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Solvent extraction in a separation funnel is a very common method in preparative organic chemistry. But sometimes you don't get a nice phase separation between the organic and the water phase.

What are the possible causes that can prevent the formation of two distinct phases, and how can you force a phase separation when you encounter such situations?

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    $\begingroup$ sometimes I try just waiting, acidifying the solution, heat, an ultrasonic bath, centrifuge, filtering through sodium sulfate... even gasp adding salt. $\endgroup$
    – Chris
    Apr 26, 2012 at 8:35
  • $\begingroup$ For "possible causes", I should think that having a surfactant present would up the chances of forming a stable emulsion, preventing separation. I don't imagine that anyone would be knowingly trying to purify a surfactant by solvent-solvent extraction, but it's something worth bearing in mind if some surface-active-agent were produced as a by-product / was an unreacted reagent / was present from an earlier reaction step and had not been fully removed. $\endgroup$
    – owjburnham
    Jan 25, 2017 at 12:06

3 Answers 3

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One such cause, where separation does not take place is the nature of the organic solvent. Its better to check the miscibility of the solvent with water. There are several websites, where one can get this info. For example 1 and 2.

Even with hydrophobic solvent, if the problem persists, one can add NaCl and then check the separation.

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    $\begingroup$ Adding salt to the mixture augments the ionic strength of water, therefore increasing the "difference" between water and your other solvent. I've found that this usually works. $\endgroup$
    – CHM
    Apr 26, 2012 at 21:15
  • $\begingroup$ Yes, assuming your solvent isn't miscible in water, adding brine (concentrated solution of NaCl in DI water) usually breaks the emulsion. $\endgroup$ Jun 21, 2012 at 20:34
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Well, shaking a sep-funnel is a great way of creating a horrible intractable emulsion with no clear phase boundary, so don't do that (also, the stopper might pop off as soon as you set it down).

As for breaking an emulsion, if you have the time, just leave the sep-funnel somewhere and wait a few days. I seem to recall the first and last time I shook a sep-funnel the lab tech added some salt to the water/diethyl ether emulsion however I'm not clear on this.

Finally, consider sacrificing part of your product by drawing the funnel off to the very base of where the emulsion begins, draining the emulsion layer entirely and then taking the remainder. If you have blobs of one phase in the other, you may be able to remove them with a pipette.

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    $\begingroup$ I'm not sure why you made the comment about creating the emulsion/popping the stopper off by shaking the sep funnel. In order to get a good extraction, you need to shake the funnel. To prevent the stopper from popping off, you need to use proper technique and vent the funnel from the stopcock periodically as you shake it. $\endgroup$ Jun 21, 2012 at 20:35
  • $\begingroup$ @scientifics - regarding venting the sep funnel, you are of course correct, and any incidents involving stoppers can be attributed to inadequate venting. However, as far as emulsifying phases are concerned, IMHO protracted swirling is preferable to shaking. $\endgroup$ Jun 22, 2012 at 3:38
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An additional option besides adding salt and having patience, is to place your funnel inside a heating oven at a temperature that is not harmful to your reaction. In my experience, it helped with the separation. To prevent buildup of pressure, remove the lid, and maybe place it upside down so it covers the funnel without really blocking vapor escape.

Also, if you plan on adding salt, it can help to empty the content in a beaker, add the salt there, and stirr with some heating for a while. Then put the content back in the funnel.

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