Drying Toluene

[Sebbass69]

I've been reading about toluene, and I understand the distinction between dry and wet toluene. However, I was wondering how much water toluene actually absorbs, and if it is really important to have dry toluene for general chemistry. If it is absolutely critical what is the easiest way to dry toluene?

[UC]

http://macro.lsu.edu/HowTo/solvents/toluene.htm

 

For general chemistry, truly dry toluene is not necessary, but specialized reactions require truly anhydrous conditions. I believe refluxing over sodium with benzophenone as an "indicator" is the standard drying method.

Edited July 9, 2009 by UC

[John Cuthber]

The easiest way to dry toluene (provided that you are in a lab environment and have the kit) is to distill some of it. The water comes off as an azeotrope at a lower temperature than toluene.

This gets it dry enough for most things, it doesn't add any potential impurities (like benzophenone) it avoids using sodium (which is fun to play with but should be avoided where you can).

Of course, once you have distilled out the water you can change the receiver and produce freshly distilled toluene.

Since toluene only dissolves about 0.05% water it's often dry enough anyway.

[insane_alien]

The water comes off as an azeotrope at a lower temperature than toluene

 

tsk tsk john, when its an azeotrope, they come of at the same rate at the same temperature.

[John Cuthber]

The azeotrope (a particular mixture of toluene and water) comes off at about 85C if I remember rightly; the toluene at about 111.

Last time I checked 85 was less than 111.

If they came off at the same temeperature then you couldn't separate them. You can.

[insane_alien]

yes, but the mix will stay at the same fraction of toluene and water and the boiling point will remain 85*C. this is what an azeotrope is.

 

i suggest that you go refresh your memory.

[John Cuthber]

I don't think I need to do any more study on this.

If you distill wet toluene the first thing that comes over is the azepotrope at 85C. Once that has removed all the water the next thing to come over is dry toluene at 111.

They do not comee off at the same temperature.

 

Toluene is not the same as a mixture of toluene and water (about 20% tol 80% H2O IIRC) They have different boiling points and one of them settles out into 2 layers.

 

As I pointed out, there's never much water in toluene (about 0.05%) so the azeotrope will remove all of it.

 

What's your problm with this? It's been used as a method for drying toluene for ages.