[Histonet] Re: Formalin Neutralizing

Tony Henwood (SCHN) tony.henwood <@t> health.nsw.gov.au
Sun Jan 15 16:23:30 CST 2012

Thanks Rob for reminding me.

It was John A. Kiernan who originally posted the message that I refer to below on Histonet back in 2002.

As everyone will agree John is a great teacher and one of our "National Treasures" in Histotechnology.

Tony Henwood JP, MSc, BAppSc, GradDipSysAnalys, CT(ASC), FFSc(RCPA) 
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 

-----Original Message-----
From: histonet-bounces <@t> lists.utsouthwestern.edu [mailto:histonet-bounces <@t> lists.utsouthwestern.edu] On Behalf Of Bob Richmond
Sent: Saturday, 14 January 2012 4:43 AM
To: histonet <@t> lists.utsouthwestern.edu
Subject: [Histonet] Re: Formalin Neutralizing

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 respirator.
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 ammonia.
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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