Bedlasky
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Anhydrous oxalic acid
Hi.
I am making some anhydrous oxalic acid now. I read about this few threads on SM forum and I choose water bath method. I don't want directly heat
oxalic acid because I am little bit scared about overheating and decomposition in to CO, CO2 and formic acid. And I don't have Dean-Stark trap, so
heating on water bath seams like a good method.
I started with 70g of oxalic acid dihydrate. After cca 2 hours of heating is dehydratation on half a way - I have 60g of oxalic acid (anhydrous should
weight 50g). But I am little bit afraid about sumblimation of oxalic acid. Water vapor are little bit irritating and when I attach nose close to jar,
I smell oxalic acid (but I this stuff smell also at room temperature). Temperature of water bath is between 90-100°C, temperature of oxalic acid in a
glass jar is between 80-85°C. Is there chance to get considerable losses due to sublimation? I also do it inside (with open window, but still...) and
breathing oxalic acid isn't good thing.
[Edited on 5-7-2020 by Bedlasky]
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Ubya
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i found on the web that it starts subliming at 157°C, so you are probably safe with good ventilation
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feel free to correct my grammar, or any mistakes i make
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mackolol
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I don't know if dehydrating any compound on WATER bath is a good idea. After all hydrates come from more or less hygroscopic compounds and water
vapors will do the reverse...
And the second thing 100C is quite low temp, the dehydration will be very slow.
It's better to crank up the heat and deal with some sublimation loss, oxalic acid is dirt cheap after all.
And as for breathing oxalic acid, come on just don't attach your nose to the jar, there are more dangerous things than oxalic acid to deal and it's
not even a gas, which means it doesn't spread in the air, but just settles on things. Neither it is super toxic or super irritating. If your window is
open then it should be enough. If not get your gasmask on, or even some coronavirus type material one, it should protect you from tiny sublimated
crystalf of oxalic acid.
Here you have few, I think better procedures on orgsyn: http://www.orgsyn.org/demo.aspx?prep=CV1P0421
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woelen
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I doubt whether the method with carbon tetrachloride is better than Bedlasky's method. CCl4 is not easy to obtain and it is very toxic. I have read
that oxalic acid can fairly easily be dehydrated (with indeed some loss, which one takes for granted), by heating it in an oven at 110 C or so (keep
it between 105 and 115 C, not higher, due to excessive losses, not lower, because of very slow dehydration).
After doing this, the oven must be heated empty to 200 C or so for a few tens of minutes to remove any traces of sublimed oxalic acid if you don't
want any long-term slow corrosion in the oven.
Just try it with 20 grams or so and if this works satisfactorily, then you can scale up.
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Tsjerk
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I have used a microwave to dehydrate oxalic acid. Put the acid in a glass container (borosilicate or soda-lime, both work) and put something glass
over the opening. Put the microwave to something approximately half power, the hydrated acid will absorb radiation and dehydrate. The anhydrous acid
will desublime on the cooler glass above the level of the heated acid. Glass and dehydrated acid both don't absorb microwave radiation, allowing the
to stay relatively cool. Just make sure not to overfill the vessel.
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Bedlasky
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Thanks for all sugestions.
After 6 hours heating I had in jar 51g of oxalic acid. I stoped heating because 1g water shouldn't do too much issue.
Next time I'll try different method, because this one is slow.
Half of it I'll use in next days for preparation of dimethyl oxalate, second half I keep for the future synthesis of diethyl oxalate.
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mackolol
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Quote: Originally posted by woelen  | I doubt whether the method with carbon tetrachloride is better than Bedlasky's method. CCl4 is not easy to obtain and it is very toxic. I have read
that oxalic acid can fairly easily be dehydrated (with indeed some loss, which one takes for granted), by heating it in an oven at 110 C or so (keep
it between 105 and 115 C, not higher, due to excessive losses, not lower, because of very slow dehydration).
After doing this, the oven must be heated empty to 200 C or so for a few tens of minutes to remove any traces of sublimed oxalic acid if you don't
want any long-term slow corrosion in the oven.
Just try it with 20 grams or so and if this works satisfactorily, then you can scale up. |
There are few procedures described, CCl4 is indeed a weak one, but there with oven and the one with H2SO4 as well.
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