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

[Bryan Hewlett]

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