Sciencemadness Discussion Board

Simple distillation question

prole - 17-3-2006 at 07:03

I'm performing a simple distillation of toluene to separate it from the impurities that came with it. Yesterday I had a nice rolling boil going (using Teflon boiling stones), and collected a fair quantity for my efforts. Today, I recharged the flask with fresh solvent, and set to boil. My point of confusion now is this: the solvent will not boil at all, but it is distilling over very rapidly, a constant flow through the condensor and a fast drip into the receiving flask. The temp at the therm. is a solid 110 C. Why is my toluene not boiling, but rapidly coming over? Heat too high? Same temp I used yesterday. I'm glad to see things progressing so quickly, but I don't want any impurities to come over as well, if that's possible.:o

TheBear - 17-3-2006 at 07:42

Don't know about impurities but you need to add another dry boiling stone. Add it when you see that the solvent is just about to start to boil, when it's turbulent and it's starting to condense higher and higher in the apparatus. Watch out, if you've heated the solvent high above its boiling point you might get surprised by a solvent fountain. If you're unlucky and don't add a boiling stone the solvent might start thumping (right expression?).

jack-sparrow - 17-3-2006 at 08:10

You have to remove the old boiling stones and put fresh ones. Boiling stone create a bubbling action by releasing gas trapped inside them and serving as a nucleation point for new "bubbles" of gaseous solvent. When the solvent cools down, the cavities in the stones fill with solvent and are not useful anymore. The separation of components in a distillation is dependant on a number of factor. The number of theoretical plates (steps of condensation-gaseification) between the pot and the receiver. The reflux ratio (1 to 5 is usualy enough to keep the equilibrium on the distillation column). Also the nature of components because there are some chemicals that cannot be purified by distillation. In the case of ethanol, you always distill out toluene and water with it. I recommend you to read the distillation section of any practical organic chemistry (like Vogel's). But you will learn more and get a better feeling by experimentation.

Good luck

DrP - 17-3-2006 at 08:17

Also for purity of the toluene, if you need it dry, we used to put sodium wire into the toluene to be distilled to remove any water.

solo - 17-3-2006 at 08:24

The poster noted teflon stones, hence the problem with trapped **** doesn't apply ?. also you may hava an azeotrope mix and the temp will differ....check out the recent post by S.C. Wack , where he posts the list of useful azeotropes ......solo

https://sciencemadness.org/talk/viewthread.php?tid=4713&...

prole - 17-3-2006 at 08:52

Thanks for all the replies. I've been reusing my boiling stones, so that may be the problem. Can they be dried out and 'recharged'? I try not to send any lab waste to the landfill and recycle what I can.

I followed TheBear's suggestion and dropped in a single fresh boiling stone while it was hot (he said to wait before it heats up, which is wise). I'm using a 1L RBF and there was about 150 mL toluene left, and yes, you get a solvent fountain. I wouln't have done it this way if there was more solvent in the flask. But after it calmed down, all the stones were now sending bubbles up nicely. (The fountain never left the flask).

Regarding impurities, the toluene I'm using came from the hardware store, as 100% toluene. But we all know there's more crap in there, as I've seen on many occasions. This is why I distill my OTC solvents before using them. I'm usually left with a pale golden colour with wispy particles suspended in the mix. It need not be dry at this time, and I don't think there's an azeotrope in there, all the solvent came over at 110 C.

[Edited on 3/17/2006 by prole]

vulture - 17-3-2006 at 12:25

You are aware of the fact that more volatile contaminations (like acetone or ethylacetate) will also end up in your distillation flask?

I'm saying this because those are common contaminants in hardware grade toluene.

Ideally you should discard of the first 10% (It's good for boosting the octane level of gasoline when used wisely).
Another good idea is adding MgSO4 or any other salt capable of absorbing water to your boiling flask to get rid of water contamination.

[Edited on 17-3-2006 by vulture]

[Edited on 17-3-2006 by vulture]

prole - 18-3-2006 at 08:13

Vulture wrote:
Quote:

You are aware of the fact that more volatile contaminations (like acetone or ethylacetate) will also end up in your distillation flask?
Quote:

Ideally you should discard of the first 10% (It's good for boosting the octane level of gasoline when used wisely).


