# Experimental determination of pH [closed]

I am trying to determine the experimental pKa for two weak acids that were titrated against 0.20M NaOH.

I have read elsewhere that you can take the point where the graph becomes steep and divide the value of base added by two the corresponding pH value would then be the pKa, but how do i choose which value since it may not be obvious which point the graph becomes steep.

Below I can see that the pka for acetic acid should be close to the theoretical value calculated of 4.76 and the Tris-HCl pka should be approximately 8.3 but there must be a better way than just guessing from a graph.

My textbook doesn't explain how to experimentally find pH just that it's the point where $$[A^-]/ [HA]$$. I am hoping someone can give me an equation to work with or guide me in the right direction.

Thank you,

$$\frac{d}{d(V_{base})} pH(x_i) \approx \frac{pH(x_{i+1})-pH(x_{i-1})}{x_{i+1}-x_{i-1}}$$ you can find the point of greatest change, or the equivalence point. Obviously, better data in that region will give you more clarity. I used to do one titration quickly so that I would know the approximate region where I would need to be and then slowly titrate the rest.