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I know that buffers have a capacity after which they are not able to maintain a relatively stable pH. This capacity is exceeded when we add too much acid or base but I was wondering why does this happen?

Suppose we have the buffer CH3COOH + CH3COONa If I were to keep adding HCl, the solution would actually lose its pH maintaining ability. Is it because when we add HCl, its H+ ions will start to get consumed by CH3COO- produced by the dissociation of CH3COONa and after some time, eventually, no CH3COO- ions to maintain the pH?

If this is the case then it means that the buffer solution actually has two capacities?

  1. The capacity to maintain pH after acid is added
  2. Capacity to maintain pH after base is added

Is this true?

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    $\begingroup$ Review the search results for site:stackexchange.com OR site:libretexts.org OR site:hyperphysics.phy-astr.gsu.edu OR site:wikipedia.org pH buffer capacity $\endgroup$
    – Poutnik
    Commented Nov 8, 2023 at 12:29
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    $\begingroup$ Well, yeah, pretty much that. $\endgroup$
    – Mithoron
    Commented Nov 8, 2023 at 13:38
  • $\begingroup$ It is right that a buffer solution has the capacity of maintaining pH after a little acid or some base is added. But this capacity has a limit. The amount of acid or base added must be smaller than (about half) the amount of acid and base present in the buffer solution. $\endgroup$
    – Maurice
    Commented Nov 8, 2023 at 15:49
  • $\begingroup$ The Henderson-Hasselbalch equation provides numerical insight. The pH of the solution is roughly close to the pKa since the adjustment factor is modulated by the log of the ratio between the acid and conjugate base. However, once this ratio becomes very large or very small, even the logarithm cannot keep it small enough. $\endgroup$
    – Zhe
    Commented Nov 8, 2023 at 16:36

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