elles
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Registered: 11-1-2013
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aspirin extraction
hello!
I would like to ask for some help with a problem.
When you're doing a fluid-fluid extraction for aspirin, what would be the correct pH to do this at?
I know that the log P for aspirin is -1,1 (Clarks database) and pKa is about 3,5.
Which means that in octanol-water it will favor the water phase and aspirin in the normal form is acidic.
When you're doing a fluid-fluid extraction with one phase being ethyl acetate would it be wise to do this at a low pH to push it to the fatty phase or
at a high pH to deprotonate it and drive it to the water phase considering the log P?
I would think you should aim for the water phase because of the log P. It favors water, so if you drive it to the fat phase then it would be more
difficult/impossible to get a high yield. Is this the correct conclusion?
Also my textbook doesn't specify but would the 2 phases be? water vs ethyl acetate or ethyl acetate vs some very "fatty" substance like petroleum
ether?
Since ethyl acetate is smack in the middle of the solvent polarity chart it could go either way in my opinion.
Thanks so much for your time!
[Edited on 11-1-2013 by elles]
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Nicodem
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That value does not make sense. The SRC PhysProp Database gives a referenced experimental logPo/w value of 1.19, which is more in accordance to the molecular structure.
There are plenty of information on how to purify aspirin, so please UTFSE. If you just want to practice extraction techniques then fine, but such
liquid/liquid extraction is unnecessary for the commercial aspirin pills (at least for those that only contain the binder). A recrystallization is
more appropriate.
| Quote: | | When you're doing a fluid-fluid extraction with one phase being ethyl acetate would it be wise to do this at a low pH to push it to the fatty phase or
at a high pH to deprotonate it and drive it to the water phase considering the log P? |
With aspirin it wouldn't be wise to go either direction with the pH due to the obvious reason. Aspiring is an aryl carboxylate.
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