Sciencemadness Discussion Board

how to seperate Methonal from H2O

whodkins - 28-7-2006 at 00:00

i was woundering if there is any way to seperate methanol from water?

chromium - 28-7-2006 at 00:06

Distillation. If you use column you can go to 100% with one time, otherwise you need several distillations. If no column is used one can get somewhat better separation if lot of NaCl is added to initial mixture.

IIRC if mix of methanol and water is saturated with K2CO3 then methanol separates as upper layer.

[Edited on 28-7-2006 by chromium]

not_important - 28-7-2006 at 00:49

The trick of adding salt to the mix is useful when you have only a simple distillation rig, the alcohol rich layer will be on top while the water rich brine forms the lower layer. This goes for most any salt, be it NaCl, K2CO3, or whatever. The brine may still have some alcohol in it, it is worth distilling it until ~80 C is reach and adding the distillate to the next batch to be processed. The upper alcohol rich layer still needs to be distilled, but it will have less water than the original mix.

Methanol does not form an azeotrope with water, so distillation can separate them. To get a clean separation you need some fractionation, with reflux of distillate encountering the acending vapour. Or, as Chromium said, do a series of distillations. Adding salt to the distillation mix helps because it makes the organic (MeOH) less associated with the water; I don't know to what degree it help with methanol/water but it is useful with the water soluble fatty acids.

The last bit of water can be removed with anhydrous Na2SO4 or MgSO4. Don't use calcium chloride, it complexs with the alcohol.

leu - 30-7-2006 at 16:43

Quote:
Distillation. If you use column you can go to 100% with one time, otherwise you need several distillations.


Actually, according to Riddick in Organic Solvents the usual level of purity of methanol achieved by fractionation in efficient columns is 99.99 % :P Methanol will adsorb water from the air, and the formation of formaldehyde occurs as well :) Generally, the commercial synthetic product is about 99.85 %; if the methanol is produced by destructive distillation of wood, acetone is a major impurity :D

Vitus_Verdegast - 30-7-2006 at 19:53

If it's just to dry your methanol from small amounts of water, 3A molecular sieves do the job well.

0.18g H2O is taken up by every gram of sieves.

Of course, the sieves can be regenerated by heating them at 250°C for a couple of hours, followed by nuking them a short time in the microwave

[Edited on 31-7-2006 by Vitus_Verdegast]

unionised - 31-7-2006 at 11:40

Make sue to let the methanol evaporate first or you may put your oven in orbit.