Sciencemadness Discussion Board

Fractional freezing to GAA

DocX - 30-4-2022 at 12:22

So I have a lot of 60% acetic acid and I need Glacial Acetic Acid. I've read up somewhat on fractional freezing, which seems like a perfect way to go. Or is it?
I understand that a 60% solution of acetic acid and water should freeze out at about -10 C, but what do I really get? Frozen water in liquid acetic acid or vice versa?

What is the real practical way to get rid of the water from 60% AA through fractional freezing?

[Edited on 20222222/4/30 by DocX]

aab18011 - 30-4-2022 at 13:07

You can try making the sodium salt, dessicating to remove excess water, then using sulphuric acid to make glacial acetic acid. However, I dont remember if acetic acid forms inseparable hydrates.

DocX - 30-4-2022 at 13:38

Nah, I was wondering about the fractional freezing method. Not the ones using expensive and rare SA.

aab18011 - 30-4-2022 at 13:45

Sulfuric acid is neither expensive nor rare. Where are you from?

DocX - 30-4-2022 at 22:28

I'm from the EU, let's leave it at that. And yes it is. It's on the narcotics precursor list since several years, and not available for purchase for private individuals. But this is not what I made this thread to discuss, please respect the theme of the thread which I stated many times now.

Fery - 1-5-2022 at 00:03

DocX I suggest you to concentrate 60% acetic acid to e.g. 98% by a distillation and then perform fractional crystallization.

DocX - 1-5-2022 at 00:38

Ok, that might be a valid way. But it would be easier and less time-consuming to just shove it in the freezer and decant off the liquid part, as mentioned in this thread. But you think distilling is wiser?

macckone - 1-5-2022 at 01:01

acetic acid water mixtures tend to supercool.
so the better way to do this is freeze it solid then let the water portion melt.
this is slow and tedious.

There are alternative methods that might work better.

1) sodium acetate and sodium bisulfate (not restricted in the eu but I could be wrong - sold as ph down for pools)
2) sodium diacetate - 1M sodium carbonate + 4M acetic acid - boil off water slowly - then drive off acetic acid
3) sodium acetate and phosphoric acid (naval jelly)
4) buy glacial acetic acid - last I checked you can still get it from places that sell photographic chemicals it is a common reagent

DocX - 1-5-2022 at 01:38

Glacial acetic acid is right there on the narcotic precursor list along with sulphuric acid. Doesn't necessarily mean its impossible to buy though, acetone is also there but definitely sold OTC.

Anyway, I have this 5L jar of 60% acetic acid, and fractionally distilling it in portions in my small glassware would be a hassle. So just shoving the jar into the freezer would be much simpler.

[Edited on 20222222/5/1 by DocX]

[Edited on 20222222/5/1 by DocX]

unionised - 1-5-2022 at 01:50


Let the experiment be made.


Put some in the freezer + see what happens.
Use a plastic (HDPE) bottle so any change in volume doesn't break the container.

What's the worst that can happen?

You can follow the concentration changes by density or titration or refractive index or whatever else you can measure easily.

SWIM - 1-5-2022 at 03:25

Quote: Originally posted by unionised  

Let the experiment be made.


Put some in the freezer + see what happens.
Use a plastic (HDPE) bottle so any change in volume doesn't break the container.

What's the worst that can happen?

You can follow the concentration changes by density or titration or refractive index or whatever else you can measure easily.


Yeah, save the save the supernatant liquor and at worst you're back where you started. zero risk, all it costs is freezer space.

Amos - 1-5-2022 at 05:26

If you don't have much success freezing it out, this could be the perfect excuse to buy a larger boiling flask so you could distill it in large portions. I'd recommend keeping a good-sized portion of 60% for making metal acetates, as you need some water in there anyway(unless adding hydrogen peroxide solution).

Fully neutralizing acetic acid just to laboriously precipitate and dry the salt to anhydrous only to then convert it back to acetic acid is an incredibly time-consuming process and not even worth contemplating. Plus you'll lose like 35% of your yield along the way.

[Edited on 5-1-2022 by Amos]

Tsjerk - 1-5-2022 at 07:01

I know from experience freezing is a good method to get the last 1-2% of water out of acetic acid. I don't know how 60% acetic acid is going to behave. Can't you add anhydrous MgSO4, decant/filter, distill and then freeze precipitate the acid?

