[Nhcoll-l] Mixing EtOH

Wed Sep 7 11:17:37 EDT 2022

I think it is worth underscoring John’s recommendation: “that alcohol that is poured into a tray be checked for concentration before being returned to a container”. The rate at which a solution falls in concentration as it evaporates is inversely proportional to the depth of the solution. If that depth is only a few mm then the concentration will be become very low after just half the depth has evaporated. Even more drastic for more shallow layers.
Rob

From: John E Simmons <simmons.johne at gmail.com>
Sent: September 7, 2022 10:49 AM
To: Robert Waller <rw at protectheritage.com>
Cc: Haff, Tonya (NCMI, Crace) <Tonya.Haff at csiro.au>; nhcoll-l at mailman.yale.edu
Subject: Re: [Nhcoll-l] Mixing EtOH

Another reason to let the alcohol and water mixture sit for 24 hr before using it (in addition to Rob's explanation of mixing vs dissolving) is to allow it to cool off to room temperature. You have probably noticed that when the alcohol and water are mixed the container becomes slightly warmer, commonly called "heat of mixing," but more properly called enthalpy (https://en.wikipedia.org/wiki/Enthalpy_of_mixing).

Several people over the years have questioned the use of compressed air to mix the alcohol, as Andy described, assuming either that this would cause more bubbles to form on specimens or that it would tend to acidify the alcohol from the reaction with oxygen. In the years I was at Kansas we did not experience either phenomenon with the alcohol. Bubbles rarely formed on specimens, the alcohol is a steady 70% coming out of the spigot on the container, and none of the problems common to acidic alcohol mixtures ever were apparent.

The binary azeotrope is what causes alcohol to evaporate from the mixture faster than the water, hence the phenomenon of "the angels share" that evaporates from whisky barrels in storage. In an azeotrope, each substance retains its own properties, so the alcohol evaporates faster than the water. For this reason, I recommend that containers of ETOH never be left open while specimens are being examined (always replace the lid) and that alcohol that is poured into a tray be checked for concentration before being returned to a container. It does not take long for the alcohol concentration to drop significantly. The higher the alcohol concentration, the faster the drop.

Paul and I have discussed concentrations before and we disagree on this point. I prefer 70% for most uses because at that concentration alcohol is an excellent biocide. Higher concentrations dehydrate specimens more, and can interfere with the passage of alcohol through the tissues by dehydrating the outer layers rapidly. Preservation in alcohol is always a trade-off between good preservation and dehydration of the specimen. In my experience, for most purposes, higher concentrations of ETOH (above 70%) are not appropriate.

Our local micro-distillery, which is an excellent venue (https://www.bigspringspirits.com/), sells a t-shirt that says "Alcohol is not a problem, it's a solution" which of course is incorrect It should say "Alcohol is not a problem, it is a mixture..."

--John

John E. Simmons
Writer and Museum Consultant
Museologica
and
Associate Curator of Collections
Earth and Mineral Science Museum & Art Gallery
Penn State University
and
Investigador Asociado, Departamento de Ornitologia
Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Lima

On Wed, Sep 7, 2022 at 10:32 AM Robert Waller <rw at protectheritage.com<mailto:rw at protectheritage.com>> wrote:
Hi Tonya,
Good for you in doing those tests to discover that!
The key is to remember that “mixing” and “dissolving” are two distinct processes that occur at different rates.

  1.  The mixing creates a mixture of 95% ethanol and 0% ethanol (water). This is evidenced by a lack of optical clarity (waviness) in the mixture.
  2.  Those two separate components of the mixture gradually dissolve into each other creating a homogenous solution that will not separate.
  3.  Before a true and inseparable solution is formed, ethanol poor parts of the mixture can sink below ethanol rich parts and ethanol poor solution can sink below ethanol rich solution.

Therefore, the key is to continue mechanical mixing until the mixture has formed a homogenous solution throughout. As you have discovered, dissolution may take more on the scale of hours while mixing requires just minutes. Sorting this out will require some repeated testing of layers, as you have done, each time allowing some time after mechanical mixing for any possible separation (I suspect a few hours), until you are confident your mixture has become a homogenous solution.
I suspect many people on this list will be interested in your results so please do share. Could also make for an interesting poster at our next SPNHC meeting and an entry in STASHC.
Best,
Rob

From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu<mailto:nhcoll-l-bounces at mailman.yale.edu>> On Behalf Of Haff, Tonya (NCMI, Crace)
Sent: September 6, 2022 9:40 PM
To: nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu>
Subject: [Nhcoll-l] Mixing EtOH

Hello all,

I have a question about mixing storage strength (70%) undenatured EtOH. Typically we add the correct proportions of 95% EtOH and water to a container and paddle or invert the container repeatedly to mix the liquids, and allow it to sit for ~24+ hrs. The containers we like best to use for dispensing 70% EtOH are 20L plastic water containers with a tap at the bottom. Recently we used our digital alcohol meter to test the alcohol concentration from the top and bottom (tap) of one of these containers and found the alcohol concentrations wildly different  - ~80% at the top and ~60% at the bottom – despite having been mixed more than 24 hours earlier. This makes me really concerned that we could be regularly using concentrations that are much above 70% with specimens. I wonder if any of you have had a similar problem, or if anyone can suggest a solution? Is there a better way of mixing or of ensuring the solution is properly combined? Any thoughts appreciated.

Thank you!

Cheers,

Tonya
-------------------------------------------------
Dr. Tonya M. Haff
Collection Manager
Australian National Wildlife Collection
CSIRO
+61(0)419569109
https://www.csiro.au/en/about/facilities-collections/collections/anwc

_______________________________________________
Nhcoll-l mailing list
Nhcoll-l at mailman.yale.edu<mailto:Nhcoll-l at mailman.yale.edu>
https://mailman.yale.edu/mailman/listinfo/nhcoll-l

_______________________________________________
NHCOLL-L is brought to you by the Society for the Preservation of
Natural History Collections (SPNHC), an international society whose
mission is to improve the preservation, conservation and management of
natural history collections to ensure their continuing value to
society. See http://www.spnhc.org for membership information.
Advertising on NH-COLL-L is inappropriate.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.yale.edu/pipermail/nhcoll-l/attachments/20220907/9b95c774/attachment.html>