Sciencemadness Discussion Board

Does sodium hydroxide decompose ethanol?

YoctoByte - 7-3-2022 at 03:35

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?

woelen - 7-3-2022 at 04:08

Ethanol does not become a tar with NaOH. I think that the purification was not complete.

DraconicAcid - 7-3-2022 at 08:37

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).

woelen - 7-3-2022 at 12:07

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).

DraconicAcid - 7-3-2022 at 21:01

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".

unionised - 8-3-2022 at 03:39

It's worth remembering that cleaning baths get dirty.

YoctoByte - 8-3-2022 at 09:47

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]

macckone - 10-3-2022 at 08:23

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.