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Author: Subject: Making glacial acetic acid
bbartlog
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[*] posted on 17-3-2010 at 19:43


After doing ethanol and water distillation I was so pleased with my 300mm Hempel column that I decided to take a stab at separating acetic acid and water by mere distillation. I did the following:

Added 248g of anhydrous sodium acetate to a 2l RBF
Added 331g of 31%HCl (aqueous) to this, and stirred briefly
Heated this at reflux for 45 minutes
Began distillation. Over a period of a few hours, 149g of distillate passed over. The head temperature never went over 100C, and the ratio of liquid dripping back into the flask to that coming over was about 3:1. At this point I was dismayed by the very strong vinegar smell of the distillate, and decided to examine the result.

Titration of the distillate with 1M Na2CO3 indicated that it was about 24% acetic acid :( . After weighing other constituents I concluded that the acid remaining in the RBF was likely no more than 56% on net (considering the entire flask contents), although settling seemed to result in separation of a salt+NaCl layer so that the acetic acid on top was more concentrated. I poured off this top layer, which amounted to 107g, and set it aside for later drying and/or evaluation.

Sooo... why are acetic acid and water *so* hard to separate by distillation? They do not form an azeotrope (apparently), and while the separation in boiling points is not huge (118C vs 100C) I've achieved fairly clean separations of various other fluids with similar differences in boiling point, like the aforementioned ethanol and water. I thought about this a bit after looking at the way the liquid composition boiling point curve and the vapor composition dew point curve separate and come together for an azeotrope.
Is there such a thing as a 'near azeotrope', where a pair of such curves drawn for the range of mixtures had neither intermediate maximum nor minimum, but the curves still deviate from a straight line to the point where the boiling temperature remains *close* to the minimum over an extended range of mixture compositions? For example, when boiling 50/50 water:acetic acid, could the boiling point be much closer to 100C than to the linear interpolation of 109C?
Or is it more likely that I had problems because the water/NaCl phase in my RBF formed a separate phase, which being a saturated brine had a boiling point above that of plain water? I'm skeptical of this explanation just because most of the separation occurs in the column, after the solutes are no longer relevant.

All in all somewhat disappointing (even if what I poured off is fairly concentrated acetic acid I'll still only end up with about 50% of theoretical yield), and it's not like a few tweaks to the column will make much difference (I figure it's the equivalent of about six plates when used properly, which would suggest 20+ plates equivalent to achieve decent separation of acetic acid and water... :o )
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[*] posted on 17-3-2010 at 20:13


The liquid-vapor equilibrium curves for the water/acetic acid system are close together. Ie, the relative volatility is close to 1. So while there is no azeotrope per se, the number of "theoretical plates" required for separation by fractional distillation may indeed be high.

My graphical estimate indicated 16 theoretical plates are required when starting with 4 mole% acid. Starting at 20 mole% acid or higher in the pot should be a great help.

There's a lot of information and experience here:
http://www.sciencemadness.org/talk/viewthread.php?tid=2194&a...

[Edited on 18-3-2010 by Magpie]

[Edited on 18-3-2010 by Magpie]




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[*] posted on 2-4-2010 at 14:59


Quote: Originally posted by Panache  
(cut)... The acetate does not decompose below 150, so use an oil bath for the distillation if you're having trouble regulating temperature and never let it go past 150.


I've run this a few time now.Each time using 1-2 mole acetate and approx 2 times excess H2SO4 in a 1 litre flask.I find it difficult to drive off all the acid at this temperature restriction,with the pot still smelling very strongly of acetic acid and yeilds of only 60%.

My last attempt where i allowed the pot temperature way up did in fact lead to some type of decomposition as I was getting a fraction at about 150C.Seemed waxlike and mostly deposited in the condenser.Any idea at what this could be?(acetate and acid both reagent grade).

Any hope of improving yeilds and recovery?My initial thoughts are to use a smaller flask for the pot but I worry at the initial vigourous reaction.Or perhaps apply (aspirator) vacuum when the distillation stalls.

Comments and suggestions appreciated.