Thanks for making those points. I usually discard only the first couple dozen drops or so when the bp rises to the target temp right away. I suppose greater purity is assured when 10% is sacrificed for purity.

So I took my litre of distilled toluene to remove the 10% forerun. I redistilled in two 500mL batches using fresh stones for the first run, and reusing the same stones for the second run. First run took under an hour (oil bath), so I figured the stones should be fine for the second. Wrong! I wouln't repeat this if it hadn't got interesting. The stones in the second run weren't bubbling as before, causing me to question my hotplate's integrity and other useless assumptions. These were practically brand-new stones! Distillate started coming over rapidly and no boiling action. Suddenly, the stones decided to work. Flask was half full (always optimistic), and started bubbling with a big POP. Violent, raging storm of hot toluene in vitro that eventually died down to a rolling boil. Damn, I nearly died on the spot! But it contained itself and the rest proceeded without a hitch. All in the name of purity. The moral here is, always use fresh boiling stones!

unionised - 21-3-2006 at 13:25

The moral is add fresh boiling stones FIRST.
Don't wait until it's nearly boiling or you risk mis-judging it and adding a nucleation source to a superheated liquid.
Thebear is talking dangerous nonsense about waiting until it's near boiling (and if he wishes to argue about that ,the bumping you saw proves my point.)

Magpie - 21-3-2006 at 16:19

I heartily second that point! Never add a stone to a solution near boiling! If it is superheated you can have a virtual explosion! :o

Put the stones in before applying heat. If I forget I stop, cool down, add stones, and then start again.

Always use new stones for the reason described above. When a used stone cools it will suck liquid into the pores. Therefore there will be no space for air. So what good is it to reuse them?

I broke up a small non-glazed piece of pottery to stone sized pieces (BB size). This supply will last me a long time for virtually no cost.

TheBear - 22-3-2006 at 07:02

@unionised: yes, I agree with you. I expressed myself in a bad way. However, in practise I always use chips of broken pottery. Commercial boiling stones are usually made of Teflon as noted earlier. I have only a little experience with them so I can't say much about them. I've experienced that if I throw in my boiling stones from start, I mostly get a superheated solution due to the gas filled cavities of my boiling stones have since long been filled with liquid, the reason for this is unclear for me, could someone enlighten me? Perhaps I should buy some commercial boiling stones.

In order to avoid this I've been adding the boiling stones a couple of degrees below boiling point (or around the b.p. of the lowest boiling component in the case of a solvent mixture). It might not be particularly wise but as I said, if you make sure that you haven't superheated the liquid (a thermometer might come in handy? :) ) I really can't see what’s so dangerous. Or am I putting myself in unnecessary danger? When I was referring to the liquid condensing higher and higher in the apparatus I was of course assuming that he was distilling an impure toluene based solvent where the main contaminants are, as earlier pointed out acetone and ethyl acetate. But I really should have rephrased that. The reason for why I put it that way was because I too distilled some OTC toluene about two weeks ago, experiencing the same problems with bumping.

If one isn't in a hurry the best way is as Magpie pointed out adding stones first, try to distil, if the boiling stone isn't providing nucleation point and your liquid is bumping: let it cool down, add fresh stones and heat up and repeat the procedure if necessary.

I have a question of my own: In the "king of chem book" (or something like that) on the ftp a fractional column is improvised by mounting a condenser vertically on the boiling flask. Is this practical or should one fill the condenser with some appropriate material to serve as disks, or is both just a waste of time compared to buying a real fractional column?

[Edited on 22-3-2006 by TheBear]

unionised - 22-3-2006 at 09:59

"In order to avoid this I've been adding the boiling stones a couple of degrees below boiling point (or around the b.p. of the lowest boiling component in the case of a solvent mixture). It might not be particularly wise but as I said, if you make sure that you haven't superheated the liquid (a thermometer might come in handy? ) I really can't see what’s so dangerous. Or am I putting myself in unnecessary danger? "

Do you know what an azeotrope is?
Boiling stones are cheap. Use lots, that way some of them are bound to work.
If you wash and dry them they can be reused too.