[Histonet] Re: Formaldehyde vs Formalin (Is it formol, not formal?)

[John Kiernan]

Dear Bryan,

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
chance.
                              John.
-------------------------------
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1
______________________________________________________
Bryan Hewlett wrote:
> 
> Tim,
> 
> "Formal" is quite different, it is another name for Methylal
> (dimethoxymethane) a flammable colourless, volatile liquid used as a
> solvent.
> 
> Bryan
> 
> ----- 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
> (lengthy)
> 
> > 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
> as
> > a fixative in 1893! It never ceases to amaze me that this should be so,
> the
> > issue has been repeatedly addressed in all major Histotechnology texts
> since
> > 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
> us
> > as 37%, 44% and 50% aqueous solutions which may contain up to 15%
> methanol.
> > These commercial grades are called Formalin.
> >
> > Formaldehyde solution (Merck Index)
> > The USP grade is about 37% (37-40%)  w/v formaldehyde gas in water,
> usually
> > 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
> within
> > 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
> depolymerization
> > and evolution of formaldehyde occurs. Thus an aqueous solution containing
> 4
> > 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
> of
> > 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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