Preparation of GAA by azeotropical distillation

[Chromic]

Activated charcoal can not be used to form glacial acetic acid--just to remove impurities in the GAA (similar to how you remove fusel oils from EtOH with activated charcoal and 50% EtOH). Actually, acetic acid is preferentially is adsorbed onto activated charcoal (i.e. preferential to the water).

From what I understand, you absolutely need to do azeotropic distillation with an column containing a large number of theoretical plates to concentrate dilute acid... here is the phase diagram data for water/AcOH from Perry's (to the right is w/w %'s I calculated from the mol frac data):

   mol frac      w/w   
Temp   liquid   vapour   liquid   vapour
118.3   0   0   100   100
110.6   0.1881   0.3063   93.5   88.3
107.8   0.3084   0.4467   88.2   80.5
105.2   0.4498   0.5973   80.3   69.2
104.3   0.5195   0.658   75.5   63.4
103.5   0.5824   0.7112   70.5   57.5
102.8   0.675   0.7797   61.6   48.5
102.1   0.7261   0.8239   55.7   41.6
101.5   0.7951   0.8671   46.2   33.8
100.8   0.8556   0.9042   36.0   26.1
100.8   0.8787   0.9186   31.5   22.8
100.5   0.9134   0.9409   24.0   17.3
100.2   0.9578   0.9708   12.8   9.1
100   1   1   0   0

If you take the time, and plot it, you'll quickly see why you can't just do a simple distillation to sufficiently purify the acid.

If you use a very long column with a good packing and with a high reflux ratio (ie nixon-stone type reflux head), then... yeah, you could go from 80% to nearly 100% without doing an azeotropic distillation. Do you guys have this sort of equipment? likely not... if you did, you wouldn't be asking this question.

A simple nixon-stone type arrangement can be made using a 24/40 RB flask attached to a very long 24/40 tube (male/female ends) that works as your column (filled with inert packing material), then a dean stark adapter (with stopcock), then a claisen adapter to monitor the temperature, then a reflux condenser ontop of that...) Set it to a healthy boil, so that the reflux rate thru the column is fairly regular, then only open the stop cock a bit to collect a rate of distillate considerably less than the rate of reflux.

Let us know how it goes... I'd love to find a better way. I've got a ton of this diluted acetic acid myself.

[ning]

To nings coarse ears, the idea of sulfuric acid "damaging" acetic acid is rediculous. What do you think is formed when you add sulfuric acid to sodium acetate? I don't think the two ways are going to bee different in any way, except one makes a lot more salt than the other  

It's a good idea. Very useful.

And what if someone were to investigate the production of acetic anhydride by using concentrated sulfuric acid on acetic acid?

What would happen if one were to add SO3 to acetic acid?

[elfspice]

methinks, if we are talking maximum concentration sulphuric acid, then this could dry the water off and prevent the azeotropism similar to how the Na-acetate decomposition likewise allows you to remove the water before distilling the acetic acid.

theoretical of course

[Organikum]

And what if someone were to investigate the production of acetic anhydride by using concentrated sulfuric acid on acetic acid?

What would happen if one were to add SO3 to acetic acid?

In the first case one would find out that conc. H2SO4 doesnt produce acetic anhydride from GAA.
In the second case one would find out that the good reverend Drone was right: SO3 actually produces AA from GAA.

But: Will SO3 produce AA from sodium actetate?

I am still not a bit convinced that the addition of conc. H2SO4 is a practical way to GAA from 80% acetic acid, but I may be wrong. The procedures for GAA from sodium acetate call for anhydrous conditions as I remember. I guess that for drying 80% acetic acid so much conc. H2SO4 will be needed that the procedure gets highly ineffective.

[gsus]

a few books (not by Professor Buzz or Uncle Fester) say that if one distills conc. acetic and sulfuric acids (at least 70%H2SO4 is ok though) then one gets some black crap, C, CO2, and SO2. everything is fine until you distill, they say. how little water must there be? dunno, never tried it. yes, i know the decomposition of HCOOH is usually mentioned, without comment on acetic. no mention of dehydrating abilities of the 70% H2SO4, just a patented method for destroying formic contaminant without hurting the acetic. i always see some blackening in pockets when i drop 93% H2SO4 onto dry Ca acetate, but not a whole lot. this is not the same as distilling the conc. acids anyways. there must be a reason why taking off that last few percent of H2O with sulfuric is not an established procedure AFAIK. i'll do an experiment tomorrow, though with my crap-grade acids this may only be of interest to OTC material bees, if anyone.

  there are several patents that use drying agents. copper sulfate and vacuum is popular for many acids. metaphosphoric acid. solvent extraction with benzene, an ether, chloroform, ethylene dichloride...though these are probably not helpful in this case and off-topic. there are a number of alternatives, but AFAIK involve acetate salts, not what was wanted here. just had to add to the sulfuric posts, though i realize conc. sulfuric probably wouldn't hurt in this case, since the acetic isn't so concentrated.

  i've heard of (old, not on espacenet) patents that describe addition of SO3 to GAA, addition of this to Na acetate, to give acetic anhydride in moderate yield at low temps, btw. perhaps useless rumours.

