[Nhcoll-l] FW: use of air to bubble through ethanol to mix solutions.

Tue Jun 22 03:11:51 EDT 2021

Good afternoon Tonya,

interesting question - do you use pure ethanol, or denatured ethanol, 
and if the latter, which denaturant is added?

If you use the bubbler to aerate the ethanol to fix it, my first thought 
would not be the oxygen (assuming that you do not use pure, bottled 
oxygen for mixing), but the carbon dioxide, which surely leads to an 
acidification of the remaining water fraction, even more so, if 
distilled or bi-distilled water is used to dilute the ethanol, because 
distilled water has a very high affinity for carbon dioxide. Even though 
a bicarbonate-puffer system establishes, the pH is rather at the low end 
and usually around pH 4-5 in distilled water, so rather acidic. This 
surely is different in tap-water (assuming it is of certified quality 
without any additives such as chlorine), especially if it is calcareous. 
But still, when mixing both fluids, you shift the dissolving 
equilibrium, and what you can spot is a lot of fine air bubbles that 
escape from the mixture (carrying capacity for the physically dissolved 
gases shifted), and (usually) a fine white precipitation (calcium 
carbonate).

Thus the carbon dioxide likely has more effect than the oxygen on the 
acidity of the aqueous fraction of the mixture.

Next step: secondary reactions like oxidation.

While ethanol is a rather stable molecule, less stable molecules would 
be attacked and oxidised first, because their unsaturated polar bindings 
have the higher affinity to attract electrons. For example, residual 
formaldehyde escaping from specimens might be oxidised to formic acid. 
Same applies to highly polar denaturants like ketones, which are more 
susceptible to be oxidised. The more such molecules are present and 
oxidised, the less the ethanol will.

And you less likely can prevent this by mixing the ethanol with N2, 
because stirring the mixture will also allow other gases to dissolve 
(unless this is done in a hermetically closed atmosphere). And even if, 
upon initial filling or topping, the fluid will have a lot of contact 
with the surrounding air and will be stirred up, and carbon dioxide and 
other gases will dissolve etc. ...

We you use a big stick to stir the fluids when mixing the ethanol in a 
drum, we usually mix 60 litres and let it rest for two days until we use 
it. We have two of these drums, which is sufficiently enough for our 
daily work, and we are careful when filling jars to avoid any air 
bubbles, i.e. we fill in the ethanol slowly.

Hope this helps,
Dirk

Am 22.06.2021 um 05:12 schrieb Haff, Tonya (NCMI, Crace):
>
> Hello all,
>
> We typically use a fish bubbler to mix ETOH and water together when we 
> mix up 70% ETOH. We are in the stages of planning a new building, and 
> I recently got the below feedback about doing this – the dangerous 
> goods consultant says that this has the risk of acidifying the 
> solution, and that using N2 to mix the solution might be a better 
> idea. Do any of you have thoughts on this? If so, I would really 
> appreciate hearing them!
>
> Cheers,
>
> Tonya
>
> *Email:*
>
> As per our conversation last week, you mentioned that CSIRO plan on 
> using air to bubble through their ethanol solutions to help mix the 
> solution. Can you please confirm if this is correct with CSIRO?
>
> Normally when you mix/bubble air through ethanol solutions, you may 
> run the risk of oxidation of the ethanol, although this reaction is 
> very slow, you may generate some acetylaldehyde and eventually 
> ethanoic acid.
>
> Can you confirm with CSIRO if using nitrogen is better? This would 
> still help mix the solutions and would have the added benefit of 
> degassing the ethanol by removing dissolved oxygen from the solution. 
> This long term, would help minimise the risk of oxidation of the 
> ethanol and generation of acetylaldehyde and eventually ethanoic acid.
>
> Regards
>
> Paulo Da Silva
>
> <https://www.cetec.com.au/>
>
> 	
>
> Dr Paulo Da Silva
> PhD, BSc(Hons), M.RACI, M.AIDGC,
> NABERS Accredited Assessor
> Principal Consultant - Dangerous Goods
>
> +61 458 639 888 | +61 2 9966 9211
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>
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-- 

Dirk Neumann

Tel: 089 / 8107-111
Fax: 089 / 8107-300
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---------

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neumann(a)snsb.de

postal address:

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