YoctoByte
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Does sodium hydroxide decompose ethanol?
I'm trying to make some pure ethanol via fermentation. After one distillation the ethanol smelled really bad so I tried to clean it up with sodium
hydroxide. For dioxane this works so I thought, why not try it with ethanol? A black/brown tar precipitated so it seems to have removed most of the
other compounds in the ethanol. I left it for a month and distilled it again to remove the tar.
But now I took a sample and added some NaOH to it and it turned yellow again within a few hours. I'm gonna leave it for a few days and distil it again
to see if it again turns yellow with NaOH.
Could it be that the first NaOH attempt did not remove all impurities? Or does ethanol slowly tar with sodium hydroxide added?
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woelen
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Ethanol does not become a tar with NaOH. I think that the purification was not complete.
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DraconicAcid
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If you leave a solution of NaOH in ethanol exposed to air for extended periods of time, it will form an orange tar/oil (back in the day, we had a
large NaOH/ethanol "base bath" for cleaning glassware. It would get gross all by itself after a while).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Texium
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Thread Moved
7-3-2022 at 11:54 |
woelen
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Quote: Originally posted by DraconicAcid  | | If you leave a solution of NaOH in ethanol exposed to air for extended periods of time, it will form an orange tar/oil (back in the day, we had a
large NaOH/ethanol "base bath" for cleaning glassware. It would get gross all by itself after a while). |
This only happens, when also oxygen is present in the mix (i.e. from air). The same is true for amines, which turn brown after some
time, but also only, when they are exposed to oxygen. In the latter case their own basic properties are sufficient to make them turn brown. Without
oxygen, ethanol does not turn tarry, not even with NaOH (it does so, however, with conc. H2SO4).
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DraconicAcid
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| Quote: Originally posted by woelen | | Quote: Originally posted by DraconicAcid | | If you leave a solution of NaOH in ethanol exposed to air for extended periods of time, it will form an orange tar/oil (back in the day, we had a
large NaOH/ethanol "base bath" for cleaning glassware. It would get gross all by itself after a while). |
This only happens, when also oxygen is present in the mix (i.e. from air). |
Yeah, I know. That's why I specifically said "exposed to air".
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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unionised
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It's worth remembering that cleaning baths get dirty.
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YoctoByte
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Thank you all for your replies!
Only the first treatment with NaOH produced tar. The second time it only turned yellow within a few hours and orange after a day or so. This seems to
be expected so I guess the ethanol is mostly "clean" now. Only some minor methanol/acetone/etc impurities.
[Edited on 2022-3-8 by YoctoByte]
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macckone
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ethanol and air will yield acetic acid and acetylaldehyde.
acetylaldehyde like all ketones and aldehydes for colored tarry residues with acids or bases due to aldol condensation.
A single distillation will not clean up alcohol very well. Fermentation creates a witches brew of compounds. This includes aldehydes, ketones,
acids, higher and lower alcohols (butanol and methanol are the worst), esters as well as amines and amides. Having the first distillation slightly
acidic will bind down the amines. Having the second distillation basic will take care of acids and break down a lot of the esters. Doing a third
distillation with a more acidic solution should clean it up really well. I use citric acid for the acid and sodium carbonate for the base. For
non-consumable alcohol, sodium hydroxide might break down more esters. If each of these is a fractional distillation with a good column, you may need
to add water on the second and third distillations to get good separation.
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