Just for detail sake, methanal (formaldehyde) does NOT decompose in water, it just reacts with it to become methanediol (methylene glycol) and the amount not reacting and remaining as formaldehyde has been estimated in about 0.1% of the total in a 4% formalin solution.
The alcohol methanol is added NOT to prevent the formaldehyde hydration, but to as a stabilizer, to retard its polymerization, so it is very likely that between the moment a 4% formaldehyde solution is prepared using paraformaldehyde to the moment it starts to penetrate, bind and cross-link it will be very close to 4%
--- On Fri, 8/29/08, MKing wrote:
Subject: [Histonet] (para)formaldehyde
Date: Friday, August 29, 2008, 10:14 AM
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.
UF Pharmacology & Therapeutics
Date: Wed, 27 Aug 2008 17:26:19 -0400
From: "Monfils, Paul"
Subject: RE: [Histonet] (para)formaldehyde
Content-Type: text/plain; charset="iso-8859-1"
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.
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