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!

12 comments:

Anonymous said...

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....;)

Anonymous said...

I often find that holding a clean white piece of paper behind the sep funnel often can reveal the phase boundary as well

Thomas said...

http://sanjayat.wordpress.com/2006/10/03/the-nmr-cap-trick/

Anonymous said...

So you see the phase boundary. No problem, except in my experience you then get plasticiser leaching :(

mevans said...

Fascinating! Love it!

Anonymous said...

Adding a few springles of Celite also works in most cases and then can be filtered off typically when using a drying agent

pmgb said...

What about dark color solutions, probably a white NMR cap is needed..

Daniel Sejer said...

Yes, a white cap makes things easier when you have two perfectly black layers. D!

Anonymous said...

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.

Ψ*Ψ said...

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...)

Medicinal Chemist said...

nice info. though simple but cool and helpful

ABC13 said...

A flash light behind the funnel usually does the trick as well. Especially with dark/black solutions.