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GreenD
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[*] posted on 2-5-2012 at 13:22
DCM distillation


I had to distill off some DCM recently.

I put heat on low (~30% of variac) and let sit. No boiling was observed. About 40mL of DCM was removed in 2 hours and nothing condensed in a ice-water cooled apparatus.

Needless to say I'll be acquiring some grade A cancer soon, but whats the deal. At atmospheric pressure, shouldn't the DCM condense with ice water?




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99chemicals
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[*] posted on 2-5-2012 at 13:33


DCM has a very low vapor pressure. If the reciving flask was warm the DCM may have vaporized from the flask. Did you smell any of the chemical in the room?

Can you describe your apparatus for us?
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GreenD
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[*] posted on 2-5-2012 at 13:39


Quote: Originally posted by 99chemicals  
DCM has a very low vapor pressure. If the reciving flask was warm the DCM may have vaporized from the flask. Did you smell any of the chemical in the room?

Can you describe your apparatus for us?


I was using a short path. I couldn't smell it at all! Not that DCM is particularly strong, but I was so confused what was going on. It was simply just disappearing, no condensation, no smell.

I did the first half with a vacuum, reduced pressure.
Took off the vacuum same situation - just a continual lowering of the solvent level, no condensation, no smell. lol

Magic?




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smaerd
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[*] posted on 2-5-2012 at 14:10


Not magic at all...

Using a vacuum and high-heat to distill DCM is a bit ridiculous(hopefully you didn't wreck your vac?). A variac at '30%' is in all likely-hood too high. I can distill ethanol with my variac at about 8 without issue, using a 500mL RBF/glas-col. One thing I was first taught about distillations is a slow temperature change is important for a good separation. Yes this means waiting an extra 5 minutes for a mantle to get to temperature. I didn't think variac's used percentages as mine and all the ones I have seen go beyond 100. Then again I am pretty ignorant maybe some do.

Short-path as in, compact distillation? Or as in you decided to use a short-path condenser? Compact distillation is simply a matter of not thinking.You are going to want to use a nice condenser, maybe a coiled one, with ice-water, and have the receiving flask submerged again in ice-water. Ideally dry-ice/acetone would be used in a cold-finger type set-up, however not many have the luxury or the care to go that far.

Not witnessing any nucleation(bubbling) could be a result of a few things. Did you care to use boiling chips of any kind? Well I guess you were doing it under vacuum initially so that's fine.

Take a gander at the vapor pressure of DCM as said above... If you are worried about getting cancer don't be improperly distilling solvents indoors without good air circulation. Anyone who can read a can of paint stripper should be able to imply this knowledge. Not trying to talk down on you, but a lack of planning can turn something trivial and relativistically safe into a disaster or a health concern. Luckily it wasn't something more dangerous.

[Edited on 2-5-2012 by smaerd]




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[*] posted on 2-5-2012 at 22:46


The variac goes beyond 100 because it's indicating voltage. A variac is just a variable AC transformer.

I have attempted to distill methanol through a short-path condenser (about 15 cm or so) and I got no distillate. DCM is the most horribly volatile sh^t I have ever seen. It will "leak" from all but the best sealed containers and... well, it's like a compressed gas in liquid form if that means anything. I'm not sure if it's more volatile than ether, but if not, I know it's close.

Anyway, the point is: you just evaporated it out to the atmosphere. I have no clue as to why you didn't smell it but I didn't smell the ~200 mL of MeOH either.

And you should be able to boil DCM just by sucking on a hose attached to container filled by it (this is hyperbole and likely not true, don't try it). Did it not boil immediately when vacuum was applied? What kind of vacuum pump did you use? You should probably use a really good/cold trap unless you have some sort of Teflon, diaphragm pump.

Regardless, vacuum shouldn't be necessary.

Also, I wouldn't worry about breathing in DCM for a few hours every now and then. Only worry if you breath it in daily for hours at a time.




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[*] posted on 2-5-2012 at 23:38


With low boiling materials you should not use vacuum. DCM probably boils even below room temperature or maybe even at ice water temperature at sufficiently deep vacuum.

