[Histonet] Re: Formaldehyde vs Formalin (Is it formol, not formal?)
jkiernan <@t> uwo.ca
Sun Feb 6 14:54:48 CST 2005
As always, you're right. Formal is methylal.
I'd never noticed that use of "formal" before. You're
quite right of course; it's all there in the Merck Index
and elsewhere, for all to read. One implication of this
is that we probably should not write "formal-saline."
This was advocated by J.R.Baker (Principles of Biological
Microtechnique) on the grounds that formaldehyde was an
ALdehyde, and not and alcohOL as implied by "formol."
A little methylal (= "formal") is present in commercial
formalin (= aqueous formaldehyde, 37%w/w = 40%w/v) because
of the small amount of methanol that's added to retard
polymerization, which results in precipitation of
insoluble paraformaldehyde (= polyoxymethylene).
There is an inconsistency in the Merck Index (I'm looking
at the 12th edition, 1996) that amounts to an error [Gasp!]
in the "Formaldehyde" entry. There it states that cooling
or evaporation of an aqueous formaldehyde solution yields
solid trioxymethylene. That's wrong; the solid stuff is
paraformaldehyde, a much higher polymer. Trioxymethylene
is a cyclic trimer - 3 formaldehyde molecules joined in a
small ring - more often called trioxane. The Merck Index
entries for "Paraformaldehyde" and "s-Trioxane" are up
to date and correct. The former states that the name
trioxymethylene has, in the past, been wrongly applied to
paraformaldehyde. Less than 30 years ago I had at least
one jar of paraformaldehyde from a major N. American
supplier with "(trioxymethylene)" also on the label, as
if it were a synonym.
Trioxane (real trioxymethylene) is very different from
paraformaldehyde. It dissolves in water without much
depolymerization - just enough to yield safely antiseptic
amounts of formaldehyde. It does not depolymerize at all
at pH >7 and could not simply be used as a solid source
of formaldehyde for making neutral buffered fixatives.
My source of chemistry for all the above is JF Walker's
book, "Formaldehyde" (3rd ed 1964; reprinted by Krieger,
NY in 1975). I was lucky enough to find one in a 2nd
hand bookshop in Britain about 15 years ago, quite by
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London, Canada N6A 5C1
Bryan Hewlett wrote:
> "Formal" is quite different, it is another name for Methylal
> (dimethoxymethane) a flammable colourless, volatile liquid used as a
> ----- Original Message -----
> From: "Morken, Tim - Labvision" <tpmorken <@t> labvision.com>
> To: <histonet <@t> lists.utsouthwestern.edu>
> Sent: Friday, February 04, 2005 2:33 PM
> Subject: RE: [Histonet] RE: Formaldehyde vs Formalin vs paraformaldehyde
> > I think "Formalin" and "Formal" were trade names from a company and just
> > went into common usage.
> > Tim Morken
> > -----Original Message-----
> > From: histonet-bounces <@t> lists.utsouthwestern.edu
> > [mailto:histonet-bounces <@t> lists.utsouthwestern.edu] On Behalf Of Bryan
> > Hewlett
> > Sent: Friday, February 04, 2005 11:24 AM
> > To: histonet <@t> lists.utsouthwestern.edu
> > Subject: [Histonet] RE: Formaldehyde vs Formalin vs paraformaldehyde
> > (lengthy)
> > I was hoping that John Kiernan would jump in reply to this issue with his
> > usual eloquence! However, here goes.
> > Confusion in terminology has been common since Blum introduced this agent
> > a fixative in 1893! It never ceases to amaze me that this should be so,
> > issue has been repeatedly addressed in all major Histotechnology texts
> > before the early fifties ( my student days in the UK).
> > The following information is from the 10th Edition(1981) of the Condensed
> > Chemical Dictionary and the10th Edition(1983) of the Merck Index (the only
> > ones at hand).
> > Formaldehyde is a gas.
> > It is readily soluble in water up to 55% and is commercially available to
> > as 37%, 44% and 50% aqueous solutions which may contain up to 15%
> > These commercial grades are called Formalin.
> > Formaldehyde solution (Merck Index)
> > The USP grade is about 37% (37-40%) w/v formaldehyde gas in water,
> > with 10-15% methanol added to prevent polymerization. This solution is
> > considered to be full strength and is also known as Formalin 100% or
> > Formalin 40 which signifies that it contains 40 grams of formaldehyde
> > 100mL of the solution. It is this solution that produces most of the
> > confusion since it is referred to and thought of as 100% Formalin.
> > Paraformaldehyde (Merck Index)
> > A white crystalline powder of polymerized formaldehyde, obtained by
> > concentrating formaldehyde solution. Upon solution in water
> > and evolution of formaldehyde occurs. Thus an aqueous solution containing
> > grams of paraformaldehyde is essentially the same as a solution of 4%
> > formaldehyde. There is NO such thing as a solution of paraformaldehyde.
> > Right John?
> > The concentration of formaldehyde used for fixation has been the subject
> > much confusion (see above). The concentration of formaldehyde in compound
> > fixatives varies widely - ranging from 0.5 to 15% w/v. The majority of
> > fixatives, using formaldehyde as the sole fixative agent, have a
> > concentration of formaldehyde between 2.5 and 4% w/v. The concentration of
> > formaldehyde in a fixative should be stated as the percentage by weight of
> > the gas, rather than as a percentage of the
> > formalin(sic) or paraformaldehyde(sic) used to prepare it.
> > Thus:
> > "4% formaldehyde" - not 10% formalin.
> > "4% formaldehyde, from paraformaldehyde" - not 4% paraformaldehyde. "NBF
> > means Neutral buffered formaldehyde" - (not formalin) and is 4% w/v
> > formaldehyde in phosphate buffer pH 7.0- 7.2.
> > Bryan Hewlett
> > Consultant Technologist
> > QMP-LS
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