[gaschroking]

I tried to  extract dibenzothiophene  using ethyl acetate. I added  10 ml of water   in 10 mg of  dibenzothiophene. After which I added 10 ml of ethyl acetate and shaked it  vigorously for 5 minutes.  Centrifuged at 10,000 g for 2 minutes. There is a clear separation of phases. My question is that Ethyl acetate has a density of around 0.9 g/cm^3, is the  solvent phase the upper layer and the aqueous phase the lower layer? I am confused with  other literatures  which say that there is a density inversion in solvent extraction. Could you enlighten me onthis matter?

Another concern.. What is the purpose of adjusting the pH to say 2.0 prior to  mixing?

These forum is always my refuge  whenever I got problems with my experiments. Please pardon me for asking very basic questions  that may be a common sense for most you. I  had a Biology degree in my undergrad but now taking MS courses in environmental chemistry.

Any help is appreciated..

[chiralic]

[quote ] My question is that Ethyl acetate has a density of around 0.9 g/cm^3, is the  solvent phase the upper layer and the aqueous phase the lower layer? I am confused with  other literatures  which say that there is a density inversion in solvent extraction

Could you post information about you literature?...

[Ψ×Ψ]

Add a mL or two of solvent to the water phase or water to the solvent phase and look for separation.  This should tell you which is which.

[gaschroking]

this is a link to the site that I visited before about density inversion..

http://goldbook.iupac.org/D01591.html

I just wonder what quantity of solute is necessary  to  have density inversion..

[Borek]

First, let's recapitulate what we know. Both phases have different densities, one that is lighter is always on top. However, solution density changes with the solute concentration - see density tables for examples. When both phases have similar density at the beginning of the extraction it may happen that when solute moves from one phase to the second, density of both phases changes - and phase that was lighter now becomes heavier. That's when the density inversion may happen.

In your case concentration of solute is so small (1mg/1mL, about 0.1%) that you may safely assume no density changes at all. Water at the bottom, ethyl acetate on top. Check density tables on the page I have posted - note that for 1% solutions density changes rarely reach 1%.

Water phase is often acidified or basified when the substance is a weak acid or weak base, to move dissociation equilibrium as far to the left as possible. For example weak acid in acidified solution gets protonated, thus it is mostly in non-ionic form - which is soluble in non-polar solvent like ethyl acetate. That helps in extraction. It may be also used other way around - if you want to get as much of the acid from the non-polar phase, you will use basified water, to neutralize acid.

That being said, I have no idea why this particular solution is acidified - dibenzothiophene doesn't look like an acid to me...