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Topic: Why am I acquiring the second pKa value before the first?  (Read 755 times)

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Offline RyanR

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Why am I acquiring the second pKa value before the first?
« on: January 04, 2024, 12:47:48 PM »
I am carrying out an experiment to investigate the effect of increasing temperature on the acidity of citric acid, and I hypothesize that the pKa will decrease as temperature increases. My results support this, so that is not the problem. The problem is, that I am acquiring 4.8 (the second pKa value of citric acid) instead of 3.1 (the first pKa value). I believe that I should be getting 3.1 since it is the first pKa value, and it only makes sense that I should reach that first. Therefore, is my belief right that I should be getting 3.1 first (and I have a flaw in my experimental design etc.) or is it perfectly fine to get 4.8 instead? Please respond ASAP
Thank you!

Offline Borek

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Re: Why am I acquiring the second pKa value before the first?
« Reply #1 on: January 04, 2024, 01:59:02 PM »
Sorry, impossible to answer not knowing HOW you are determining pKa values. Plenty of methods, each with its own quirks.
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Online Babcock_Hall

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Re: Why am I acquiring the second pKa value before the first?
« Reply #2 on: January 04, 2024, 04:52:40 PM »
There are many commercial forms of citric acid.  Exactly which one are you using?

Offline Vidya

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Re: Why am I acquiring the second pKa value before the first?
« Reply #3 on: January 06, 2024, 07:33:27 AM »
Quote
I am carrying out an experiment to investigate the effect of increasing temperature on the acidity of citric acid, and I hypothesize that the pKa will decrease as temperature increases. My results support this, so that is not the problem. The problem is, that I am acquiring 4.8 (the second pKa value of citric acid) instead of 3.1 (the first pKa value). I believe that I should be getting 3.1 since it is the first pKa value, and it only makes sense that I should reach that first. Therefore, is my belief right that I should be getting 3.1 first (and I have a flaw in my experimental design etc.) or is it perfectly fine to get 4.8 instead?
As Borek has said that it is not clear what are the conditions and method you are using to determine the pKa values. First pKa is more acidic and may be your experimental conditions are basic enough the deprotonate the first acidic H.

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