Sciencemadness Discussion Board

Question about distillation

xfusion44 - 30-9-2015 at 20:54

Hi,

I needed some toluene but since it's hard to get it in pure form in my country, I purchased 1l bottle of paint thinner, that has toluene and acetone in it - that are the only two ingredients that were listed on the bottle label, but I think there are probably some other compounds in this mixture. Toluene and acetone are probably just the main two ingredients (that are present in largest quantities)...

So, I distilled the thinner and when the temperature got to 111°C, there was about 25-30% of the liquid left in the flask (if there wasn't any other compound with higher BP than toluene's, that should be pretty pure toluene, right?).

And here goes my first question: distillation started at about 66°C, but shouldn't it start at 56 (BP of acetone)? I know that mixing some compounds can change the boiling points, but does that happen in this case too? Also, at about 70 or more deg. C, I noticed the phenomenom on the picture, below... Don't know what it's called, but this is where I think the second (mysterious) compound, with different density started to distill. Am I right?

Second question: when you have a mixture of let's say toluene and acetone, why doesn't all of the acetone distill at it's boiling point, instead of gradually increasing temperature, when there is still acetone in the mixture? Is that why we can't distill compounds with BP's closer than 20°C together, without using fract. column?

Thanks for your answers! :)



IMG_20150930_114014.jpg - 769kB

[Edited on 1-10-2015 by xfusion44]

Deathunter88 - 30-9-2015 at 21:01

Quote: Originally posted by xfusion44  

I noticed the phenomenom on the picture, below... Don't know what it's called, but this is where I think the second (mysterious) compound, with different density started to distill. Am I right?


You are probably right, however it may just be that your condenser is either hotter or colder than room temperature. Some liquids, (such as sulphuric acid) changes densities very significantly at different temperatures, so those distortions could just be due to your distillate being a different temperature to what is already in the beaker.

xfusion44 - 30-9-2015 at 21:19

Quote: Originally posted by Deathunter88  
Quote: Originally posted by xfusion44  

I noticed the phenomenom on the picture, below... Don't know what it's called, but this is where I think the second (mysterious) compound, with different density started to distill. Am I right?


You are probably right, however it may just be that your condenser is either hotter or colder than room temperature. Some liquids, (such as sulphuric acid) changes densities very significantly at different temperatures, so those distortions could just be due to your distillate being a different temperature to what is already in the beaker.


Well, that could be possible. I started with ice water, which probably warmed up significantly after distilling about 600ml of liquid at 66-111°C.

The distillate smelled pretty much like acetone, so the acetone is most probably large part of it.

Thanks!

xfusion44 - 30-9-2015 at 21:34

Update: I tried to burn the distillate and the flame released lots of soot. But why? Was this because of that mysterious compound(s) or was there some toluene that made it through?

THX :)

Praxichys - 1-10-2015 at 04:45

If it's the toluene you are after, shake the raw material with about half its volume of water 4 or 5 times and keep the top layer each time. The acetone (being fairly polar) will adhere to the highly polar water and leave toluene, which can then be distilled for purity. This process will remove a lot of other ingredients that are commonly in the product you describe, including methanol, ethanol, ethyl acetate, and acetone.

Distillations are complicated. If you distilled two mixed components which had zero interaction with each other, you could theoretically get clean, pure separation of both as long as they have different boiling points. In the real world, even things as small as hydrogen bonding affect the separation of even the most nonpolar of components, and some will carry each other through depending on the closeness of their boiling points. Mixtures with polar components are worse, especially if their boiling points are similar, because they tend to be attracted to each other and the more volatile one will carry the less volatile through the still. Many compounds have such strong interaction that they form azeotropes, or mixtures of two or more substances that cannot be concentrated by boiling since both substances boil off at the same rate.

Toluene forms azeotropes with water, ethanol, and methanol. By experience (and without MSDS... speculating here) you have one or more of these components in your raw material, and it is carrying over as the azeotrope. Use the washing method before you distill and you will have your pure toluene.

aga - 1-10-2015 at 08:28

Washing as Praxichys sggested certainly works.

I followed that advice and the two layers separate almost instantly.