Re: [Histonet] RE: formalin
Hi Hedley and All,
Over the years we have adopted in the US and a great deal of literature the use
of formalin to cover most formaldehyde solutions. Yes, it is wrong however, it
is sometimes difficult to change.
What is interesting to me is paraformaldehyde powder solutions are generally
called 4% as it is made (4gm to 100mL of 60C water). It is difficult to get an
understanding that anything starting as paraformaldehyde powder will end up as a
formaldehyde solution. I tend to think of formaldehyde as 37 to 40% gas in
water and formalin as what is usually called 10% Neutral Buffered Formalin, as
it is 10mL of 37% to 40% formaldehdye in 90mL of buffer or water. I know it is
really approximately 3.7% however it is not clear to a lot of people that this
what we are using. In speaking with people as a technical person these terms
were always sticky and required an explanation so we could talk about the same
concentration no matter what the starting material. Even the histology
textbooks do not explain this well.
Also John said on Friday that formaldehye contained 10% methanol. I have done a
good deal of referencing on this subject for a seminar and it can range from 5%
to 15% methanol depending on the manufacturer. It is the same with pre-made
10% NBF. The methanol precentage will depend on when the methanol is added and
if it is only in the 37% formaldehyde or added to the final NBF solution at the
time of manufacture. Reading the MSDS to determine exactly what amonut of
methanol is contained in a solution is critcal for some processes and should be
carefully monitored when vendor changes are made and problems arise.
Quoting "Glencross Hedley (RW3) CM&MC Manchester"
> Hi everyone
> It has always been my understanding that "formalin" like "Hoover" &
> "Sellotape" (and countless other words) is just a trade name. This
> describes a 37% solution of formaldehyde gas in water, and we should be
> talking about 3.7% (4%) formaldehyde and not 10% formalin.
> Hedley Glencross
> Manchester Cytology Centre UK
> For all my years I have believed that formaldehyde was the 37%
> concentrate without buffers, and that formalin was 10% buffered... learn
> something new every day!
> Thanks John!
> -----Original Message-----
> From: John A. Kiernan [mailto:email@example.com]
> Sent: Friday, May 13, 2005 11:31 AM
> To: Weems, Joyce
> Cc: firstname.lastname@example.org
> Subject: Re: [Histonet] quick question about formaldehyde
> Dear Joyce,
> Formalin = 37% formaldehyde; so no, it doesn't
> make a difference. Formalin is not buffered; it
> does contain about 10% methanol, which is put
> in to retard polymerization. When diluted to
> make a 4% formaldehyde fixative, the methanol
> concentration is 1%. Buffering of the dilute
> solution offsets pH changes due to the
> Cannizzaro reaction. It also inhibits the
> formation of blood-derived "formalin pigment"
> which forms after fixation in an acidic
> formaldehyde solution.
> Tim Morken is correct in saying we don't know
> the extent of chemical change in 12 year-old
> formalin. The fact that there's no expiry date
> sugggests that it's not much. For what it's
> worth, I've used formalin that's more than 5
> years old and fixation has been OK.
> John Kiernan
> london, Canada
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