Sciencemadness Discussion Board

Ethanol Purification for Synthesis

nimgoldman - 22-4-2019 at 12:31

I never had any problems with self-produced ethanol in any syntheses so far as well as using it as extraction and washing solvent.

However, I would like to see your opinion on the purification process I currently use and any ideas on the topic.

I start with so called sugar wash, the mixture of lukewarm water, white sugar, alcohol resistant yeast and optionally a yeast nutrient (mostly DAP, or diamonnium phosphate).

Once the fermentation is over, the mash is decanted/filtered, and distilled in so called stripping run. This is just a simple distillation to remove most of the solid impurities (dissolved salts, undigested sugar, dead yeast etc.)

I then dilute the alcohol to 40-50% ABV as stronger alcohol cannot be efficiently filtered with carbon (at least I have been told so). I measure pH of the diluted alcohol (another reason for adding water is to be able to actually measure acidity). It is usually slightly acidic so I add sodium bicarbonate to neutralize.

I've read at homedistiller.org that one can also add potassium permanganate to remove some of the impurities (I assume this is by oxidation). Is this necessary for a sugar wash? I've read in relatively recent article a proposed purification method by ozonization. I have a small ozonizer unit so maybe it can be used instead of the permanganate. But I am not sure how effective any of the chemical purification methods are though.

I filter the alcohol, add about 5% of the volume in activated carbon and stir it for 10+ minutes with an overhead stirrer. I filter it again through a bed of Celite. At this point, any residual smells are removed and the alcohol smells sharper, chemical, like it should.

Now I do a second distillation run (40 cm Hempel column packed with Rashig rings on top of 60 cm Vigreux column), leading to ~86% ethanol. This is redistilled to yield 92-94% ethanol.

I add 3A or 4A molecular sieves to break the azeotrope and reach about 97% ethanol. Then I filter the alcohol (to remove the sieves) and distill under vacuum (simple distillation on rotavap to speed things up).

Finally, I dilute the alcohol with DI water to 95% ABV and store it.

I often split the 97% alcohol in halves, one half is processed to 95% as described above and to the other half I add more sieves to reach 99+% for applications where anhydrous ethanol is required.

I am however not sure if ethanol made with such process can be assumed pure or there are any significant impurities that could pass all the way through the process (something like aldehydes or ketones - like acetone).

Do you have a similar process or any ideas for improvements?

draculic acid69 - 23-4-2019 at 05:03

One idea of mine is to purify methylated spirits to a drinkable substance using cryogenic crystallization and distillation.other ppl have done this and they say it works out five times cheaper than buying yeast and sugar.point being try metho as a starting point.

[Edited on 23-4-2019 by draculic acid69]

PirateDocBrown - 23-4-2019 at 06:44

Here's my route to lab ethanol:

I start with this:

http://www.kleanstrip.com/product/green-denatured-alcohol

I reflux it several hours with NaOH, to hydrolyze the EtOAc denaturant, and to promote aldol condensation of other impurities. I cut off the head of the first distil before the high 60s, to remove most of the methanol, then collect the main run up to the high 90s, to leave behind as much of the MIBK denaturant as possible. This is done with a short (20cm) Vigeraux. I then repeat this, over fresh NaOH (to hold back as much water as possible), with a tighter focus on the boiling point in the upper 70s. The product is collected on anhydrous MgSO4, and is then filtered into 3A sieves. Hydrometry shows it 99+% EtOH.

Assured Fish - 23-4-2019 at 21:33

I suppose it really is quite dependent on exactly what you are going to use your ethanol for, if its just for making ethers and esters then you should be fine with pretty crude ethanol.
Though I can certainly see why preparing the purest cleanest ethanol is desirable.

I have a friend who distills a lot of ethanol and is quite the expert at it (he also drinks alot as well).
He uses a 1 meter collumn packed with steel turnings, with a layer of copper wool at the bottom, the copper is for removing sulfides and must be freshly cleaned with citric acid.
I know he use to use activated carbon but has done away with it as of late as it doesn't do much if your distilling ethanol as pure as he is.
He undoubtedly makes the cleanest vodka to ever grace my lips it has virtually no flavor at all, nothing but the characteristic sharpness of the ethanol. He even achieves the azeotrope of 95.6% buy my own density measurement.

