[macman104]

Sometimes when I'm doing a solvent extraction, after shaking I may get a homogeneous milky mixture, or I get a film in between my aqueous and organic layer.  Typical procedure is then to add brine (sat. NaCl) to the solution, which usually clears up the solution immediately.  I can't find an explanation for what the addition of the salt does to the solution to clear up the solution like this.

Any thoughts?

[agrobert]

All organic solvents have some misciblity with water.  When you have an organic compound in your extraction this is complicated even more.  An emulsion or mixture with water is cleared with the addition of salt because NaCl is insoluble in organics and very soluble in water.  If you saturate the water with salt (brine solution) you force most all organics out (including solvent and compounds) of water.  The only place for organics to go is back into the organic layer.

[agrobert]

For the same reason, this is why THF must be salted out because it is very miscible in water but not brine.

[macman104]

All organic solvents have some misciblity with water.  When you have an organic compound in your extraction this is complicated even more.  An emulsion or mixture with water is cleared with the addition of salt because NaCl is insoluble in organics and very soluble in water.  If you saturate the water with salt (brine solution) you force most all organics out (including solvent and compounds) of water.  The only place for organics to go is back into the organic layer.

Ah, ok.  I thought it probably had something to do with the salt saturating the water, but I wasn't exactly sure.  I just know that the typical procedure is:

1) Dilute with water
2) Extract with x number of organic washes
3) Wash organic layer with some salt (last time it was KHSO₄)
4) Rinse with brine and dry

Thanks for the info.  I knew the procedure but it was just mindless following of directions instead of really getting why I was doing this particular order.

[movies]

Point of clarification: two solvents are only miscible if they mix in any proportion and for a homogeneous mixture.  So, acetone and water are miscible, but hexanes and water are not.  A solvent may have some solubility in water (which all organic solvents do, to varying degrees), but it is only miscible if it is infinitely soluble.  There is no partial miscibility.