All organic chemists have experienced it. Your reaction is done and your transfer it to a separatory funnel with an aqueous and an organic phase. You do your shaky, shaky bit and the phases separate but it is impossible to tell where the phase boundary is. The problem is most commonly encountered when very dark phases are obtained but it can also occur with perfectly clear phases (see picture). The solution to this problem is very simple. Simply tip a clean NMR tube cap into the separatory funnel and observe as it settles on top of the bottom phase. Problem solved. D!Wednesday, April 09, 2008
The Invisible Phase Boundary
All organic chemists have experienced it. Your reaction is done and your transfer it to a separatory funnel with an aqueous and an organic phase. You do your shaky, shaky bit and the phases separate but it is impossible to tell where the phase boundary is. The problem is most commonly encountered when very dark phases are obtained but it can also occur with perfectly clear phases (see picture). The solution to this problem is very simple. Simply tip a clean NMR tube cap into the separatory funnel and observe as it settles on top of the bottom phase. Problem solved. D!
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12 comments:
Good Tip.
But it works only organic solvent with a density lower than water i.e. AcOEt, Ether or Toluene, that does not work with chlorinated solvents...
But That is nice....;)
I often find that holding a clean white piece of paper behind the sep funnel often can reveal the phase boundary as well
http://sanjayat.wordpress.com/2006/10/03/the-nmr-cap-trick/
So you see the phase boundary. No problem, except in my experience you then get plasticiser leaching :(
Fascinating! Love it!
Adding a few springles of Celite also works in most cases and then can be filtered off typically when using a drying agent
What about dark color solutions, probably a white NMR cap is needed..
Yes, a white cap makes things easier when you have two perfectly black layers. D!
Turn the lights off and use a UV light. Different light absorption/refractive index or whatever sometimes makes the boundary visible. Especially with dark solutions.
TLC lamp can work wonders. I had an extraction from hell a while back, full of crud, dark icky brown all over...except something in the phase I wanted fluoresced bright green. (I never figured out quite what it was, since it wasn't the stuff I was trying to make...)
nice info. though simple but cool and helpful
A flash light behind the funnel usually does the trick as well. Especially with dark/black solutions.
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