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nimgoldman
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Diluting Ethanol to 95% (Azeotropic)
Since ethanol I am able to obtain by distillation is only up to 92% ABV, I dry the alcohol to >95% using molecular sieves. Then I filter and
distill the alcohol (to remove dust) and measure its density using tared volumetric flask. Then I look up its w/w concentration from table and calculate how much water to add to reach azeotropic concentration, which I believe is 95.5% w/w according to Wiki, though sources vary.
I assume the azeotropic concentration is the most stable one and the concentration most procedures call for (if not specified otherwise).
Unfortunately, my measurements vary from theory and I am not sure why. The ABV alcohol meter (0-100% range with 2% precision) shows the alcohol is 96%
ABV and density measurement shows 0.810 kg/L which should be something like 93% w/w.
Since 95% ABV corresponds to 95.5% w/w, this means either the alcohol meter shows higher concentration than actual, or the density measurement shows
lower concentration than actual.
I tested the alcohol meter on pure ethanol from pharmacy and it shoved 96% ABV. It also correctly shows 40% concentration on store-bought vodka so I
don't think the alcohol meter is off.
I also don't think there is significant amount of admixtures in my ethanol, that would skew the results. I filtered the ethanol with activated
charcoal and Celite and fractionally distilled 3 times, always taking the middle fraction running between 78-82 °C.
What also puzzles me is that even excess amount of lab grade molecular sieves (Carl-Roth brand) aren't able to produce 99% ethanol. I use over 20 g of
sieves per gram of water (way over the specified absorption capacity). I really have to use crazy excess (like half the volume of alcohol) to produce
reasonably anhydrous ethanol for certain syntheses. This then leads to a significant loss of ethanol. I usually let the sieves adsorb water for 1-2
days.
How would you achieve azeotropic ethanol from almost-azeotropic or over-azeotropic one?
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Tsjerk
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What is your source of ethanol?
Edit: can you do a Karl Fischer?
[Edited on 7-2-2019 by Tsjerk]
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SWIM
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If you want azeotropic ethanol from ethanol that's over 95%, how about just leaving the cap off for a while?
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Tsjerk
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Quote: Originally posted by SWIM  | | If you want azeotropic ethanol from ethanol that's over 95%, how about just leaving the cap off for a while? |
You will lose a significant amount of ethanol before reaching azeotropic.
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streety
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What type of alcohol meter do you have? Is the 2% precision over the entire range or is that at a specific concentration?
What volume did you weigh? If a small volume, could differences in the shape of the meniscus throw off the volume? What about the temperature?
Leaving the cap off may be wasteful but you could partly distill and then use what doesn't distill over.
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Heptylene
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Have you considered using a large volumetric flask and a scale? Volumetric flasks measure volume very precisely, scales measure mass very precisely
too (if properly calibrated). I'd assume the result would be better than an alcohol meter.
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nimgoldman
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I make the ethanol from sugar wash (white sugar with Alcotec Turbo Yeast 23%). After fermentation, I fractionally distill it using 60 cm Vigreux
column, then twice with Vigreux and packed columns with activated charcoal filtering between distillations.
I used 1L volumetric flask and a scale with 1 g precision.
I measured same density with 250 mL flask and scale with 0.01 g precision and got same result.
The amount of liquid under and over meniscus have negligible effect on the result being the concentration measured by density is very different from
that given by ABV meter.
The alcohol meter is has 2% steps over whole range, here is one image of its use:
The problem is the ABV meter says my alcohol is 96% ABV which should be more than 95% w/w, but the density table says it's only 93% wt% ... I don't
know how to explain the discrepancy. I even tried adjusting temperature of the alcohol to 20 °C but with same result.
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Heptylene
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Maybe your scale is off? If it has not been calibrated, there can be an error of about 0.5 % just due to earth gravitational field differences between
where the scale was made and where it is being used. And that's not counting other sources of errors that might contribute.