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[*] posted on 2-4-2010 at 20:55


I've always heated my acetate + H2SO4 flask until distillation stops somewhat ignoring the temperature. Should not decompose until well into the 400 degrees Celsius range since anhydrous acetate melts in the 300 range. I do find that my product is very contaminated with Sulfur Dioxide and requires further treatment with anhydrous copper sulfate to dehydrate and remove the sulfur dioxide.

I've given up this route to glacial acetic acid because it is too much hassle since I found a biodiesel supplier supplying ACS grade Glacial Acetic Acid for dirt cheap.
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[*] posted on 7-4-2010 at 09:45


Quote: Originally posted by starman  
Quote: Originally posted by Panache  
(cut)... The acetate does not decompose below 150, so use an oil bath for the distillation if you're having trouble regulating temperature and never let it go past 150.


I've run this a few time now.Each time using 1-2 mole acetate and approx 2 times excess H2SO4 in a 1 litre flask.I find it difficult to drive off all the acid at this temperature restriction,with the pot still smelling very strongly of acetic acid and yeilds of only 60%.

My last attempt where i allowed the pot temperature way up did in fact lead to some type of decomposition as I was getting a fraction at about 150C.Seemed waxlike and mostly deposited in the condenser.Any idea at what this could be?(acetate and acid both reagent grade).

Any hope of improving yeilds and recovery?My initial thoughts are to use a smaller flask for the pot but I worry at the initial vigourous reaction.Or perhaps apply (aspirator) vacuum when the distillation stalls.

Comments and suggestions appreciated.


its slow tedious and shit but that's tthe best way i've found to ensure purity, definietly use a vacuum and have your oil bath on 150. Believe me it will still only come over sedately. Using a 5l flask it takes 8hrs to get ~1L glacial. After about 4hrs, cycling the vacuum on and off really helps.
I have been trialling a new method using citric acid(anhydrous) and sodium acetate, the two are combined, well ground in a blender then packed into a cromatography column. The column leads into a collection vessel that is protected with a cacl2 tube. On top of the tightly packed column i inflate a large double ballon with dry argon. I leave it in the corner and forget it, occasionally reinflating the ballon. Thus far it isn't working great but i need to have a proper look at it, there's approximately 1/3 of the expected glacial through in around 10days however it must be tested. If it doesn;t get better i will try hot argon as a carrier gas, this should work well. In theory some water in this system should help things along without getting into the final product as long as everything is kept below 40C as the trihydrate of citric acid forms and is an excellent dessicant.




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[*] posted on 7-4-2010 at 16:58


Interesting Panache.You must keep us updated on that one.I've been looking at anodic oxidation of ethanol.Apparently its been pretty sucessful with platinum or gold and I'm wondering if PbO2 coated lead has the required overvoltage to get the job done.



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[*] posted on 1-6-2010 at 07:13


Has anyone here tried to separate butyl acetate + acetic acid mix by distillation with a column?

I failed many times... it cannot be that hard:(

Here is what I did:

Bought a paint stripper OTC with the following composition on the label: butyl acetate ~60w% butanol ~20w% acetone ~20w%

I shook the paint stipper with water, after the extraction the water had an alcoholic smell, but no acetone. I also fractionated the mixture, but only one component could be detected. with a B.P. of 124°C... so the stripper is mainly made of butyl acetate.. I presume..

In the next step, i took two litres of "acetic acid escencia" which is sold in my country in supermarkets, and it is 20w% acetic acid + some organic crap, and I azeotroped the water off by means of the stripper + a dean stark.

Then I poured the water free butyl acetate (?) and acetic acid mix into a RBFL for fractionation, and started to separate them.. but no matter what I did... I adjusted the reflux, the temp of the reboiler, even replaced the packing in the column, I got the same result every time... a liquid with a strong smell of acetic, but it is not totally miscible with water, IIRC 50v/v% is lighter than water and floating on top like hexane, and has a stong smell of.. nail paint..

I am a bit puzzled any help would be appreciated!
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[*] posted on 4-6-2010 at 09:13


Has anyone any experiance with the acid hydrolysis of Vinyl acetate? Its used in alot of wood glues as the polymer and as a better source I believe would be spackles which is composes around 68% of the spackle the remaining being light organics and water mostly. Drying the spackle produces the polymer I belive so removing water would be no problem.

