I'm doing titration experiment, where I record the volume of base added and the pH value, then use these two variable to make a titration graph. I'm suppose to extrapolate any data needed from the chart, and find the acid dissociation constant at 0% to equivalence point, 50% to equivalence point, and 75% to equivalence point. The ka value I determined respectively are 0.000165, 0.000457 and 0.000239. The first, and possibly the last value (by a stretch) are the only acceptable values. So my question is, does ka stay constant or can it fluctuate during titration, before settling down to the literature ka value?

I'm leaning towards it stays constant regardless and that the discrepancy is due to errors I made, but then I also think that during titration, not all of the molecules neutralizing each other are going to meet and react instantly, and that could also affect something somehow.

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    $\begingroup$ As the name implies, it is constant. When the only thing happening is proton exchange, the equilibrium is reached pretty rapidly. $\endgroup$ – orthocresol Nov 2 '15 at 20:00
  • $\begingroup$ There is a pretty good explanation of titration and how you can use it to determine Ka on the "Titration curve" Wikipedia page. The pKa can be determined as the pH value measured at the "50% to equivalence" point. To be accurate, you might want to repeat the experiment a few times. I am not seeing how you would use the 0% and 75% values. $\endgroup$ – iad22agp Nov 3 '15 at 17:42

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