When you freeze 98-99% acetic acid and let it thaw a bit you get a porous brick of glacial acetic acid from which you can decant the less concentrated acid with most of the water.

Fery - 1-5-2022 at 10:53

Look at the curve of mixture what you get when freezing 60% acid and look the freezing point too. Concentrate it by distillation first as I suggested already.
I got good result when putting 1 L bottles of 98% acid into fridge to +4 C where most of the acid crystallized and about 100 ml stayed as diluted acid which I simply poured out of a bottle. I had 2 bottles each 1 L of 98% acetic acid for decades so I wanted to try whether such concentration by freezing works. I used the diluted acid poured off the bottles for some neutralizations in organic chemistry experiments.
I also observed the supersaturation as already mentioned in the thread.
When I was recently buying glacial acetic acid I had to fill some documents for what purpose I will use it... I did not know it is on precursor list, now I understand why the filled documents. I'm living in EU.

Fery - 1-5-2022 at 10:56

I forgot to mention that acetic acid when freezing it does NOT expand in volume unlike water. So both glass bottles stayed intact.

Tsjerk - 1-5-2022 at 10:59

I bought glacial a couple of weeks ago, no documents needed.

There are very few liquids that expand on freezing, but 60% acetic acid might be one.

DocX - 1-5-2022 at 21:22

Ok, experiment will be done with a smaller amount of 75% acetic acid this week (I have a smaller bottle of this lying around). I'll get back to you with results. For the experiments sake I won't be distilling it beforehand in this first run, just try the freezing.
What is the best way to calculate concentration? Indicator titration?

[Edited on 20222222/5/2 by DocX]

Tsjerk - 1-5-2022 at 23:17

Did you check the freezing curve for water/acetic acid like Fery suggested?


Edit: I will just leave this here to take away any hope freeze crystallization will get you anywhere when starting with 60-70% acetic acid:

Attachment: 10.1.1.402.642.pdf (495kB)
This file has been downloaded 436 times

It was also the first hit on Google with the first search term I used.

[Edited on 2-5-2022 by Tsjerk]

DocX - 2-5-2022 at 12:03

Will you look at that, right at the 60% mark! Well, we have different first hits then, this was mine before I started this thread: https://chemistry.stackexchange.com/questions/2506/acetic-acid-freezing-distilation

But in that thread its clearly stated that the graph is only for visualisation and that the exact composition of the eutectic mixture is not known to the author- followed by a post that claims it to be a bit above 70%, if I don't misunderstand it.

Anyway, I'll be trying with 75% to start. Lets see how it goes. If that works, but not the 60%, I'll know to boil off some water first.

[Edited on 20222222/5/2 by DocX]

DocX - 13-5-2022 at 04:22

Ok, so a rudimentary experiment has been made.
I measured up 200 ml of 75% acetic acid, and put it in the freezer. I brought it down to -14, at which point I got a slurry of crystallised acid and water/acid.

To separate the two was harder than I imagined, since the acid would melt during the process. All I had for equipment right now was a metal sieve, but I scraped out all of the remaining crystals and returned them to my beaker and melted them again, yielding 80 ml.

Unfortunately my good scale is lost, but with what I had I could determine the density to be around 1.07 g/ml, which should correspond to around 80%.
But my scale really is too crappy for this.

Also, the density curve of acetic acid peaks at 70-80%, then slopes down again, meaning that anhydrous acetic acid has a lower density than 75%, and more or less the same as 40% acetic acid.

So reasonably I should measure concentration through some other mean. I guess it's titration time. I seem to be out of burettes though ...

Tsjerk - 13-5-2022 at 04:46

Maybe you should read the PDF I posted again, at least if your goal is to concentrate the acid any further than it is already.

[Edited on 13-5-2022 by Tsjerk]

DocX - 13-5-2022 at 09:11

Oh, I did read it. It says that distillation of acetic acid to anhydrous is practically impossible, right? And that freeze distillation is preferred.

Then, in their experimental, they freeze separate 2% acetic acid up to 58%. They do this by freezing out the water, which you can do up to the eutectic point. Beyond that, you freeze out the acetic acid.

Then there's a figure in which the eutectic point is at 60% and -26.
In that figure, at 75%, the freezing temperature should be around -14C, which is what I got, and produce solid acetic acid and a solution of acetic acid and water. Which is what I got.

