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My textbook, Harris 7th Edition Quantitative Chemical Analysis, says:

In titrations involving 1:1 stoichiometry of reactants, the equivalence point is the steepest point of the titration curve. This is true of acid-base, complexometric, and redox titrations as well.

My question is, does this apply to all acid-base titrations (strong-strong and weak-strong)? My intuition seems to say that it only applies to strong-strong, but I may be incorrect.

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Your intuition is not sufficient in this matter. The equivalence point is the steepest point of any titration curve, however weak or strong is the acid and the base.

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    $\begingroup$ Within titration precision, it is true, but mathematically, I would not be so sure even for strons/strong case, as even here is not the curve perfectly symmetric. I remember there was similar question recently. I also remember some potentiometric titration chart, where equivalence and inflection points were explicitly marked as not identical. $\endgroup$ – Poutnik Dec 25 '19 at 18:47
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    $\begingroup$ @Poutnik A discussion of equivalence vs inflection points in titration curves can be found here: chemistry.stackexchange.com/questions/124603/…. $\endgroup$ – kappi Dec 25 '19 at 19:08
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There are two sides to this. We can take a look at the shape of titration curves, but also look into the choice of the end point (where the indicator changes colour).

Let’s first take a look at the titration curve itself. There are several good answers on the internet that explain the shape of a titration curve. See for example The reason behind the steep rise in pH in the acid base titration curve, where several ways of explaining are used. The same reasoning goes for redox titrations (check pages 328-332 of your textbook) or even precipitation titrations (pages 127-131 of the same book). But let’s focus on acid-base titrations for now.

Ok, so we know what titration curves look like and why. But it’s important to know that the end point is chosen to be the steepest part of the titration curve (which happens to be at the equivalence point). That’s because at the steepest point of the curve, the effect of the titration is measured with the greatest precision. Near the equivalence point, a small amount of titrant added will cause a relatively large change in pH. This can be easily observed if a suitable indicator is used. The fact that the equivalence point can thus be measured so precisely makes it especially suitable for analysis.

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    $\begingroup$ Then there can be even 3 points: the equivalence point, the end point, the inflection point. $\endgroup$ – Poutnik Dec 25 '19 at 19:12

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