Proving inflection points - Titration

In summary, the concept of determining the inflection point on a titration graph with 2 pKas is that it is the average of the 2 pKa values. This can also be proven through calculations, by doing a 1st derivative plot and finding the point where the delta pH/delta volume is at its maximum. This is also demonstrated through a titration of glycine with KOH, where there are two endpoints at pH 6.02 and 11.01, with the inflection point being at the average of the two pKa values.
  • #1
apchemstudent
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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...
 
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  • #2
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
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.
 

1. What is an inflection point in titration?

An inflection point in titration is the point at which the pH of a solution changes rapidly due to a change in the concentration of the acid or base being titrated. This is often seen as a point on a titration curve where the curve changes from a gentle slope to a steeper slope, indicating a rapid change in pH.

2. How do you determine the inflection point in titration?

The inflection point in titration can be determined by plotting the volume of titrant added against the pH of the solution. The inflection point will be the point at which the curve changes from a gentle slope to a steeper slope. Alternatively, the inflection point can also be determined by calculating the second derivative of the titration curve and finding the point where it crosses the x-axis.

3. Why is it important to find the inflection point in titration?

Finding the inflection point in titration is important because it allows us to accurately determine the equivalence point, which is the point at which the moles of acid and base are in equal amounts. This information is crucial in determining the concentration of an unknown solution or the pKa of an acid or base.

4. What factors can affect the location of the inflection point in titration?

The location of the inflection point in titration can be affected by several factors, including the strength and concentration of the acid and base being titrated, the type of indicator used, and the presence of any interfering substances in the solution.

5. How can you improve the accuracy of determining the inflection point in titration?

To improve the accuracy of determining the inflection point in titration, it is important to use a suitable indicator that changes color at the desired pH range. It is also important to perform multiple titrations and take the average of the results. Additionally, making sure to carefully add the titrant drop by drop near the expected inflection point can also improve the accuracy of the results.

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