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This is a practical question about the use of rotary evaporators ("rotovap"). So I'm asking for answers from people who actually use the things.

A rotovap typically uses a cold condenser to condense the solvent into a collection flask. But the distillation is typically done under weak vacuum as well. So the solvent sitting in the collection flask seem susceptible to re-vaporizing and getting sucked into the vacuum pump. Now I have a cold trap in front of my pump that should be cold enough (-50C) to trap most solvents under weak vacuum.

I think the answer is that if the vacuum is weak enough to vaporize the solvent under the hot water bath, but not under the cooler ambient air temperature, then the loss to the pump should be minimal. Indeed, this is what the instructions for the rotovap suggest.

Question: In practice, what are the risks to losing solvent into the pump?

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    $\begingroup$ We always kept the collection flask cooled with an ice bath; with volatile solvents we used a dry ice/acetone bath. The ice bath was just so easy to do, so why not. $\endgroup$ – ron Apr 7 '15 at 13:53
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Honestly, I never bothered to actually measure the volume of the condensed solvent in order to compare it with the original volume.

The concept described in the question, that is using a cooling trap with dry ice + acetone, is good practise and typically good enough to retain the evaporated solvent and protect the membrane pump. In rare cases, I have used a second, flat Dewar filled with dry ice + acetone on a "Laborboy" to cool the condensate-collecting flask.

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  • $\begingroup$ Thanks--this confirms that having a trap in front of the pump, in addition to the condenser cooling, is probably a good idea, along with cooling of the receiver per ron's comment. Perhaps not vital, but why not, as ron says? $\endgroup$ – user467 Apr 8 '15 at 0:38
  • $\begingroup$ @trb456 ron's suggestion is actually a good one! The only "argument" against it is laziness. It requires an additional movement (adjusting the height of the cooling bath for the receiver) when you evaporate from flasks of different size ;-) $\endgroup$ – Klaus-Dieter Warzecha Apr 8 '15 at 3:39
  • $\begingroup$ The idea of cooling the collection flask sounds good, but is actually useless. Makes no difference to the partial pressure of the solvent at the cooler, unless you let the rotary evaporator run for an extended time after the flask in the hot water bath is already dry. $\endgroup$ – Karl Nov 29 '15 at 23:26
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In my experience, no further cooling other than the condensor above the collection flask is necessary. Remember that you are selecting a pressure which is low enough to boil the entire liquid at $40~\mathrm{^\circ C}$ (your flask’s temperature) but high enough so that it will recondense on cold water (I’m going to guess $15~\mathrm{^\circ C}$). Dripping down into the collection flask, the solvent will retain that temperature or at maximum reach room temperature again which should be just below a temperature for sufficient evaporation. If it does evaporate, it will be caught by the condensor.

Any solvent ‘pulled through the pump’ should be caught in a collection flask below a condensor on the high-pressure side of the pump. In $99~\%$ of my evaporations, nothing is there and the remaining percent are special cases.

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Your cooling water should have around 10°C, and as long as you're not impatient and use a lower vaccum as recommended for your specific solvent, you should not loose much of it to the pump. Using an additional cold trap in front of it is not usually necessary. The manuals don't recommend or require it either.

Unless of course you (mis)use the roti to dry a product from solvent residuals. ;-)

Btw. The common membrane pumps are quite resistant to small amounts of solvent. What breaks first (if) are usually the seals, which are easy to replace. Don't soak it in chloroform, of course, and let it run dry if it got some. Put on empty flasks, leave the manual ventilation valve slightly open, and let it pump for ten minutes or so.

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