I myself distilled chloroform using a normal distillation setup (with NS24 ground joints) with a Liebig water cooler, without vacuum. It worked quite well. Some CHCl3 escaped (smell), but only a small fraction of it. The boiling point of CHCl3 is similar to that of DCM, so it certainly is possible to do a distillation of DCM. You, however, need water cooling and a cold receiving flask and should NOT use vacuum.




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[*] posted on 3-5-2012 at 01:34


Woelen, I wouldn't say that chloroform and DCM have "similar" boiling points. I'm pretty sure DCM is ~40 and chloroform is ~60. I guess I'm just arguing semantics...

I always found it interesting that, in the case of DCM vs. CHCl3, the increase in boiling is primarily due to the increase in mass as they have almost identical dipole moments (that's why I remember the b.p.s).




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[*] posted on 3-5-2012 at 05:56


I recently distilled DCM from paint stripper, and made a video of it here: http://www.youtube.com/watch?v=c3ppYMTAqWM (shameless self-promotion). It went quite well with no problems at all, without using vacuum. I did indeed use ice water in the condenser and an ice water bath at the receiver. I stored my product in an amber glass bottle with teflon-lined cap, screwed on tightly with teflon tape on the threads, and put it in the lab fridge. However, I feel like I'm still steadily losing my DCM through evaporation. Is there any particular way to store such a volatile liquid? I feel like this is almost going to be like storing bromine :/
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[*] posted on 3-5-2012 at 08:01


Believe it or not but the only thing I remember working perfectly was a bottle of beer called Grolsch with a bit of teflon placed on the top in between the "closer" and the mouth of the bottle.

If only there was a good "professional" bottle that used the same mechanism. There might be one.




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[*] posted on 3-5-2012 at 10:25


That's funny. When I get around to distilling DCM from my can of stripper, I'll ask my dad to try Grolsch beer.

Is it a good idea to distill from the can, but to use a stopper and glass tube to direct the vapors into an icewater-cooled condenser? Theoretically this should improve efficiency and avoid gunking up flasks. The methylene chloride vapors might ruin the stopper.
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[*] posted on 3-5-2012 at 20:24


The DCM will ruin the stopper but only slowly and they're cheap. I think that's the best way to do it unless you enjoy cleaning out large container's with frustratingly small openings.

If you really feel like it you can invest in a teflon stopper with some good silicone/fluoropolymer grease to make a good seal.

Now that I think about it, other than teflon I have yet to come across a plastic that DCM didn't at least dissolve slowly. Even with those Nalgene, HDPE, wash bottles I notice residue upon drying the DCM on a piece of glass. Nobody believes me though and they just think I'm being overly worried about purity. I just think it's a good idea that if you're using the DCM for critical syntheses, don't store it in plastic other than PTFE.

I could be wrong, of course.




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[*] posted on 3-5-2012 at 20:49


@GreenD

DCM is one of the easier distillation among volatile solvents... easier in that you don't need much heating and the solvent is inflammable.

As noted by Woelen, you do not need vacuum. It boils at 40C. A hot water bath is all you need.

You can distill right off the can...I've tried that many years ago because I didn't want to ruin my RBF. However it is a hassle to drill the hole and fit it with copper tubing and plumber puddy and to set everything up.

Now, I use an old RBF and I stop the distillation before the goopy material gets all dried up. I do a quick rinse with hot water and shake and vent and shake and vent... you get the point! The key to clearing the distillation flask is to clean it right away after your distillation is done before it has a chance to cool down.

As for using rubber stopper. DO NOT use rubber stopper!!! DCM will dissolve it away.

Anyway, search SM and read up on this subject and do some research on it before doing it. I read everything on this site and others before launching into cleaning various OTC solvents. It's a rewarding time and you learn alot of techniques and some "secrets" of the trade about distillation and solvent cleaning, etc.


Good luck!
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[*] posted on 3-5-2012 at 21:37


Quote: Originally posted by 99chemicals  
DCM has a very low vapor pressure.


I think you meant low boiling point. The vapor pressure is relatively high.

As jamit says, searching will be worth the effort:

http://www.sciencemadness.org/talk/viewthread.php?tid=1083&a...




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GreenD
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[*] posted on 4-5-2012 at 06:05


Cool. I'll keep the vacuum hidden next time.

I use 99% DCM from a supplier, no paint strippers.




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