Some time ago I received an email from a reader regarding some problems keeping a constant vacuum during a distillation:
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"...What type of grease (if any) do you use for the ground glass joints during high (oil pump) vacuum distillation? I use the 'regular' Dow Corning High Vacuum Grease that comes in a funky aqua colored tube. Sometimes when I am carrying this out, all of the sudden partway through the distillation, the pressure will suddenly go from about 0.06-0.1 Torr (where it normally is) to maybe 0.6-1.0 Torr. It doesn't always happen but the effect on keeping track of the boiling point is irritating with the pressure all over the place..."
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This is a common problem and it's not necessarily a problem associated with the grease. Let's talk grease first. I have never used the grease mentioned above but I'm sure that there are many excellent greases available and this particular grease may also be good. However, I do know that if you stick to the correct Apiezon grease for your application you won't have any grease related problems. There is a whole range of Apiezon grease and for a high vacuum distillation I would recommend either Apiezon T or H. All the information you need to select the correct Apiezon grease is available here. Don't overdo it when you apply your grease otherwise you'll contaminate your material during distillation.
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Interestingly, I recently had a similar problem to that described above when setting up a high vacuum manifold. However, in this case the problem was not the grease but the use of inappropriate vacuum tubing. The vacuum I was operating at was much lower than that described above but the same vacuum surge phenomenon was observed. The solution to this problem is to buy Tygon tubing. It's expensive but it's worth all the money and you usually don't need more than a few meters anyway. Aldrich sell 10 foot pieces of Tygon. It's transparent, nonreactive, flexible, not brittle and doesn't leak. It's simply the best. There's a good post at Wikipedia on grease. Notice the smart way of applying grease using a syringe. Also if you are setting up a high vacuum manifold you should consider reading this post at Org Prep Daily. D!
We had an issue a while back in one of those undergrad lab classes where you have to characterize & identify unknowns. A lot of students ended up with plasticizers, which shouldn't have been their products by any stretch. We think they leached out of the Tygon, but it's also possible that there was some crud at the bottom of the DCM tank in the stockroom--someone from Pharmacy mentioned having to set up a still.
ReplyDeleteI have in the past used the grease in the syringe trick, but I am still looking for a way to get the grease in the thing without a high proportion of it ending up on me.
ReplyDelete(re Tygon: The regional MS facility hereabouts got real tired of analyzing dibutylphthalate for graduate students who were convinced they had discovered some new reaction after finding this unknown in small amounts in their reaction workup.)
Curlyarrow: Thank you for the plug. (Also, if the spam posts are bothering you here, Wordpress has a quite easy-to use and efficient filtering system, and its free).
ReplyDeleteSometimes the leak in the system is from poorly matching joints. When one needs to go to such high vacuums (below 0.1 Torr) there should be as few connections as possible and as strong pump (=with high pumping speed) as possible.
Despite the fact that the joints are standartized, sometimes they don't match perfectly and you get a tiny channel forming even in a properly greased joint. Apiezons are better than Krytox or silicone or a generic petrolatum grease but even the best grease will not solve the joint mismatch problem or a leak in a tubing. And avoid rubber hoses for highvac application - they leak. Butyl rubber is less permeable but Tygon rules.
I don't think the plastisiser came form the Tygon tubing. To get significant amounts of plastisiser in your sample you need to get the tubing in direct contact with an organic solvent for some time. I have experienced some pretty annoying plastisiser contaminaions over the years. Once some clever individual decided to use rubber tubing when filling the hexane bottles up from the tank. Recently, someone decided to save some money and get the cheap ethyl acetate in plastic bottles. Regarding the trick with putting your vacuum grease in a syringe. Think about it man. Covered in grease once every decade or once evey week? D!
ReplyDeleteMilkshake is making some very good points. Keep connection points to an absolute minimum. Have as few joints as possible and make sure that you get the perfect fit and use the correct Apiezon grease. If you suspect your system is leaking pull it apart and attach the vacuum gauge to the pump. Check the final vacuum. Connect the next bit and check the vacuum again. Keep doing this until you find the weak spot. It can take some time to get it right but when you finally have that high vacuum manifold set up perfectly it'll be running beautifully for a long time. D!