Just curious if anyone has tryed even dilute AcOH from PVA. I believe it has been mentioned but never heard of anyone attempting it.





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[*] posted on 4-6-2010 at 12:45


Quote: Originally posted by JohnWW  
It is possible that some additional concentration may be obtainable by distillation with benzene or something,


----
Tis - it is claimed - done this way on an industrial scale, Azeotropic
distillation.


By da Thorpe sez you can get from 4% vinegar to 40 by freezing,
without loosing flavour.

[Edited on 4-6-2010 by The WiZard is In]
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[*] posted on 5-6-2010 at 22:43


thats real handy for pickeling my chillies wiz thanx
and octopus :)

[Edited on 6-6-2010 by Ephoton]




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[*] posted on 6-6-2010 at 10:01


But we have discussed these means Over and over again in the past many many times.....

What about Vinylacetate hydrolysis? If the price is right on the PVA or Vinyl acetate product this could prove rather cheep and not really bulky either.





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[*] posted on 6-6-2010 at 11:12


you can also produce acetic anhydride from vinyl acetate. No one has gone into detail on this forum but I have a reference in the reference section.



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[*] posted on 6-6-2010 at 11:45


can you PM me a link please? ill look on the SE but if you know where its at then that might be quicker.




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[*] posted on 23-7-2010 at 10:23


Sedit you have you managed to make acetic acid by ageing the alcholod with dilute nitric acid?

I managed to decompose the denaturating agent by keeping the diluted alcohol + HNO3 mixture at 60°C for a day, but again, nothing acetic.
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[*] posted on 23-7-2010 at 11:38


Never tryed dilute HNO3 only insitu from KNO3 and H2SO4. The start is violent but nothing unmanageable with a bucket of ice water as the first little bit of EtOH was added. Don't know how safely it could be scaled due to the large amount of heat and NOx generated in this synthesis. Perhaps outside in a snowbank you could bucket chemistry it but thats still to be seen.

However it is indeed large amount of AcOH but I have yet to work it up out of fear. After seeing reports of Nitric acid and AcOH exploading with heat im kind of reluctant to distill it for obvious reasons so its been sitting there bottled up buried up to its neck in sand because I dont know what the hell to do with it right now and I fear it could go off. I doubt it would spontanously but im not a big fan of "what ifs".

I also do not think what I made is glacial either since it has shown no signs of being so. I got side tracked right after performing the synthesis in reworking the entie layout of my lab to something that resembles a lab almost so its largely been put on hold.

You will find more then likely BTW that if you heat your mixture until NOx is produced it will start to bubble like soda pop and sustain a reaction from that point on. As magpie and the papers provided have shown there is an activation temperature needed before this reaction starts else you need NO2 present already to lower this activation temperature down to almost nothing. What concentration are you speaking of here anyway?





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[*] posted on 23-7-2010 at 17:34


Does anyone have solubility vs temp curves for the systems Acetic Acid/Citric Acid/Sodium Citrate/Sodium Acetate or any combination therefore?



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[*] posted on 22-7-2012 at 21:35


I put some 1500ml of vinegar in flask and 500ml of chloroform and then about 50 grams of table salt(to save the 8 grams of chloroform that would be dissolved into every liter of water otherwise) and mixed the contents for 5 minutes and let it sit for a while, then filtered the contents into separator and separated the lower layer and distilled off the chloroform, and about 60ml of acetic stenchy stuff remained. I expect it to be somewhat concentrated acetic acid, still need to test the freezing temp. The salted water that remains, has trace smell of vinegar, so the chloroform does pull it off.

Anyways, does anybody happen to know the solubility of acetic acid into chloroform? I have found out that it can dissolve up to 40-50% of its volume of acetic acid, so counting from this, 500ml chloroform could be used to reform about 6-7 liters of vinegar. Ok, I could test this out by myself since I have some pure reagent grade GAA and chloroform but just if someone has the data already?

Does the acetic acid break down when distilled? The commercial vinegar contains all kind of soluble shit and gunk which would be easy to get rid of by distillation.

[Edited on 23-7-2012 by chucknorris]

[Edited on 23-7-2012 by chucknorris]
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