July 08, 2020, 09:21:34 AM
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Topic: Why is the pH of a 1:1 carb/bicarb sol'n 9.4, when the pKa is 10.3?  (Read 985 times)

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Offline vnaraya3

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This was in a lab experiment in class. All the groups had a pH of about 9.4-ish (using digital pH meter) when they made the 1:1 buffer of NaHCO3 and Na2CO3, even though the pKa of HCO3 is around 10.3. Why was it consistently lower than the theoretical pH?

Offline Arkcon

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Re: Why is the pH of a 1:1 carb/bicarb sol'n 9.4, when the pKa is 10.3?
« Reply #1 on: February 16, 2015, 07:34:53 PM »
Its true that theoretically, the pH of an equimolar mix of conjugate acid and conjugate base equals pka (because 1/1=1 and log101=0).  In order to explain your experimental results, you'll have to figure out why.  How did you prepare the 1:1 buffer?  Are you sure there's nothing else that could be present to affect the pH?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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