In terms of improving your set up i will give you what advice i have from my own experience (you may already know these things).
The first advice would be to take multiple cuts, ideally as many as you can, one for the heads and atleast 4 for the body, then about 2 for the tails.
The tails is generally what contains the most impurities (higher boiling alcohols) and the most water as you probably already know.
Another note is temperature control in your pot, if your pot doesn't come with a built in temperature controller it might be a good idea to get a variable transformer, these arn't cheap but they are extremely useful.

To be honest from what you describe you are already preparing ethanol that is more than adequately pure for any laboratory uses.

nimgoldman - 24-4-2019 at 11:12

Quote: Originally posted by Assured Fish  
I suppose it really is quite dependent on exactly what you are going to use your ethanol for, if its just for making ethers and esters then you should be fine with pretty crude ethanol.


I use it mostly for Victor-Meyer synthesis, i.e. for making bromoethane/iodoethane and to wash and dry silver nitrite (not nitrate!).

Another use is as a solvent for various syntheses.
Quote: Originally posted by Assured Fish  
Another note is temperature control in your pot, if your pot doesn't come with a built in temperature controller it might be a good idea to get a variable transformer, these arn't cheap but they are extremely useful.


I use heating mantle with a thermocouple that is immersed in the liquid. I found the separation is pretty good.

Thanks for all the advice. I already thought my procedure is good enough but I still finding new and new tips (e.g. the permanganate purification, neutralization, effects various types of charcoals, ozonization, gas stripping etc. etc.), which makes me feel like the ethanol produced, even very carefully, could be horribly impure.

[Edited on 24-4-2019 by nimgoldman]

markx - 1-5-2019 at 07:47

Ozonation is way too agressive and shall convert your EtOH to acetal aldehyde. The most reasonable purification method for fermentation based mixes is a decent rectification column with a condenser head that allows adjustable reflux rates. A strip run is performed on the wash with neglidgible reflux rate, followed by rectification run at azeotropic conditions under higher reflux rate. That’s it and the product is perfectly palatable in terms of either consumption or chemistry. The intermediate messing around with carbon filtration, ester hydrolysis and oxydative treatments are pretty much a waste of effort and amount to pretty much nothing in terms of improving the end product compared to what a well tuned rectification column can achieve.

nimgoldman - 6-5-2019 at 18:37

Thanks for reassuring about the sufficiency of good distillation.

I tried potassium permanganate as a chemical pre-cleaning but I think it reacts too much with the ethanol lowering the yield. I've read that one can at least add some baking soda to neutralize acid (mostly acetic acid) in the alcohol. Another impurity of concern is ethyl acetate.

I never had problems using EtOH purified with just distillation in any synthesis so far.

I however found the carbon filtered ethanol smells differently than unfiltered even after 2 fractional distillations. I guess the smelly impurities are minute but still...

Unfortunately, I don't have a reflux head (but I think about adding a 10 cm reflux condenser with adjustable coolant flow to improve reflux) ... so I just use 40 cm Hempel column (packed with Rashig rings) on top of 60 cm Vigreux column, both insulated.

The stripping run (simple distillation) gives me 65% v/v EtOH.
I dilute it to 45%, stir with AC for 10 minutes, filter through Celite (to remove carbon containing trapped impurities) - I also make a carbon bed on top of the Celite bed.
Then I fractionate (with a 5 g/L sodium chloride to improve fractionation), removing "heads" and "tails", keeping 82-84% EtOH.
Then I fractionate again, getting to ~92-93% EtOH.

I add 3A or 4A molecular sieves to get 96-97% EtOH.

Finally, I redistill that (to remove zeolite dust) on rotavap and dilute to 95% with distilled water.

Some of the 96-97% EtOH is further dried to anhydrous EtOH for washing and some syntheses.

This is the procedure that currently works for me.