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streety
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It sounds like you have been very thorough. I would expect 1L and 250mL volumes to be accurate. It might be worthwhile weighing a liter of water just
as a sanity check but I think I would trust your density measurements.
For the hydrometer I'm not sure I would assume the accuracy is +/-2% just because the gradations are 2%. The hydrometer you have also has a very wide
range. I would expect a more limited range for a high precision hydrometer.
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S.C. Wack
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If accuracy is proven one may perhaps seek a different sort of proof, like the different kinds of chromatography or color tests. AFAIK carbon does
little for small molecules...BTW I've seen fractionated samples from the same yeast turn yellow on reflux with KOH, from condensations.
There's probably a good reason why you have no references for the drying of 95% alcohol with sieves.
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unionised
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Azeotropic concentration depends on atmospheric pressure.
I'd check the balance and the flasks by weighing 250 and 1000 ml of water.
The density of alcohol/ water mixtures varies with temperature too so you need to measure that.
Hydrometers are usually calibrated for aqueous solutions. There's a small effect from surface tension if you use them for other materials.
If you use the same balance and the same flask to weigh 250 ml of water and 250 ml of ethanol then calculate the density from tehratio (so it doesn't
matter much if your flasks wrong or the balance is miscalibrated) you should get a figure that's more reliable than any meter.
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Tsjerk
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I have the feeling your ABV meter is off. I would trust density measurements over this meter as I have a vague recollection of these meters being
imprecise in the very low or very high range.
I also think the way to go is checking your balances with water. If they are accurate density is the way to go as I don't see turbo yeast with white
sugar producing much else than ethanol.
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happyfooddance
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No it doesn't. Not once is temperature mentioned anywhere, never is anything calibrated, checked against a standard... I wouldn't expect any accurate
anything from the techniques and suggestions in this thread.
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nimgoldman
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Yes temperature has been checked and one of the scales (the 0.01g one) has been recently calibrated using two point calibration. Note that both
scales give the same result.
Also when I load the ethanol with sieves to make it practically anhydrous, the hydrometer reads 100% - since both the vodka reading and the anh.
ethanol readings seem accurate, I might assume the hydrometer is also.
Well the problem is that I just want 95% ethanol since this seems to be the standard concentration. I don't want to overshoot or undershoot the amount
of ethanol in a synthesis that calls for 95% and I also don't like changing the percentage label every time a distill a new batch of ethanol :-)
One problem might be that I measure inside a somewhat slim 100 mL graduated cylinder. Maybe a wider container would be better... Another source of
skew might be atmospheric pressure and temperature (likely pressure).
Okay I will try weighing de-ionised water first just for control.
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streety
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happyfooddance, we are all going to have different views on what constitutes thoroughness. I would certainly appreciate reading how you would approach
this problem.
A concern I had was the potential use of a small volumetric flask coupled with an imperfect reading of the meniscus could have been one source of
error. I was pleased to read that a 1L flask had been used. The use of a second flask and balance exceeded my expectations.
Although not mentioned in the initial post, both temperature and calibration have been mentioned.
nimgoldman, I've been assuming the sieves are 3A. Is this correct?
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happyfooddance
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Thorough would be weighing pure H2O first, not later. Always maintaining 20 degrees Celsius. Using a high precision analytical balance.
A slim volumetric will give you better results than a wider one.
How I tackle this problem (I use lots of ethanol in various grades) is I take etOH and dilute it to roughly 92-94%, and I distill it slowly through a
400mm vigreux, collecting the fraction that distills at 78.1°C. I usually cut the distillation short when the pot gets above 79. Bottom of column
never goes beyond 78.1, and since I live by the beach I stay close enough to 1atm for my calculations.
I have never used an alcohol abv meter but I would expect that it would be not very accurate, and if I wanted a more accurate measurement of alcohol
concentration I would do a titration against dichromate (again, standardized solutions using de-ionized, de-gassed water at 20°C...)
