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Registered: 6-5-2019
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Protecting vacuum from H2SO4 vapors?
In the interest of exploring flavor compounds, I needed to make food grade sulfuric acid. The hardware store stuff is not acceptable for something I
intend to taste.
Using a bath of steel shot and a gas burner, I distilled and collected around 80 mL of clean sulfuric acid. However it was very time consuming and I
felt uncomfortable working with 300C+ sulfuric acid.
You would figure that people would vacuum distill it. I'm not really sure why people don't. Before attempting it though, I'd like some input on how to
keep my vacuum line safe from H2SO4 vapors.
I'd use a water aspirator, and I have an aluminum check valve, various brass fittings, silicone tubing; all of which would probably have a bad time
sucking sulfuric acid. I think this is an actual concern because holding pH paper at the vacuum takeoff (when distilling at atmospheric pressure)
shows the air leaving is very acidic.
I was thinking of building a sort of in-line scrubber. Maybe filling a wash bottle with sodium carbonate and pulling a vacuum through that (I don't
think I would get very good flow rates, my Na2CO3 is very fine). Or, filling the wash bottle with a saturated solution of Na2CO3 and pulling through
that. Maybe hydroxide would be better, I don't know how significant the CO2 production would be.
So, from the vacuum takeoff, into the basic gas wash bottle, then into the vacuum. Maybe there's a concern of boiling the water in the scrubber, I
could put it in ice, or bleed off my vacuum if needed.
Does that sound reasonable?
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TLutman
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That is what I did. Pick your vactubing wisely. I had 3/8” clear and the heat tends to soften it, letting it collapse.
I have a cold finger that is mounted to the rear of my hood and I just use a beaker with ice and left enough room to add if needed.
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Gammatron
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You can buy PTFE tubing really cheap on eBay, I've used it for distilling HF at over 200C with 0 issues and it can take much more. It's not flexible
but it would be suitable to go between your distillation and scrubber and then you can use any type of tubing to go to your pump.
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Deathunter88
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If you are using a water aspirator, all of that is unnecessary. In fact, water aspirators are specifically chosen when corrosive vapors are expected
because they get washed out by the stream of water. Besides, aluminum, brass, and silicone are all resistant to sulfuric acid at the concentrations
they will come in to contact with.
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Quote: Originally posted by Deathunter88  | | If you are using a water aspirator, all of that is unnecessary. In fact, water aspirators are specifically chosen when corrosive vapors are expected
because they get washed out by the stream of water. Besides, aluminum, brass, and silicone are all resistant to sulfuric acid at the concentrations
they will come in to contact with. |
That is a fair point, but part of the reason for asking is to get a new tool for my toolbox which I could use with different reactions, or different
setups. I'd be in the same position if I chose to use my nicer pump, for instance.
Or if I'd need to be cognizant of suckback. Intuitively, I don't think suckback should be a problem with H2SO4 because of how low the volatility is,
but it might be worth considering when trying to apply the same system to other reactions.
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