[Novice]

Somehow I think the sulphuric acid will hold on to all the water it can find, even at 120 C... If one adds a slight molar excess to the 80% AA, it should yield GAA upon distillation. Wouldn't it seem unreasonable otherwise?

If you consider delta H for the reaction H₂SO₄ + H₂O --> H₃O⁺ + HSO₄^-... I get the feeling that there are no side reactions (such as AA + H₂O) that are even near as favoured as the one between sulphuric acid and water.

[ning]

Organikum, I was thinking he should first distill the acetic acid, or freeze it, or whatever to get to 95-98% purity, then distill from sulfuric acid to remove that last bit of water.


I doubt pure SO3 could make acetic anhydride from NaOAc. You need some hydrogens there, I think. IIRC, SO3 is not acidic at all when pure. But hey, maybe if there was just a little bit of water, (oleum?) it could start a chain reaction.

H2SO4 + 2 NaOAc --> 2 HOAc + Na2SO4
2 HOAc + SO3 --> Ac2O + H2SO4

and repeat.

I don't see any reason why not. Only problem is, you'd have to titrate your SO3 fairly accurately, otherwise distillation could get interesting...

Say, maybe you wouldn't need to distill. probably Na2SO4 isn't soluble in Ac2O. perhaps you could just pour it off.

[Nicodem]

Ning, the reaction of SO3 and NaAcO is formally possible:

SO3 + 2 MeCOONa => Na2SO4 + (MeCO)2O

...though I have no idea if it is practically possible.
You see, CO2 is also not acidic but it will nevertheless react with solid Ca(OH)2, for example, forming CaCO3 and water. Even more, it will also react with CaO forming only CaCO3. These are not acid-base reactions anymore.

[elfspice]

Generally, sulphuric acid pushes all other acid ions out of the conjugations, and in this situation, the main two products of the reaction would be sodium sulphate and free acetic acid.

Sodium sulphate is a very strongly hygroscopic salt and the acetic acid hasn't got a snowflake's chance in hell of holding the water, 80-90% sulphuric acid hardly has any water in the first place, and the sodium sulphate will form the decahydrate... Every mole of sulphuric acid will pull this ratio of things out of the system - here it is with the total amount of water that could be present before the acetic acid could actually grab onto it:

(H2O)10 + (NaCH3COOH)2 + H2SO4 --> Na2SO4 + (H2O)10 + (CH3COOH)2

here's the formula with the weight of one mole of each chemical corresponding with that formula:

180 + 164 + 98 --> 142 + 180 + 120

This means, by weight, sulphuric acid dissolved with a weight ratio of 98:180 sulphuric acid to water (what's that - 50%ish) will be retained by the formed sodium sulphate. So any more concentrated sulphuric acid than that will most definitely completely dehydrate the sodium acetate, sans decomposition temperature requirement of the dry acetate distill.

at least, in theory.

[ning]

I think it would probably be very much like reaction of CO2 with Ca(OH)2....CO2 could bee called "carbonic anhydride", and much like SO3, it's not acidic at all....until it encounters water. Then there is formed the famous HOCOOH, which can immediately react acid-base wise with Ca(OH)2 to form the carbonate.

I wonder, will dry CO2 react with dry CaO? Even when both are perfectly anhydrous?

Way I think of it is, CaO is like a basic anhydride. CO2 is an acid anhydride. Whether they actually become the acid or base transiently when they react is not so important as realizing that the reaction will run down the energy hill.
CaO + H2O --> Ca(OH)2 = Energetically favorable
CO2 + H2O --> HOCOOH = hmmm. Not so favorable, not unfavorable. Equilibrium.
Ca(OH)2 + HOCOOH --> CaCO3 + 2 H2O = Energetically favorable

Imagine there was only a very, very small amount of water. It would still enable the reaction to take place,  with the overall equation CaO + CO2 --> CaCO3. And if it still works under perfectly anhydrous conditions, so what? You could still figure out the reaction was energetically favorable from acid-base equations.

So, while I agree acid/base chemistry isn't the end, it sure is useful.  

So whether the reaction SO3 + 2 NaOAc --> Na2SO4 + Ac2O actually involved transient hydration or not, I bet it will still work. SO3 is a very, very angsty molecule.  

Organikum recently forwarded me a very interesting idea that avoids the need for getting up close and personal with Mr. T (Trioxide, that is):

http://www.sciencemadness.org/talk/viewthread.php?tid=9&page=2

near the bottom, there is mentioned a patent that says  sodium pyrosulfate can dehydrate and free from salt sodium acetate.

