Sciencemadness Discussion Board

Azeotrope question

yoyoils - 14-4-2012 at 21:45


I'm wondering IF when isolating methanol from a solution of nitromethane in methanol, (using distillation) would you be able to obtain pure methanol?

because I've read how they form an azeotrope, but I gotta be sure the methanol I save wont contain any trace nitromethane. At All

[Edited on 15-4-2012 by yoyoils]

weiming1998 - 14-4-2012 at 23:59

If you've read that they form an azeotrope, and what the book says is true, then you cannot separate them by distillation using conventional methods. The only 2 ways I could think of is to either react the nitromethane with something that won't react with ethanol, or make an ester of an acid with methanol, distil the ester/nitromethane away, then hydrolyze it back into methanol and the corresponding salt using a base.

yoyoils - 15-4-2012 at 11:04

I know distilling nitromethane from methanol, i'll always will result a mixture no matter how i try but can you distill pure methanol and leave all the MeNO2 behind??

weiming1998 - 15-4-2012 at 16:33

Quote: Originally posted by yoyoils  
I know distilling nitromethane from methanol, i'll always will result a mixture no matter how i try but can you distill pure methanol and leave all the MeNO2 behind??


I don't know what you mean by that. Why would you need to distil pure methanol? And if you are distilling pure methanol, why would it leave behind nitromethane? As I said before, ordinary or fractional distillation does not work for a mix of methanol and nitromethane, no matter what order you distil in or how many times.

Magpie - 16-4-2012 at 07:23

Quote: Originally posted by yoyoils  

I'm wondering IF when isolating methanol from a solution of nitromethane in methanol, (using distillation) would you be able to obtain pure methanol?


You have told us that there is a methanol/nitromethane azeotrope. But you have not told its concentration, its bp, the bp's of methanol and nitromethane, nor the methanol concentration of your starting solution. We can't answer your question without this information. Are we supposed to look all this up?

[Edited on 16-4-2012 by Magpie]

[Edited on 16-4-2012 by Magpie]

Organikum - 16-4-2012 at 23:49

Apply reduced/elevated pressure and/or ternary azeotropes for separation.

fledarmus - 17-4-2012 at 04:34

Methanol (bp 64.7C) and nitromethane (bp 101.2C) form a low boiling azeotrope (bp64.6C) containing 9% nitromethane. Wikipedia, Azeotrope(data)). This means that when you try to distill a mixture of nitromethane and methanol, the lowest boiling fractions that you collect will be those containing 9% nitromethane. If you start with a solution that contains less than 9% nitromethane, you will collect pure methanol once all the nitromethane has been removed as a 9% solution in methanol; if you start with a solution that contains more than 9% nitromethane, you will collect pure nitromethane once all the methanol has been removed as a 9% nitromethane solution.

The boiling points of the azeotrope and the pure methanol are so close together that it would be very difficult to tell when all the nitromethane was gone and you were collecting pure methanol.

You could add cyclohexane to your mixture and boil off an even lower boiling azeotrope (54.2C) containing 62.8% cyclohexane. Then a water extraction will remove the methanol from the cyclohexane, and a final distillation of the methanol from water will give you pure methanol.

Reducing the pressure will change the boiling points of all the components of the mixture, but will not change their rank order. You still won't get any pure material, only the azeotrope at a lower temperature.

[Edited on 17-4-2012 by fledarmus]