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[*] posted on 3-6-2013 at 14:53
Methanol Testing


Hello. Say one had a mixture of methanol and ethanol and wanted to test for the presence and/or concentration of methanol in the mixture. How would one go about testing for this using home-chemistry lab chemicals/methods.

Thanks




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[*] posted on 3-6-2013 at 21:36


I asked this question a while back and got no answers that helped. I was told gas chromatography but that doesn't help a home chemist without equipment.

Try reading about the iodoform test, I know it precipitates from ethanol but not from methanol so maybe you can work out an estimate concentration by the amount that doesn't precipitate that would have if it was pure? I have no idea how to do that, if it will work or even have a good reference. This is only a guess.

There must be a simple presence indicator...




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[*] posted on 4-6-2013 at 03:00


What about the complete oxidation of the mixture with excess acidified potassium dichromate? The methanol will become oxidized all the way to the gaseous CO2 (oxidation of formic acid), while the ethanol will only oxidise to acetic acid and stop there. So by measuring the volume of carbon dioxide produced, the concentration of methanol can be (roughly) determined.
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[*] posted on 4-6-2013 at 04:20


This test is not practical. The complete oxidation of methanol by acidified dichromate is slow and it will take a long time before all methanol is oxidized to CO2. You certainly will not get bubbles of CO2 from this reaction.

A qualitative test for methanol is the following:
- Take appr. 2 ml of the ethanol/methanol mix
- Add a 5 to 10 ml of dilute acid (e.g. 15% sulphuric acid or 15% nitric acid).
- Add 200 mg or so of solid sodium nitrite to the mix.
If methanol is present, then the mix will start bubbling vigorously. Wait some time and then keep a flame near the mix. The gas ignites and burns with a pale grey (or orange, due to presence of sodium ions).
Use cold chemicals, allow the methanol/ethanol mix to cool down in a fridge and do the same for the acid.

When nearly no methanol is present, then there only is slow production of gas, or no production of gas at all. If some gas is produced in that case, then it is non-flammable.

The gas is methylnitrite. Its formation is very fast and its solubility in water is low. Ethanol also forms a nitrite (ethylnitrite), but this compound does not escape easily as gas from cold water.

[Edited on 4-6-13 by woelen]




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[*] posted on 4-6-2013 at 05:43


Thanks for your replies. Both methods sound interesting. Isn't dichromate like impossible to find though and it's very toxic, no?

Ethanol has a boiling point of 78.37c (http://en.wikipedia.org/wiki/Ethanol) and methanol has a boiling point of 64.7c (http://en.wikipedia.org/wiki/Methanol). If I keep the temperature of steam under 67c for some time, can I separate any methanol from solution? Then I can raise the temperature to 78.37c to collect mainly ethanol? Does anyone know if ethanol forms an azetrope with methanol?





[Edited on 4-6-2013 by binaryclock]




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[*] posted on 4-6-2013 at 06:19


Of course ethanol forms an azeotrope with methanol. If it didn't, it would be almost pointless using methanol as a denaturant for ethanol; people would just distill it off.
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[*] posted on 4-6-2013 at 06:23


Quote: Originally posted by adamsium  
Of course ethanol forms an azeotrope with methanol. If it didn't, it would be almost pointless using methanol as a denaturant for ethanol; people would just distill it off.


Ahh yes, of course! Damn it :) But lets say there is a 1% to 2% contamination of methanol in an ethanol solution. If keeping the temperature below 67c, would the core of the methanol be extracted first, leaving a lower percentage of methanol in the ethanol that is distilled later at 78c?






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[*] posted on 4-6-2013 at 07:41


Actually, now that I think about it, I wonder if it's not actually an azeotrope, but just very difficult to separate by distillation due to the very close boiling points. Either way, purifying ethanol denatured with methanol via distillation would be very difficult.

I thought I'd read somewhere that it was an azeotrope, but it's possible that I either read something that was incorrect or I'm just remembering wrongly.
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[*] posted on 4-6-2013 at 07:48


Well the way the moonshiners seem to do it is by using multiple 150ml jars. They label the jars, 1, 2, 3, 4, etc, then go by taste to figure out where the purest ethanol is, which is usually the jars in the middle of the distillation. It seems rather crude and unscientific, but that's what they do. They claim there is a bit of methanol in all ethanol distillations and it won't harm you.

But I'm trying to separate the ethanol to use in the lab, not drink so I'd rather it be as pure as possible. There's gotta be a different way than using multiple 150ml fractions throughout the distillation. Would a fractional Vigreux column help any?

PS - woelen thanks for that. I didn't see that post when I posted my next reply. That sounds like a wonderful methanol testing method if it works as described.

[Edited on 4-6-2013 by binaryclock]




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[*] posted on 4-6-2013 at 07:49



Ethanol and methanol do not form an azeotrope. However, the boiling points are close enough that separating them by distillation would require a pretty good column.





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[*] posted on 4-6-2013 at 07:54


The boiling points are not very different, but the freezing points are:

http://pubs.acs.org/doi/abs/10.1021/ie50258a029

It will not allow you to measure a small % of methanol, but if you have a substantial amount it should allow you to estimate roughly the ratio MeOH/EtOH.

I hope you are not intending to purify methanol-containing ethanol for consumption... Bad idea!

[Edited on 4-6-2013 by phlogiston]




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[*] posted on 4-6-2013 at 07:57


Well, Methanol - 64C, Ethanol - 78C. So separation would not be that hard if there is no azeotrope.



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[*] posted on 4-6-2013 at 08:42


Quote: Originally posted by phlogiston  

I hope you are not intending to purify methanol-containing ethanol for consumption... Bad idea!

[Edited on 4-6-2013 by phlogiston]


No consumption. This is mainly to explore the science and have a small amount of reagents for my troubles, acetone/methanol mix, ethanol.

I just find it curious that the shiners on the distillery forums haven't found a better method of purifying ethanol either. There's got to be a better scientific method than just to fraction using 150ml beakers/jars then smell/taste each jar and combine the best ones in the middle.


[Edited on 4-6-2013 by binaryclock]




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[*] posted on 5-6-2013 at 05:27


If you are trying to determine the concentration of each, there are actually a number of ways to do this. A simple one is the specific gravity, aka density, which is close for both, but can be determined easily to give an idea of if a sample is MeOH, EtOH, or about what mixture. It is likely not exactly linear, but a simple calibration curve could be made or found in the literature. Just put 100 ml in a small volumetric flask, weigh the flask, subtract the tare weight, and see what the density is. I have been able to determine mixtures of liquids that way to within a few %, which were verified by GC.

NMR could get the conc within 5%, but is not very good at home. GC is the ideal way, but also not readily avail to all, but many businesses use it. A simple thermal conductivity sensor would work, which is cheap and easy to build.

Running a small distillation will give a rough idea of the mixture. I am pretty sure that there are ways to analyze involving oxidation to the aldehydes and then determining formaldehyde verses acetaldehyde via some colorimetric assay, but I can't find that off hand. And index of refraction is often used to determine amounts of alcohol in water, I would guess that you could do the same for mixtures of alcohols, but have not tested that. A simple apparatus for that is available at many home brew stores. (I think I have one somewhere in my pile of stuff, but I don't know where right now...)



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