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Topic: How to find pka from absorbance?  (Read 17777 times)

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Offline breathoffire

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How to find pka from absorbance?
« on: July 09, 2008, 10:40:20 PM »
I am suppose to be able to calculate the pka of a buffered phenol red solution. I have no idea where to even start. The absorbance is 0.334 with a pH of 7.44. The %base is 40.09 and the % acid is 59.01. The equation the problem says to use is pka= pH-log([Base]/[Acid]).
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Offline Yggdrasil

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Re: How to find pka from absorbance?
« Reply #1 on: July 09, 2008, 11:18:42 PM »
Quote from: breathoffire on July 09, 2008, 10:40:20 PM
I am suppose to be able to calculate the pka of a buffered phenol red solution. I have no idea where to even start. The absorbance is 0.334 with a pH of 7.44. The %base is 40.09 and the % acid is 59.01. The equation the problem says to use is pka= pH-log([Base]/[Acid]).

Well if you know pH and the ratio of base to acid, you should be able to get the pKa using the formula given (Henderson-Hasselbalch equation).  The absorbance likely helps you figure out the ratio of acid and base (even though you've given this information already)
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Offline Borek

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Re: How to find pka from absorbance?
« Reply #2 on: July 10, 2008, 03:33:56 AM »
I suppose your problem is what is a base, what is an acid here. See Bronsted-Lowry theory, it should help.
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