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Sulaiman
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I have one of these cheap hydrometer kits
https://www.ebay.co.uk/itm/3pcs-Hydrometer-Accurate-Alcohol-...
Three scales and a thermometer with correction tables.
So far I can't find a fault with this kit.
Density changes with temperature cause significant errors if not compensated for.
e.g. "probably about 20oC" ... is not accurate enough.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Tsjerk
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| Quote: Originally posted by nimgoldman | | Then I look up its w/w concentration from table and calculate how much water to add to reach azeotropic concentration, which I believe is 95.5% w/w
according to Wiki, though sources vary. |
Can you tell us how you do this calculation?
The difference in density of liquids at "roughly 20 degrees" is negligible, at least it is when trying to determine between 95% and 96% ethanol.
[Edited on 10-2-2019 by Tsjerk]
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Sulaiman
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From the temperature compensation tables provided with the kit that I pointed to above;
a reading of 95 indicates 95%ABV at 20oC and 96%ABV at 15.5oC
a reading of 96 indicates 96% at 20oC and 95% at 24.5oC
so a 4.5oC difference is equivalent to a 1%ABV change in reading.
(6oC change for 99/100%AVB)
CAUTION : Hobby Chemist, not Professional or even Amateur
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S.C. Wack
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Thorough is proof by challenge...an easy and informing challenge would be comparison with gradually diluted undenatured ACS ethanol, AKA calibration.
Follow the directions and use lime like everyone else if you want to dry...no you can't just dump in MgSO4 or something and it'll all be cool...
[Edited on 10-2-2019 by S.C. Wack]
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happyfooddance
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While I have to agree with this, I will say that ball-milled anhydrous MgSO4 has been one of my favorite drying agents as of late. It is quick, has a
high capacity, and yanks water out of solutions that are already reasonably dry.
The only thing is having to ball mill it, and also storage requires a little planning and care.
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macckone
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The meter shown in the picture is not a high quality, high proof meter.
The recommended range of the meter and temperature matter, a lot.
I suspect that it is pretty accurate though.
Another thing that may throw off the measurements is ethyl acetate and associated impurities.
If you can't get azeotropic by distillation then you probably aren't getting rid of those either.
It takes 5 or more passes through simple distillation to get azeotropic.
With a packed column, you should be able to do it in 2 passes with reflux.
But you stated you were doing a 4C range, and that is way too wide for final distillation.
I would work with a slower distillation and better reflux with a packed column.
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nimgoldman
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I see.
I should have stated the question differently: How to get "standard" 95% ethanol, provided that you have ethanol/water mixture of arbitrary ratio?
The impurities in my ethanol could be one cause of the discrepancy. I am thinkin about removing ethyl acetate, aldehydes and possibly other impurities
chemically before proceeding to second distillation.
I run the fermentation in a closet with controlled temperature (24 °C) so I doubt there will be typical impurities such as fusel oils, but ethyl
acetate is dfinitely a possibility. The yeast also come with plenty of yeast nutrient (DAP) of which some components might possibly come over.
Of course I don't need ultrapure ethanol, just the "standard" 95% ethanol usable for most amateur lab syntheses (making diethyl ether, alkyl halides,
washing and drying products etc.)
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Sulaiman
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Concentrating or diluting ethanol to azeotropic concentration may be a useful exercise,
but I would try for anhydrous ethanol because;
. some reactions need to be anhydrous
. adding H2O is easy
CAUTION : Hobby Chemist, not Professional or even Amateur
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DavidJR
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| Quote: Originally posted by Sulaiman | Concentrating or diluting ethanol to azeotropic concentration may be a useful exercise,
but I would try for anhydrous ethanol because;
. some reactions need to be anhydrous
. adding H2O is easy |
I agree with this. I use molecular sieves 3A to get the last few percent of the water out - they are slow but they are also highly effective,
providing you use a sufficient amount. Store over more fresh sieves to keep the ethanol anhydrous, as absolute ethanol is quite hygroscopic.
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