2 NaHSO4 --heat--> Na2S2O7 + H2O

Na2S2O7 + 2 NaOAc -----> Ac2O + 2 Na2SO4

I guess NaHSO4 can bee made by carefully neutralizing sulfuric acid only half way.
Read the original post, that sounds too tasty. Perhaps even a one-pot procedure?

[Organikum]

elfspice, how do you come to the funny belief that sodium sulfate is more hygroscopic than acetic acid or sulfuric acid?

Because we could easily dry acetic acid by NaSO4 if this is true ya know?

Life is so easy when you just have to talk about, and its so hard when you actually have to do it.

[elfspice]

I think that would be quite likely actually

H2SO4- + (4HCOO-)--> (CO2)4 + H2S + (H2O)4

the formic acid would reduce the sulphuric acid's oxygens until it formed H2S, and there would be a whole series of intermediate sulphur acids in between.

Still seems to me that decomposing sodium acetate still comes out on top.

The only caveat to that method is occluded water, water of crystallisation. It would cross with the acetic acid.

In reality it is Na+.CH3COO-.H2O, and thus the stuff you get at in the receiver is CHCOO-.H2O

that's 60:18, or in other words, 70%


So, i think the best option would be to put a drying tube in between (pardon me for repeating already known information if this is already known), containing a dehydrated salt like CaCl or sodium sulphate or whatever.

Or can the water of crystallisation be driven out of the sodium acetate without releasing the water, say perhaps simply by holding the temperature at 100 degrees until no more moisture comes out of the salt. This could be monitored by distilling it, and when the constant boiling temperature fails to drive any more water out of the salt, the receiver is switched and one collects 98% or so perfect dry acetic acid. To ensure that this does occur, one of those cow teat type receiving adapters, and in the flask put an excess of drying agent which has the purpose of retaining all the moisture that will drip out of the condenser, and helping the internal atmosphere remain dry. A drying tube will also be neccessitated as the distillation will need venting but it cannot let water in. Once water stops distilling, the receiving flask for the dehydrated acetic acid is put in the position for receiving and the temperature raised to the decomposition temperatuer of the sodium acetate.

[Organikum]

I really dont know what you are talking about elfspice, but I assume you dont know it either so it doesnt really matter.
I admit frankly that it sounds impressing though.


Formic acid and conc.H2SO4 produces carbonmonoxide as Rhodium told already.

[Grignard]

molecular sieve could be used to bind the water? CaO is often used to absorb the water i EtOH to obtain 100% but i dont think this could be used in this case, because it form the hydroxyl with water. anion exchange resin could be used to concentrate the gaa, but you would need a resin that dont deformate under high ionic concentration...

[ning]

Make sure you don't try using anything that will react. Like calcium oxide. Sieves? Dunno.

How about this?

Post 463967

(hermanroempp: "Re: sulfur?", Novel Discourse)

NaHSO4 + NaOAc --> HOAc (glacial)

Alternately, ning suspects that anhydrous sulfate salts (Na2SO4, MgSO4, etc) may be able to dehydrate the acid. Sodium sulfate in particular was supposed to bee very strong if fully dehydrated. (is this so?)

[Barium]

Sodium sulfate is almost useless as a deccsicant at temps above 36-37 deg C.

[Mountain_Girl]

Ning said :

Organikum recently forwarded me a very interesting idea that avoids the need for getting up close and personal with Mr. T (Trioxide, that is):

http://www.sciencemadness.org/talk/viewthread.php?tid=9&page=2

near the bottom, there is mentioned a patent that says  sodium pyrosulfate can dehydrate and free from salt sodium acetate.

2 NaHSO4 --heat--> Na2S2O7 + H2O

Na2S2O7 + 2 NaOAc -----> Ac2O + 2 Na2SO4

I guess NaHSO4 can bee made by carefully neutralizing sulfuric acid only half way.
Read the original post, that sounds too tasty. Perhaps even a one-pot procedure?

This was actually raised before ^__>

Post 462958

(Rhodium: "Na-Acetate + Na-Pyrosulfate -> Acetic Anhydride", Novel Discourse)

[gsus]

some of the patents i was thinking of were on espacenet after all. the best one wasn't, was probably too good to be true. maybe these are as well. patents can be a little vague.
acetic anhydride from acetic acid and sulfur trioxide:

Patent GB283781

acetic anhydride from acetate, sulfur trioxide with or without another salt, and GAA:

Patent GB191517920

acetic anhydride from GAA/acetic anhydride, oleum, and NaOAc:

Patent GB191312042

  and as for azeotropic distillation, the chloroform/H2O azeotrope boils at 56°. it contains 2.8% H2O, so one would need a bit of it. it contains no acetic acid and chloroform does not form any sort of azeotrope with acetic acid.

  elfspice: i believe the reaction of H2SO4 gives CO and water, and that is all. the patented reaction i mentioned earlier about formic-contaminated acetic:

Patent US1210792

it also mentions the decomposition of acetic acid by hot H2SO4.