[Histonet] RE: Formaldehyde vs Formalin vs paraformaldehyde (lengthy)

[Kathy Paton (WDHB)]

According to The Microtomists  Vade-Mecum 1937 you are absolutely
correct.
Formol, Formalin and Formalose are commercial names for the saturated
solution.
Cor lummy....you learn something new each day
Kathy Paton
Auckland
New Zealand 

-----Original Message-----
From: histonet-bounces <@t> lists.utsouthwestern.edu
[mailto:histonet-bounces <@t> lists.utsouthwestern.edu] On Behalf Of Morken,
Tim - Labvision
Sent: Saturday, 5 February 2005 08:34
To: histonet <@t> lists.utsouthwestern.edu
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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