Proving inflection points - Titration

  • #1

Main Question or Discussion Point

This is not a homework question, but my prof came up with the concept of determining the inflection point on a titration graph with 2 pKas. He simply told us that it was the average of the 2 pKa values. However, he also just mentioned that it can also be proven through calculations, but he never showed us how.

I'm just curious as to how this can be proven, and this is not a homework question. Can some one show this to me? Thanks.

The example we were talking about in class was glycine :P, again. so the COOH group's pKa value was 2.4, and the NH3(+) group's pKa value was 9.6.

I just don't know how to prove it...
 

Answers and Replies

  • #2
Borek
Mentor
28,296
2,681
Something is wrong. Glycine (or any other substance with 2.3/9.6) when titrated with KOH (0.1M both solutions) have two endpoints at 6.02 and 11.01.

Download BATE (link in signature - pH calculation) and play with titration curves - seems to me I don't what inflection point is or your prof. is wrong.
 
  • #3
I think he's just using it as an example, even though it might be wrong. The concept is still there. However, i still need help proving the inflection points. I'm just curious as to how it's done. Thanks.
 
  • #4
1,104
25
Do a 1st Derivative plot. Plot the delta pH/delta volume vs. vol. base added. You will see a bunch of nothing and then a big spike. The tip of the spike is where you inflection point is.
 

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