Howling
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Liquid extraction of acetic acid using ethyl acetate.
I have a vinegar solution containing 20% acetic acid. The remainder is water. Using a combination of liquid liquid extraction and distillation i want
to concentrate the acetic acid as much as possible. As solvent I want to use Ethyl acetate.
Could the amount of solvent needed for a certain extraction amount of acetic acid be calculated using the Distribution ratio?
Distribution ratio of the ternary system ( acetic acid/water/ethyl acetate) = 1.912
I am having trouble calculating the amount of solvent I use for this task, as I have never done this before.
Say I would use 200ml of vinegar, which contains 40ml of acetic acid, and 50ml ethyl acetate. Would this be the correct way to calculate the amount of
extracted acetic acid?
1,912=(X/50)/((40–X)/200)
x=12.9 ml AA extracted into the solute.
As Ethyl acetate has a much lower boiling point than acetic acid I should be able to use distillation to concentrate the acid.
Would like to hear your thoughts!
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Detonationology
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Correct me if I'm wrong, but I believe that liquid-liquid extraction only works when extracting something DISSOLVED in a solution using a solvent that
the solute is more soluble in (i.e. caffeine extraction from coffee using DCM). The most common, and probably easiest method of making concentrated
acetic acid is by distilling an anhydrous acetate salt with concentrated sulfuric acid.
“There are no differences but differences of degree between different degrees of difference and no difference.” ― William James
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gdflp
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Theoretically, yes this should work. It's a process used industrially to recover acetic acid, as shown in the document attached below. That document
claims a distribution coefficient of 0.84 however, not 1.912. See
<url=http://www.sciencemadness.org/talk/viewthread.php?tid=62973&page=23#pid429746>here</url> for a description on how calculate the
efficiency of an extraction.
Attachment: P150e0.pdf (220kB)
This file has been downloaded 728 times
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Amos
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Quote: Originally posted by Detonationology  | | Correct me if I'm wrong, but I believe that liquid-liquid extraction only works when extracting something DISSOLVED in a solution using a solvent that
the solute is more soluble in (i.e. caffeine extraction from coffee using DCM). The most common, and probably easiest method of making concentrated
acetic acid is by distilling an anhydrous acetate salt with concentrated sulfuric acid. |
You're right; acetic acid is DISSOLVED in water. Therefore it can be extracted with ethyl acetate.
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Howling
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Thank you for your replies.
I recalculated the extraction process with the coefficient provided by gdflp. Obviously the extraction is somewhat less favorable using the lower
ratio.
Using 50 ml ethyl acetate for each extraction from a 100ml vinegar solution containing 20wt.% acetic acid, the extraction process should give the
following results:
Given: Kd= ( Wo /Vo )/ ( Waq / Vaq )
o and aq refer to the ethyl acetate and water layer. Wo and Waq are the weights in grams of dissolved acetic acid for the respective layer.
First extraction:
Calculation: 0,84=(Xo1/50)/((20-Xo1)/100)
5.91 grams of acetic acid extracted
Second extraction:
Calculation: 0,84=(Xo2 /50)/((14,09 –Xo2)/100)
4.61 grams extracted.
Would this be correct?
Giving a total extraction worth of 10.07 grams of acetic acid in 100ml of ethyl acetate. This is a dilution of the original mixture. As the boiling
point of ethyl acetate is 77 degrees Celsius, using ( simple?) distillation I can more easily concentrate the acetic acid.
I am still figuring out the best ratio of ethyl acetate / water for the extraction. This would minimize losses, solvent use and distillation effort.
Guess there is no quick way about that, I will just calculate several scenario's.
[Edited on 7-12-2015 by Howling]
[Edited on 7-12-2015 by Howling]
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FormicAcidAnimal
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Easy way would be liquid-liquid continous extraction using a clogged soxhlet extractor etc.
You could use ~100mL EtOAc to extract a liter of conc. AcOH
If you do it by hand - most effective is always to use many extractions with small amounts of extraction solvent - the more and the smaller the
portions the more effective it get's (based one the amount of extraction solvent is used)
Addition of salts like NaCl, Na2SO4 into the 20% acetic acid might improve yield.
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Howling
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Thank you for the suggestion, I would like to do the extraction by hand.
Looking at figure 4 of the file that user ''gdflp'' attached to his post I figure that there is a minimum amount of ethyl acetate required to form a
two phase system. Given the acetic acid concentration of 20% a two phase system would form in the region of 0.6 to 0.2 ethyl acetate. Would this make
the minimum amount of ethyl acetate to be added to around 20% of the water content present?
Sorry if these questions are too general, I am completely new to this! 
Your help is much appreciated!
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gdflp
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Ethyl acetate is soluble to an extent in water, 83g/L, so there is a minimum amount of ethyl acetate required to form a biphasic mixture. Your
proposed 50ml of ethyl acetate per extraction far surpass this limit however, so there shouldn't be an issue. Also, this link should have been in my last post.
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Howling
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Thank you for the link
This would mean that if I use 100ml vinegar and 20 ml of ethyl acetate, roughly 8.3 grams of the ethyl acetate will ''forever'' be diluted in the
water.
