Concentrating acetic acid by freezing- QUESTION

[MadHatter]

I read that you can effectively concentrate acetic acid through simply freezing out the water. The acetic acid has a lower freezing point than water at any given concentration, and the more concentrated the solution is the higher the freezing point is. As you all know, anhydrous acetic acid (Glacial Acetic Acid) has a freezing point of just about 16 C. At 75%, the freezing point is around 2 C, after which the water can be decanted off.

Whereas at 60%, the freezing point according to this table should be around -10C. This confuses me somehow though. At -10 C, the water should crystallize too, so what am I really left with at that point? A liquid mix of acetic acid and water and a frozen solid of water, or vice versa?

How is this actually done, practically?

 

[WillD]

Yes, the mixture crystallizes entirely

 

[MadHatter]

As I understand it, the eutectic point is at -27 C, and occurs somewhere around 60% concentration. I wonder if this method for concentration is feasible.
I am of course trying to produce enough GAA to perform the P2P synth through the bayer-williger oxidation.

 

[WillD]

You can try distillation or extraction.

 

[Jack]

If you can obtain anhydrous sodium acetate, then via dry distillation with sodium bisulfate glacial acetic acid is distilled off.

 

[MadHatter]

Yeah, well, the problem is I need LITRES of the stuff, and any synthesis method just takes too long and costs too much. Also, buying GAA in those quantities will definitely raise suspicion.

 

[WillD]

Unfortunately, there is no convenient non-industrial method of production of anhydrous acetic acid, except distillation. Regarding your situation, I proposed other ways to solve the issue in the topic about MEK/BMK

 

[MadHatter]

Yes, and thank you for that! Even if distillation is time-consuming I think I'll go that road. Apparently distilling acetic acid to anhydrous concentrations is really difficult too without a fractional column of at least one meter, and even then. I think the solution will be to distill it to the highest achievable concentration and then fractionally freeze it.

 

[WillD]

Maybe. Let you know about the results, everyone will be interested.

 

[MadHatter]

Sure will. Freezing test has already begun, and it showed to be difficult. I'll upload pictures soon.

 

[MadHatter]

Sorry, no pictures. But the experiment has been made.

I tried freeze-drying 75% acetic acid. In the first attempt, crystallization was achieved at -14 C, and the remaining liquid decanted. From density calculation alone, no change in concentration could be determined.
This made me both sad and curious. So I started researching.
Apparently an eutectic point is reached in an acetic acid/water mixture at around 60% concentration, in which both water and acid crystallizes at around -29 C. In concentrations up to that point, water freezes first (or rather, water with a lower concentration of acid), and beyond that point acetic acid-rich solution solidifies first.
Therefore, when I'm freezing a 75% solution, the liquid part left should consist of 60% acetic acid. And be a big part of the total solution.

So for every freezing, I'm only able to shift the concentration very slightly. It's too time-consuming.
I think the best route is to try and fractionally distill it with a really long vigreux column (two 40 cm columns on top of each other) to as high a concentration as possible. Then use magnesium sulphate to push it even closer to anhydrous, and finally freeze out the last part.

I'll try that next.