[Histonet] (para)formaldehyde

[MKing]

The statement that 4% paraformaldehyde solution contains nothing but 
water and formaldehyde is simply not true: as has been discussed 
thoroughly and often on Histonet (see archives), formaldehyde in aqueous 
solution spontaneously decomposes.  This is precisely why formalin 
solutions are used, with methanol 'parking' the chemical reaction. 
Without measurement one never knows exactly how much formaldehyde is 
really present in solutions made from paraformaldehyde, although freshly 
made solutions will have concentrations closest to 4%.  It is highly 
improbable that 3.8 and 4.0% solutions would produce any detectable and 
reproducible differences on any histological procedure, although the 
methanol added to formalin might.

Mike King
UF Pharmacology & Therapeutics

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Message: 5
Date: Wed, 27 Aug 2008 17:26:19 -0400
From: "Monfils, Paul" <PMonfils <@t> Lifespan.org>
Subject: RE: [Histonet] (para)formaldehyde
To: <histonet <@t> lists.utsouthwestern.edu>
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<4EBFF65383B74D49995298C4976D1D5E03835C54 <@t> LSRIEXCH1.lsmaster.lifespan.org>
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10% formalin and 4% paraformaldehyde are interchangeable for most 
purposes (in histology at least).  However there are a couple of minor 
differences.  First, commercial formaldehyde solution contains 37% to 
38% formaldehyde.  Therefore diluting it 1:9 results in a solution 
containing 3.7% to 3.8% formaldehyde, while a 4% solution of 
paraformaldehyde in water contains a full 4% formaldehyde.  Secondly, 
commercial formaldehyde solution contains 10% to 15% methanol as a 
preservative.  Therefore diluting it 1:9 results in a solution 
containing 1.0% to 1.5% methanol.  This is not a problem for most 
histological applications, but it could be a problem in a procedure 
where sources of methylation have to be avoided.  4% paraformaldehyde 
solution contains no methanol - nothing but water and formaldehyde.