[Nhcoll-l] Question about buffered formalin, what buffer should I use?

[Bentley, Andrew Charles]

Reposting this from John Simmons on a similar question on the Natural History Collections Facebook page (because I couldn’t have said it better myself):

What is recommended is a 1:9 mixture of formaldehyde and deionized water, buffered with two salts, which is to say, use 4.0 g of monobasic sodium phosphate monohydrate plus 6.5 g of dibasic sodium phosphate anhydrate per 1 liter of a 1:9 mixture of formaldehyde and deionized water, which will yield a mixture with a pH of approximately 7.0, which is good. Tap water should not be used because its pH varies widely from place to place and over the course of the year—to make reliable formaldehyde mixtures with tap water you need to test the pH of the tap water each time and adjust your buffering agent accordingly (and even then, you will not know what buffers the tap water already contains from its processing, or if it is acidifying from chlorination, etc.). Distilled water tends to be too acidic and, in any case, its pH tends to be that of untreated tap water, which will most likely not be in a suitable range. In the field it is usually impractical to use deionized water but carry a supply with you if possible.
Unbuffered formaldehyde is very acidic. The pH of full-strength formaldehyde is around 2.5 to 3.5 and when diluted 1:9 it will still be around 3.0 to 4.6.

Andy

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Andy Bentley
Ichthyology Collection Manager
University of Kansas
Biodiversity Institute
Dyche Hall
1345 Jayhawk Boulevard
Lawrence, KS, 66045-7561
USA

Tel: (785) 864-3863
Fax: (785) 864-5335
Email: abentley at ku.edu<mailto:abentley at ku.edu>
http://ichthyology.biodiversity.ku.edu<http://ichthyology.biodiversity.ku.edu/>

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From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu> on behalf of Esther Dondorp <esther.dondorp at naturalis.nl>
Date: Friday, February 12, 2021 at 3:27 AM
To: "nhcoll-l at mailman.yale.edu" <nhcoll-l at mailman.yale.edu>
Subject: [Nhcoll-l] Question about buffered formalin, what buffer should I use?

Dear colleagues,

I need your advice on what buffer to use to fixate a large eel in formalin.  I have to order 10% buffered formalin but I get the question from the chemical company what buffer they should add.  The pH should be around 7 I guess?

Many thanks and best regards,

Esther Dondorp
Senior Collectiebeheerder
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