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Tsjerk
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I think the last bit of water can be removed with a bit of sulfuric acid, but I'm not aware of any report about doing so.
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teodor
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It doesn't work. At the boiling temperature of acetic acid H2SO4 carbonizes it. And P2O5 (under vacuum) cannot dry AcOH perfectly, subsequent
distillation/freezing is required, so I think H2SO4 (under vacuum) is even less effective.
But acetic acid esters work well to form an azeotrope with water which could be distilled off except, probably, last 0.5% for which you need
P2O5/vacuum or acetic anhydride.
[Edited on 8-12-2021 by teodor]
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Fery
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The freezing method works well, no need to contaminate acetic acid with anything. Obtained diluted liquid acid could be concentrated by distillation
and then frozen again. Freezing is good only with already quite concentrated acid. This is good skill to achieve. Glacial acetic acid got its name
from this process of its concentration by crystallization. Crystallization in common is much more powerful method in purification than distillation
(one crystallization in common purifies much more than 1 distillation).
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Oxy
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I have no access to my lab at the time so I can't try it but did anyone tried to extract acetic acid from water with DCM?
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teodor
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Quote: Originally posted by Fery  | | Crystallization in common is much more powerful method in purification than distillation (one crystallization in common purifies much more than 1
distillation). |
But I think it is less effective than the column/azeotrope by means of how many milliliters of the final product you can get from the diluted acid. As
an example: decanting half of a volume raises the melting point only by ~1C in my try (9.2 -> 10.4). So, as you said, it probably works when the
acid is already concentrated enough.
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Fery
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teodor you are right, the diluted liquid acid has such composition which melts at the temperature you used for freezing (something like 80 weight % or
maybe 70)
further concentrating 70-80% acid by distillation is more efficient than concentrating by distillation something like 99% acid
best to combine both method (for a lot concentrated acid use freezing and for less concentrated distillation)
I used the diluted acid obtained after crystallization process in experiments where the acid should be diluted (some oxidations when acetic acid + h2o
was solvent) or when acetic acid had to be added into some substance already dissolved in water
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teodor
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Fery, thanks for the information. It proofs that my experiment results were right. But as you said, AcOH is cheap. What I am really interested in is
using similar techniques to purify other carboxylic acids. For example, I've just finished the synthesis of ethyl n-butyrate and I will try to restore
the acid from the residue for subsequent synthesis. I think this acid is a more expensive one.
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Fery
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Teodor OK, I have also almost all possible aliphatic acids upto C10 (below C6 also all possible isomers like isobutyric, isovaleric, pivalic, from C6
only straight chain without branching). I have more than 1,5 l butyric acid where the label says 95% so we can try to purify this. I have also very
pure butyric acid (Acros organics) which is not worth to purify further. Nonanoic acid (pelargonic) I have only 50 ml which I synthesized by myself
and this should also be purified but for the past 2 years I did not try this (other more important priorities, lack of time, other experiments etc).
Other acids than C9 are all commercial.
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