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# PArtition coefficient. watch

1. So i did an experiment in the lab a week ago.

(1st procedure)
we were given barbiturate (weak acid) solutions and we mixed it with
a) chloroform + 0.1M HCl.
b) chloroform + buffer pH 6.6
c) chloroform + buffer pH 7
d) chloroform + buffer pH 8
e) chloroform + buffer pH 9

and then allow them to saperate before collecting the Organic layer.

(2nd procedure)
then we mix the organic layer with 20ml of NaOH and wait for saperation, again in a saperating funnel. The AQUEOUS layer was collected and the concentration of butabarbital in the aqueous layer was determined from a calibration graph (using the UV absorbance determined from the aqueous layer).

So can anyone please tell me the point of the 2nd procedure? Do we add NaOH to the organic sol so that it will extract all the butabarbital that was in the organic layer, so that we can determine how much of butabarbital was present in the aqueous layer and the organic layer?

Im getting more and more confused as I thought through it for an hour and a half!!!
2. Yeah, the whole point of using the NaOH was to deprotante the acid and form the salt which is more solutble in water than DCM. You could then calculate how much of the acid the basic wash removed from the organic layer (if you knew how much of the acid was in the organic layer to begin with).
3. (Original post by EierVonSatan)
Yeah, the whole point of using the NaOH was to deprotante the acid and form the salt which is more solutble in water than DCM. You could then calculate how much of the acid the basic wash removed from the organic layer (if you knew how much of the acid was in the organic layer to begin with).
we dont know how much the acid was in the organic layer. But we can determine the concentration of the acid in the aqueous layer from the calibration graph that we've done as pert of the experiment. The main thing i would like to know is if the NaOH will react will ALL the acid in the aqueous (collected from the 1st part)?

Thanks.
4. (Original post by speedstacker)
we dont know how much the acid was in the organic layer. But we can determine the concentration of the acid in the aqueous layer from the calibration graph that we've done as pert of the experiment. The main thing i would like to know is if the NaOH will react will ALL the acid in the aqueous (collected from the 1st part)?

Thanks.
Hard to say but theoretically yes
5. (Original post by EierVonSatan)
Hard to say but theoretically yes
ok, thanks a lot!!

So from the acid that present in the aqueous, i can determined how much the acid was in the Organic solution (from part 1) as well the amount of Acid in aqueous (in part 1). so i can calculate the P apparent. Edit : or is it P true??

Then i need to find out the Pka of Butobarbital from the literature to be able to calculate % ionisation.

Then i can get P true from :
P apparent = P true x fraction unionised

am i right?

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Updated: November 11, 2008
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