Say I would use 20g of ethyl acetate for the first extraction, I can expect to get a return of just 11.7 gram, correct? So I should calculate with the
extraction value of just 11.7 grams for the first extraction? As only the following extractions will yield the full 20gram.
Would love a response for this!
I will be doing my extraction on the 15th of this month, I will keep you guys posted.
[Edited on 9-12-2015 by Howling]
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DraconicAcid
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| Quote: Originally posted by Howling |
This would mean that if I use 100ml vinegar and 20 ml of ethyl acetate, roughly 8.3 grams of the ethyl acetate will ''forever'' be diluted in the
water.
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No, because the acetic acid will increase the solubility of water in ethyl acetate and vice versa. If you want to do this as an extraction, I'd
suggest adding a bunch of salt to the vinegar. This won't salt out the acetic acid, but it will reduce the solubility of both the acetic acid in
water and the ethyl acetate in water.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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clearly_not_atara
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Best salt for salting out is probably NaHSO4 or KHSO4. Lowering the pH of the aqueous phase prevents deprotonation of acetic acid to acetate, which
raises the effective solution enthalpy. At pH2 AcOH will migrate to a nonpolar phase but at pH 10 AcO- strongly prefers the aqueous phase.
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Howling
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Thank you for the suggestions on the salt addition. I will use NaCl as my salt of choice, as it is easy to come by and seems to do the job just fine.
I just ran some small tests, the addition of NaCl does indeed make the fase separation happen earlier. What would be the correct way to introduce the
salt? Dissolve it in the vinegar before its introduction to the funnel? The study i found the ternary diagram in did not specify this...
Part of the salt appears to dissolve in the upper solvent layer. Is this possible? If it does, how should i go about removing the salt after the
liquid extraction is complete? Filtration?
I am afraid the addition of salt is going to cause problems during distillation. As the acetic acid will not evaporate, and will be left in the
distilling flask together with the salt. The order of magnitude i will be working in is small, creating just ~8ml of concentrated acetic acid for each
run. (The distillation setup i use cannot hold more than 100ml liquid )
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chemrox
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Could you remove most of the water with refluxing toluene and a dean-stark? It wouldn't have to be soluble.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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FormicAcidAnimal
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| Quote: Originally posted by chemrox | | Could you remove most of the water with refluxing toluene and a dean-stark? It wouldn't have to be soluble. |
I think it won't work very well as acetic acids and toluenes boiling points are not that far away from each other and so aqueous acetic acid might be
collected instead of pure water. And acetic acid is not easy to destill from toluene without a good vigreux column, at least not clean. Imho it's not
worth the effort only to get some conc. acetic acid.
But maybe a good water-jet vacuum may break up the H2O/HOAc azeotrope like it does with H2O/EtOH, so that you could simply distill water from 20% acid
to reach a high concentation but that's only speculation - but maybe a speculation worth to try  .
[Edited on 14-12-2015 by FormicAcidAnimal]
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gdflp
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But you're not concerned with toluene's boiling point. Rather, it would be the boiling point of toluene/water azeotrope which is 84°C. The
distillation would need to be closely monitored though, as this would shoot up once all of the water was removed. Worth a shot on a small scale.
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Dr.Bob
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Can't vinegar simply be boiled to concentrate the acetic acid? That seems simpler. I'm just a simple person, so I aim to do things easily.
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Detonationology
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Although acetic acid has no azeotrope with water, the boiling of acetic acid is 118˚ centigrade, which is much too close to that of water to distill
by means of a simple distillation. I would guess that it could be possible to concentrate the acid to ~30-40% (no data to support this claim) using a
~400mm vigreux column; however, a very large fractionating column would be necessary to achieve glacial acetic acid.
[Edited on 12-14-2015 by Detonationology]
“There are no differences but differences of degree between different degrees of difference and no difference.” ― William James
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DraconicAcid
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If you wanted to extract it in fewer steps, perhaps you should neutralize it with sodium hydrogen carbonate, let it dry, add either sulphuric acid or
concentrated sodium hydrogen sulphate solution, and then extract with ethyl acetate.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Howling
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The results are in!
The liquid extraction process was quite erratic. The 100ml 20% acetic acid vinegar solution was soluble with ethyl acetate up to 25 ml. So the first
extraction step only yielded a small portion of solvent. The water fraction actually increased in weight by about 15%. For the second extraction the
water was already saturated with ethyl acetate, so the phase transition showed way earlier and yielded a larger portion of solvent. During the third
subsequent extraction a LOT of foam was produced during shaking. Had to let it settle for an hour before I could extract the two phases.
Did the entire liquid extraction process 4 times, did the amount of runs same for the distillation. The first 3 tests were run without addition of
NaCl, during the fourth run I added 7 grams of NaCl during the first extraction stage. This caused the phase transition to happen earlier, also the
water fraction did immediately reduce in weight.
During distillation all the solvent evaporated easily. The average result of the 3 test was 65.52wt.% acetic acid. Pretty sure the balance is made up
of entirely water, with some ethyl acetate traces. The test with the addition of NaCl yielded the highest result, 72.49wt.% acetic acid. My intention
was to only evaporate the solvent. No attempt was made to remove the water fraction.
I would like to thank you guys for your suggestions and help  . Another test I
might want to try in the future is vacuum/distillation with the same stock vinegar solution.
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