ReplyDeleteexcellent. i knew between derek (pipeline), daniel (curly arrow), and milkshake (org prep) my question would be answered in a most satisfactory manner. apiezon H (and L) and Tygon tubing just arrived. Thanks all!!
ReplyDeleteI don't know what Tygon is made of and it seems pretty solvent resistant - but the cheap kinds of vinyl tubing contains something like 60% of phtalate by weight and it does dissolve easily. Many companies sell vinyl vacuum tubing and I have seen thick "tears" of dissolved plastic oozing down inside the tubing, brought about by repeated exposure to solvent vapors.
ReplyDelete"When one needs to go to such high vacuums (below 0.1 Torr)"
ReplyDelete0.1 Torr??? I get upset when I can't get my vac. line below 10 mTorr...and in most cases am unable to fully dry my reagents. The most common sources of error for me tend to be moisture in the hoses (heat for several minutes with a heat gun while pumping to remove), poorly greased joints, not having enough liq. N2 in the traps (can make as much as a 40 mTorr difference), having organic residues in the line (flame drying for a few minutes with a bunsen burner seems to work, or clean out line if it's real bad), or glass joints in the line that aren't aligned properly. Of course, having a high-quality vacuum pump is essential (and well maintained too).
I haven't seen any difference using just one greased ground-glass joint, or having several, provided the joints aren't splayed as to leave a small opening, and they are greased well. Usually my line gets down to 3 or 4 mTorr with just the glass line and 5 to 10 mTorr with Tygon tubing connecting the line to a flask.
The question was about vacuum distillation, not about max performance of a closed manifold. Normal 2-stage oil pumpms have a nominal vacuum about 0.3 mTorr so I bet you can go lower than 4 mTorr - if you close everything.
ReplyDeleteIn my experience a fractional distillation on higvac (while keeping the vacuum below 100mTorr) can be tricky, even with just a short Vigreaux. Every tiny leak shows. (And we use Chemglass glassware, not a bargain chinese stuff.)
Rather than risking decomposition of poorly volatile material upon slow distillation, I prefer to do a quick distillation of these things on Kugelrohr, that way all nonvolatile components (like salts) are out, so are the low boiling fractions. Then I would do a careful re-distillation if needed.
As usual, Milkshake is right on. My question wasn't about the ultimate vacuum of a pump, but the vacuum that is attainable under high vac distillation. I generally use Chemglass (or Ace) stuff as well...and ive been happy with it. Also i was using a small vacuum jacketed vigreux last time when the vacuum was all over the place. but then ive done it the same way (same glassware) before with no problems. we'll see how it goes with the new grease and tygon tubing. kugelrohr's are sweet...but i don't have one here. and a spinning band column wouldn't hurt either...
ReplyDeleteanyone ever used one of those B/R instruments spinning band columns?
What is tygon? It's unfilled PVC plasticised by dibutyl pthalate, and is oxygen and water have much higher permeation coefficients than black filled PVC. Tygon is very good at stopping oxygen permeation (but not as good as black PVC), but has very high permeation of water, but still lower than poly(isoprene).
ReplyDeleteThe shame is you can't but PVDC (saran) tubing anymore. It has low enough permeation constants to be used even with high vacuum systems.
Speaking of high vacuum, I wish to note that an oil pump produces "fine vacuum". High vacuum is 10^-4 mbar to about 10^-7 mbar and requires use of a momentum transfer pump (usually diffusion of turbomolecular). Using a diffusion pump I can't bring a line down below about 1x10^-3 mbar because the pump is then equalising the permeation of water through the butyl rubber tubing (butyl rubber having the lowest water permeation coefficient of any commercially available rubber tube). However, when the rubber is excluded it pumps down rapidly into "black vacuum" (10^-4 mbar and below).
At 1x10^-3 mbar a 100 mL RBF contains about 4.2 nmols of water (assuming all ordered water has been pumped off the glass), but at 1x10^-6 mbar it's 4.2 pmols. By extension, at 0.1 mbar the flask still contains 0.42 umols of water. This can be significant in catalytic reactions.