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I read this passage:

“It is important to emphasize that, while a strong acid/strong base titration will have its equivalence point had a pH of seven, the equivalence point does not always occur at pH 7. when titrating polyprotic acids or bases, there are multiple equivalence points, as each acidic or basic conjugate species is titrated separately.”

I want to make sure I understand what is being said. Only strong acid-strong base titrations have an equivalence point at a pH of 7, if a weak acid and/or weak base is involved (either as a titrant or titrand), that is not the case. Also for polyprotic compounds (as titrands?), there are multiple equivalence points, therefore the titration of a strong polyprotic acid or base won’t ONLY have an equivalence point at pH 7 (although it is possible to have one of the multiple equivalence points be at 7 but that would be a property of the compound, ie one of the pKas would be equal to 7).

Ibby
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  • Phosphoric acid is a weak acid and none of the three equivalence points are at pH 7. Carbonic acid, i.e., carbon dioxide dissolved in water, is another weak acid. Its two equivalence point pH values are shown here. Neither is at pH 7. – Ed V Jun 23 '21 at 22:06
  • Thank you for correcting me on H3PO4. However, I was giving it as an example of a strong polyprotic acid. When I said one of the pKas could be 7, I was talking about polyprotic acids in general, as in it is possible for a polyprotic acid to have a pKa value of 7 (among other pKa values) – Ibby Jun 23 '21 at 22:18
  • To clarify I’m talking theoretically. If a polyprotic acid has one pKa value equal 7, that’s the only way for its titration to have an equivalence point at pH = 7. Is that correct? – Ibby Jun 23 '21 at 22:24
  • You need to look at this and also follow the link in the comment provided there by @Mithoron. Sulfuric acid is a bit different than you might have hoped. – Ed V Jun 23 '21 at 22:27
  • I feel like the inclusion of an example is what’s making my explanation bad, so I’ll just remove it. I’m not trying to get into specifics, I’m just trying to understand equivalence points in general. I’m asking this: are the only possible ways for the equivalence point to be at pH = 7 with the titration of a strong acid with a strong base and theoretically, if one of the pKas of a polyprotic acid or base is equal to 7? – Ibby Jun 23 '21 at 22:37
  • To directly answer the question you state in your second comment, the answer is no. Titrating HCl (aq) versus NaOH (aq) yields saltwater at the equivalence point, so the pH is 7. For sulfuric acid versus NaOH, the first equivalence point yields a solution of sodium hydrogen sulfate, which is acidic. The second equivalence point yields a solution of sodium sulfate, which is basic. No pH 7. – Ed V Jun 23 '21 at 22:41
  • Let us continue this discussion in chat. – Ed V Jun 23 '21 at 22:42

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