 units polymer of methanal (formaldehyde) and is called polyoxymethylene. When hydrated it will render a pure  formaldehyde (methanal) solution to the concentration you desire.
 
Formalin or formol, depending on the commercial name used to designate it, is the solution of methanal (formaldehyde) obtained by catalytic oxidation of methanol and bubbled through water to a concentration of 37-50% w/w to which 7-15% methanol is added as a stabilizer to retard its polymerization.
 
Formaldehyde is so reactive with water that it is present in the formalin as a concentration of  0.1% only, the rest (99.9%) is present as methanediol or methylene glycol. From this last chemical the formaldehyde has to be reconstituted during fixation, hence the slow fixation rate of formalin but its rapid penetration rate (penetration about 48 times faster than fixation).
 
In the same way as formaldehyde bubbled through water reacts and forms methylene glycol, the same reaction will take place when disolving paraformaldehyde. 
 
Both solutions are "formaldehyde solutions" the only thing is that formalin contains methanol as a stabilizer and the formaldehyde solution from paraformaldehyde does not, unless you decide to add it.
René J. 

--- On Thu, 8/21/08, Mikael Niku <mikael.niku <@t> helsinki.fi> wrote:

From: Mikael Niku <mikael.niku <@t> helsinki.fi>
Subject: RE: [Histonet] (para)formaldehyde (was: in situ question)
To: histonet <@t> lists.utsouthwestern.edu
Date: Thursday, August 21, 2008, 4:49 AM

This has probably been discussed 1000s of times, but....

I'm wondering to what extent dissolved paraformaldehyde really is
equivalent
to 4% formaldehyde or 10% formalin.
In my experience, samples fixed in formalin and "4% PFA" as we tend
to say
are different when one does immunostaining or especially in situ
hybridization. Formalin is of course traditionally used in pathology,
whereas many labs doing immunos or ISH use "PFA".

But what really causes the differences? Is it incomplete de-polymerization
of paraformaldehyde, or perhaps the methanol typically included in formalin?

-----------------------------------------------------
 Mikael Niku, PhD, university lecturer            
 University of Helsinki, Division of Nutrition
 URL: mikael.nikunnakki.info

 - What do I think of western civilization?
   I think it would be a good idea!                       
                                            Gandhi
-----------------------------------------------------
-----Original Message-----
From: histonet-bounces <@t> lists.utsouthwestern.edu
[mailto:histonet-bounces <@t> lists.utsouthwestern.edu] On Behalf Of Tony Henwood
Sent: 20. elokuuta 2008 4:42
To: Lee Crosby; histonet <@t> lists.utsouthwestern.edu
Subject: RE: [Histonet] in situ question

Which the becomes 4% formaldehyde or equivalent to 10% formalin. Not 4%
paraformaldehyde.

Regards

Tony Henwood JP, MSc, BAppSc, GradDipSysAnalys, CT(ASC) Laboratory Manager
&
Senior Scientist
Tel: 612 9845 3306
Fax: 612 9845 3318
the children's hospital at westmead
Cnr Hawkesbury Road and Hainsworth Street, Westmead Locked Bag 4001,
Westmead NSW 2145 


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