I then decanted the solution, loosing a lot of acetic acid but hopefully acquiring more concentrated instead in the form of frozen solid. Although I lack the means of determining this in any. good way right now.
The liquid that I decant off should be around 60%, since that is the eutectic point and won't crystallise until at -26 C. Theoretically, every freezing should produce a more concentrated acid.

So what is your take on the paper you posted? Am I missing something essential?


[Edited on 20222222/5/13 by DocX]

[Edited on 20222222/5/13 by DocX]

[Edited on 20222222/5/13 by DocX]

DocX - 16-5-2022 at 10:54

Quote: Originally posted by Tsjerk  
Maybe you should read the PDF I posted again, at least if your goal is to concentrate the acid any further than it is already.

[Edited on 13-5-2022 by Tsjerk]


Any thoughts on the above interpretation of the PDF you posted? I fail to see why it would be impossible to concentrate 75% further through freezing? So any help would be much appreciated!

[Edited on 20222222/5/16 by DocX]

Tsjerk - 16-5-2022 at 13:02

The liquid and the solid at the eutectic point will have the same composition. Not being at the eutectic point means you freeze one or the other component, shifting the concentration of the remaining liquid, or solid to the eutectic point. When you're already there it will give you no further separation.

Only when the curve is linear you can get an infinite separation. When you freeze 99% acetic acid, the melt will be eutectic.

[Edited on 16-5-2022 by Tsjerk]

DocX - 16-5-2022 at 13:06

Quote: Originally posted by Tsjerk  
The liquid and the solid at the eutectic point will have the same composition. Not being at the eutectic point means you freeze one or the other component, shifting the concentration of the remaining liquid, or solid to the eutectic point. When you're already there it will give you no further separation.

Only when the curve is linear you can get an infinite separation.


Yes, and the eutectic point according to the paper is at 60%, right? So 75% should be possible to concentrate further by freezing. As per my reasoning above, that at the eutectic point, freezing will not do anything but crystallise the entire mother liquid at -26C, but before or beyond that, a separation of a more concentrated phase and a less concentrated phase should be produced.

In the case of a concentration above 60%: a concentrated crystallised product and an eutectic liquid. Which can then be decanted off. Or was that faulty?

Sorry for nagging about this, but I really don't understand and it vexes me. I'm terribly vexed.

If distillation of acetic acid is not possible (which has been stated in the paper you posted and in several different posts on this forum) and concentration through freezing is not possible either, is it even possible to concentrate acetic acid?
I guess treatment with molecular sieves and/or drying agents?

Gah. I really hoped for a simple affair of shoving a jar of cleaning product in the freezer. But I guess chemistry just doesn't care about my hopes and ambitions.

[Edited on 20222222/5/17 by DocX]

[Edited on 20222222/5/17 by DocX]

[Edited on 20222222/5/17 by DocX]

Tsjerk - 17-5-2022 at 04:23

Okay sorry, I indeed didn't read your post carefully and didn't think freezing 15% (if 100% efficient) from 75% was worth the trouble. next time I will try to assume a bit less.

What you have, both the 60% and the 75% is already quite concentrated. I would indeed suggest using some drying agent like Na2SO4 or MgSO4, decant or filter and then freeze. Molecular sieves will dissolve in acid.

If you don't have anhydrous salts but do have an oven you can bake the water out of both sodium and magnesium sulfate. When you start with low temperatures and slowly go up you can prevent the salts dissolving in their own water, thereby keeping the surface area high making the process faster. Microwaving the salts is faster but I can imagine larger amounts to be hard to microwave.

DocX - 17-5-2022 at 06:35

Yes, I have MagSulf. My goal is to produce GAA, so those 15% is indeed of importance.
I'll try the freezing of a little bit larger quantities first, just need to get a new burette for titration. My old one broke. Decanting off and saving the 60% for subsequent distillation trials.
Thank you!

Fery - 24-10-2022 at 08:50

here I found well explained process of freezing a mixture of acetic acid + water:
https://chemistry.stackexchange.com/questions/2506/acetic-ac...
eutectic point is at -27 C for 88,3 wt% acetic acid

DocX - 24-10-2022 at 10:02

I actually left this project. Gave up and ordered some GAA from the ultraprofessional Chemship1978.

But that was the link I shared in one of my earlier posts. For me, it quite boldly says what Tsjerk was trying to tell me: that concentration above the eutectic point is not possible through fractional freezing. In any case, it's too much work for me.