[Nhcoll-l] Mixing EtOH

[a.j.van_dam at lumc.nl]

Dear Tonya,

When you measure with a digital densimeter, it is very important that the solution has cooled down after mixing. The thermodynamic process of mixing leads to significant rise in temperature of the fluid and the formation of tiny air bubbles. It takes a pretty long time before all these bubbles have surfaced and the fluid has cooled down. Taking a sample with a digital densimeter before these processes have settled will certainly lead to wrong read-outs, because the instrument measures the weight of the fluid sample being drawn into a folded glass tube by means of oscillation. When there is air in the fluid sample or the fluid temperature is higher than the calibration temp. the tube will be lighter and resonate at a higher frequency leading to a false and higher read-out of the ethanol concentration.

Kind regards,

Dries

From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu> On Behalf Of Robert Waller
Sent: woensdag 7 september 2022 16:33
To: Haff, Tonya (NCMI, Crace) <Tonya.Haff at csiro.au>; nhcoll-l at mailman.yale.edu
Subject: [MOGELIJK SPAM ! *****] Re: [Nhcoll-l] Mixing EtOH

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.yale.edu/pipermail/nhcoll-l/attachments/20220907/d6d1ad38/attachment.html>