Question

Addition of `"sodium hydroxide"` to an `"acetic acid/sodium acetate buffer"` only marginally increases the `pH` of the solution. Why so?

Answer 1

Because `[HO^-]` is moderated by the capacity of the buffer..........

Explanation:

From here, we learn that for a buffer solution...........

`pH=pK_a+log_10{[[""^(-)OAc]]/[[HOAc]]}`

And thus addition of hydroxide ion results in rapid formation of `"acetate ion"`..................likewise addition of `H_3O^+` would give rise to formation of `"acetic acid"`.

Given the equation, and the logarithmic term, during the titration, `pH` should remain tolerably close to the `pK_a` of `"acetic acid"`. At half-equivalence `pH=pK_a`, because `log_10{[[AcO^-]]/[[HOAc]]}=log_(10)1=0`, i.e. `[HOAc]=[""^(-)OAc]`. And such titrations are one way to measure `pK_a` for a weak acid, if you take `pH` measurements during the titration.

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