P.S. The chemist softened 50 mL of his freshly made ethanol with a little glycerol and sugar (in a form of orange juice) and found he made a pleasant booze. But it was too good to be consumed so he left it in the chemical cabinet for more serious experiments and swore he will stick to beer when it comes to celebrating achievements.

[Edited on 7-5-2019 by nimgoldman]

markx - 8-5-2019 at 06:29

Quote: Originally posted by nimgoldman  
Thanks for reassuring about the sufficiency of good distillation.

I tried potassium permanganate as a chemical pre-cleaning but I think it reacts too much with the ethanol lowering the yield. I've read that one can at least add some baking soda to neutralize acid (mostly acetic acid) in the alcohol. Another impurity of concern is ethyl acetate.

I never had problems using EtOH purified with just distillation in any synthesis so far.

I however found the carbon filtered ethanol smells differently than unfiltered even after 2 fractional distillations. I guess the smelly impurities are minute but still...

Unfortunately, I don't have a reflux head (but I think about adding a 10 cm reflux condenser with adjustable coolant flow to improve reflux) ... so I just use 40 cm Hempel column (packed with Rashig rings) on top of 60 cm Vigreux column, both insulated.

The stripping run (simple distillation) gives me 65% v/v EtOH.
I dilute it to 45%, stir with AC for 10 minutes, filter through Celite (to remove carbon containing trapped impurities) - I also make a carbon bed on top of the Celite bed.
Then I fractionate (with a 5 g/L sodium chloride to improve fractionation), removing "heads" and "tails", keeping 82-84% EtOH.
Then I fractionate again, getting to ~92-93% EtOH.

I add 3A or 4A molecular sieves to get 96-97% EtOH.

Finally, I redistill that (to remove zeolite dust) on rotavap and dilute to 95% with distilled water.

Some of the 96-97% EtOH is further dried to anhydrous EtOH for washing and some syntheses.

This is the procedure that currently works for me.

P.S. The chemist softened 50 mL of his freshly made ethanol with a little glycerol and sugar (in a form of orange juice) and found he made a pleasant booze. But it was too good to be consumed so he left it in the chemical cabinet for more serious experiments and swore he will stick to beer when it comes to celebrating achievements.

[Edited on 7-5-2019 by nimgoldman]


Yes, the persistant off smells and tastes can be generated by esters and other noxious contaminants that are unmistakenly detectable in the ppm range. Very laborious to get rid of completely by either chemical or other methods. The heavy stuff in the tails is stupid simple to cut out and does not carry over if any countermeasures are taken to monitor the apparatus. It is the low and mediocre boiling ones (esters) that are a pain to avoid and tend to carry over despite all odds.

The chemical means of purification (oxydative or alkaline/acidic treatments) are totally kosher for EtOH that is destined to synthesis purposes, but when palatable spirits are the objective, then I suggest to steer well away from these types of manipulation.
I have a long and colorful career in that field and my experience has brought me to the unmistakable conclusion that chemical means of refining are not to be used if consumable spirits are the goal. These types of treatments will be able to transform and remove some contaminants, but they also ruin some form of unknown balance in the product giving it a harsh, bitter and very unpleasant taste and feel....although no off tastes may be present. The most curious thing is that this taste profile can not be removed or altered any more after the chemical treatment. Not even by several careful fractionation runs with very high separation ability reflux column. Product remains clearly tagged and unfit to be consumed....at least by my standard. In fact I can recognize this specific taste profile in many commercial vodkas and thus conclude that they likely have been produced from spirits that have seen some form of harsh treatment during their manufacture.
Carefully refined distillation with adequately capable equipment is the only true way to really top quality product. If one feels the urge to further purify the distillates after a fractionation run, then it makes more sense to revise the distillation equipment and procedure than engaging in filtering or other (chemical) methods. That goes for palatable spirits....reagent grade can be handled without any regard to "fine structure states" :D
Once perfected, one can conjure top notch azeotrope at will straight from the column and it will need no further manipulation regarding the composition for whatever purpose it shall meet in the end.