[Nhcoll-l] Acidity in alcohol

Wed Oct 19 10:56:30 EDT 2016

This does highlight the difficulty in measuring pH in high concentration ethanol solutions and making sense of them, but ethanol is only weakly ionisable and strong acidic pH in such solutions in my experience tends to be a rare problem. Of course there are other factors at work such as type of glass (there is research that suggests soda glass can affect acidity), specimen being preserved, ratio of fluid to specimen etc. Also be careful just changing the solution because you feel you need to. Discoloration is mainly fatty acid type components, but these are only soluble to a few percent in the preserving solvent and the specimen / solvent will reach an equilibrium. This will change if you change the fluid which will just start extracting chemicals out of your specimen again! So unless there are issues such as turbidity, notable degradation in the preserved tissues or inadequate strength of preserving alcohol then probably no need to change the preserving ethanol solutions. The water you use to mix your solutions is another debate, but if its a clean supply it is not likely to have a significant effect on pH.

 - this paper may be of value;

https://urldefense.proofpoint.com/v2/url?u=https-3A__www.researchgate.net_publication_306392851-5FTHE-5FEFFECT-5FOF-5FPH-5FON-5FETHANOL-5FPRESERVED-5FMUSCLE-5FTISSUE&d=CwIGaQ&c=-dg2m7zWuuDZ0MUcV7Sdqw&r=CLFZJ3fvGSmDp7xK1dNZfh6uGV_h-8NVlo3fXNoRNzI&m=Ij9oUBA8xZZboTXH3p1gMKeUt2PFS89P-1nStN8Ja-M&s=JQIJO65vMfpN1tGelHnxpLEoMbuLhTMm1zYUH3paCv4&e= 

â€‹
Cheers

Jules
â€‹Conservator - National Museum Wales

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From: nhcoll-l-bounces at mailman.yale.edu <nhcoll-l-bounces at mailman.yale.edu> on behalf of Dirk Neumann <dirk.neumann at zsm.mwn.de>
Sent: 18 October 2016 22:10
To: nhcoll-l at mailman.yale.edu
Subject: Re: [Nhcoll-l] Acidity in alcohol

... maybe just to add: depending on which sort of specimens are under storage in your wet collection, they could either release larger amounts of fatty acids (e.g. in insect collections with fats leaching out of specimens) or acids such as formic acid which might dissolve directly out of specimens which could result in even lower pH values. You mentioned that you aim to restore on old collection: be careful with old specimen jars or insects which are sorted to small glass tubes inside jars. Low pH-values create a highly reactive surface layer on the glass which not only corrodes the jar surface (you might know this effect from dishwashers) but also specimens.

If crustaceans et al. are on storage, the pH might shift to values around 8 or 9 due to the shells, which producing an likewise unhealthy environment inside your specimen jar ....

Best would be to keep the condition stable, i.e. change the fluid only if there is a real conservation issue that needs to be dealt with.

Hope this helps
Dirk

Am 18.10.2016 um 22:54 schrieb rw at protectheritage.com<mailto:rw at protectheritage.com>:
Hi Nicolas,
Your calculation of pH is good. I get 4.22 for 12mg/L and 4.27 for 9.6mg/L so all are in the same ball park.
Roughly, a mg of acetic acid will dissolve about a mg of calcium carbonate (shell material) or a mg of hydroxyapatite (bone mineral) so that gives you a sense of how much reaction you can anticipate: one litre of solution will dissolve about 10mg of each of those materials. Depending on the composition of specimens and the fluid to specimen ratio that may or may not be significant.

I think Dirkâ€™s suggestion is very sensible. Can you find out whether or not your water is chlorinated?
Best,
Rob

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From: nhcoll-l-bounces at mailman.yale.edu<mailto:nhcoll-l-bounces at mailman.yale.edu> [mailto:nhcoll-l-bounces at mailman.yale.edu] On Behalf Of Dirk Neumann
Sent: October 18, 2016 10:32 AM
To: nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu>; Margraf Nicolas <Nicolas.Margraf at ne.ch><mailto:Nicolas.Margraf at ne.ch>
Subject: Re: [Nhcoll-l] Acidity in alcohol

Dear Nicolas,

it might be worth to compare the attached paper; my colleague did some tests which are summarized therein. Normal pH-paper tests will not work, you need to dilute the alcohol to get reliable measurements (see attached paper).

If your tap water is not chlorinated and pure but calciferous, you might dilute your 96% EthOH with tap water to 80%; the dilution needs to rest for at least one day, as calcium carbonate starts to precipitate; the residual carbonate in solution gives you some sort of buffer capacity.

Hope this helps
Dirk

Am 18.10.2016 um 16:00 schrieb Margraf Nicolas:

Dear all,

The MHNC is a small natural history museum located in the hearth of the Jura Mountains in La Chaux-de-Fonds, Switzerland. The MHNC team is currently re-conditioning the alcohol collection, changing containers and alcohol using an 80% Ethanol solution. The team was told to check the acidity of the alcoholic solution as acidity is not good for the specimens but had trouble doing so.
Is a 9.6 mg/l acidity expressed as acetic acid too acidic? (12 mg/l in 96% Et-OH diluted to 4/5)
Additional questions:
The alcohol dealer was asked about the acidity of the 96% Ethanol he provides us with. The value he gave us is 12 mg/l of acidity expressed as acetic acid. We tried to calculate the pH based on that value and obtain a value of 4.23. Does that sound right?
We tried measuring the pH of the solution with both pH-paper and a pH-meter and found conflicting values. Is the pH-paper reading valid for an alcoholic solution? Or is the pH-meter value valid for an alcoholic solution?
A pH of 4.23 seems too acidic to preserve specimens so we tried to calculate the amount of Na2CO3 necessary to neutralize the 12 mg/l of CH3COOH and obtain 10.6 mg/l. Is it correct (and sufficient) to add this amount of Na2CO3 to every liter of our 96% Et-OH solution?
Any advice or help would be greatly appreciated.
Thanks,
Sincerely,
N
*****************************************
Nicolas Margraf
Collaborateur scientifique
MusÃ©e d'histoire naturelle de La Chaux-de-Fonds
Av. LÃ©opold-Robert 63 - CH 2300 LA CHAUX-DE-FONDS
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