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Author: Subject: Problem during the synthesis of acetic acid
Angus
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sad.gif posted on 1-12-2023 at 12:26
Problem during the synthesis of acetic acid


Hello everyone,

I'm new here, and this is my first message.

I encountered a minor issue during the synthesis of glacial acetic acid using the following method: sulfuric acid + sodium acetate. I had previously successfully conducted this synthesis. My sulfuric acid is of quality and concentrated at 98%. However, the sodium acetate I used was highly impure, but that doesn't bother me.

In the middle of the reaction, the heating mantle was quite hot (I made the mistake of not taking the time to measure the temperature). A suspicious whitish crystallization appeared on the condenser walls, rapidly spreading and taking on a light yellow-green hue. A thick whitish smoke then infiltrated the receiving flask, dissolved in the acetic acid, making it cloudy. At that moment, I detected an odor of acetic acid, hydrogen sulfide, and perhaps a bit of sulfur dioxide.

Naturally, I stopped the reaction by turning off the heating. Later, without touching the setup, I noticed a black spot floating on the surface of the sulfuric acid in its dropping funnel.

What happened?

I have an idea about the cause, but I'm not sure due to my lack of experience. I believe I simply overheated, leading to the decomposition of sulfuric acid. What are your thoughts?

[Edited on 1-12-2023 by Angus]
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chornedsnorkack
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[*] posted on 1-12-2023 at 12:42


Quote:

believe I simply overheated, leading to the decomposition of sulfuric acid. What are your thoughts?

To begin with, sulphuric acid does not simply decompose on overheating or just produce any dark products, or hydrogen sulphide or sulphur dioxide. Sulphuric acid should boil reversibly, giving only colourless products, certainly under 400 Celsius.
The one way sulphuric acid can be said to "decompose" is by preferentially evaporating SO3, but that happens at concentrations above 98,3%.

But acetic acid is not resistant to hot concentrated sulphuric acid, and gives dark tars. The first step is sulphonation at alpha hydrogen, but the reaction goes on, giving dark condensation results and plausibly reducing the sulphur.
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RU_KLO
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[*] posted on 2-12-2023 at 15:06


Probably the problem was in the impure Acetic Acid. If it was from Vinegar, then some organic material was in there.



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Angus
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[*] posted on 3-12-2023 at 14:14


Thank you for your responses.
I wasn't aware of the tar issue; I'll take that into account for next time. I'll definitely try the method with sodium bisulfate.
Yes, it was vinegar. I'll choose a different brand of vinegar next time or distill it to avoid too many impurities.
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