[Histonet] Re: Formalin Neutralizing

Bob Richmond rsrichmond <@t> gmail.com
Fri Jan 13 11:43:03 CST 2012

Three years ago Tony Henwood in Australia posted a formula for
neutralizing formaldehyde with ammonia. He also alludes to using
sodium bisulfite (a.k.a. metabisulfite), but I don't know the
proportions or how the reaction works. - I've posted a copy of his
note, below.

A lot of commercial formaldehyde neutralizers are pure mumbo-jumbo. Of
course, this has become an issue where what managers and regulators
think is a great deal more important than what actually happens at the
chemical level where MBA's fear to tread.

Bob Richmond
Samurai Pathologist
Knoxville TN
Date: Wed, 25 Mar 2009 09:45:43 +1100
From: "Tony Henwood" <AnthonyH <@t> chw.edu.au>
Subject: RE: [Histonet] FW: formalin neutralizers
To: "Burton, Lynn" <Lynn.Burton <@t> Illinois.gov>,
   <histonet <@t> lists.utsouthwestern.edu>

This is what we use:
Neutralization and Disposal of Formalin Fixative

10% Formalin can be neutralised with sodium bisulfite or concentrated
ammonia. The reaction with ammonia results in the formation of
hexamethylenetetramine (commonly known as hexamine or methenamine).
This can then be safely disposed of either as a liquid fertiliser or
via the sewage system (check with your local authority).

The reaction proceeds as follows:

6 CH2O + 4 NH3  ---> C6H12N4 (Hexamethylenetetramine) + 6 H2O


1.  Before beginning, personnel must have the following safety
equipment readily available in the event of an accidental spill:
sorbent material (spill pillows, bulk sorbent) formaldehyde rated
2.  Personnel must wear a lab coat or apron, safety goggles and neoprene gloves.
3.  A pH meter or pH paper
4.  To 1000 ml of 10% formalin (= 4% formaldehyde) add 56 ml of strong
ammonia solution (27%). This will generate 31 g of hexamine
(approximately a 3% solution).
5.  Stir well.  Reaction may produce heat.
6.  Initially, the pH of the formaldehyde solution will be about 6.
As ammonia is added and stirred, a fluffy white precipitate will
result.  Addition of sufficient ammonia will raise the pH to about 8.
Because the neutralization of the formaldehyde requires less molecules
of ammonia than the apparent acid-base reaction supplies hydronium
ions, the pH change from acid to base is used as an indicator that an
excess of ammonia has been added.
7.  Let set overnight (12 hours).
8.  The smell of formalin is greatly reduced or replaced by a faint
whiff of ammonia.
9.  Schiff's reagent is perhaps the best, most sensitive and available
reagent in any lab to test for the presence of aldehydes. If the
"neutralized" formalin turns purplish with the addition of Schiff's
reagent, it is not totally neutralized and you will need to add more
10. Dispose of appropriately.

I am not sure how the bisulphite method works. I picked it up from a
reference on formalin neutralisation but have never tried it.

And would you believe that after some searching I can't even find that
reference (I probably have it on my home computer).

The notes come from my "Infamous" text book I have been writing for
the last 20 years. As my staff call it, the book that will probably
never be
published. But then the chapters are quite usefull for teaching so
they are of some use.

Tony Henwood JP, MSc, BAppSc, GradDipSysAnalys, CT(ASC)
Laboratory Manager & Senior Scientist
Tel: 612 9845 3306
Fax: 612 9845 3318
the children's hospital at westmead
Cnr Hawkesbury Road and Hainsworth Street, Westmead
Locked Bag 4001, Westmead NSW 2145, AUSTRALIA

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