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

Jun 1, 2017

Because $\left[H {O}^{-}\right]$ 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 $\text{acetate ion}$..................likewise addition of ${H}_{3} {O}^{+}$ would give rise to formation of $\text{acetic acid}$.

Given the equation, and the logarithmic term, during the titration, $p H$ should remain tolerably close to the $p {K}_{a}$ of $\text{acetic acid}$. At half-equivalence $p H = p {K}_{a}$, because ${\log}_{10} \left\{\frac{\left[A c {O}^{-}\right]}{\left[H O A c\right]}\right\} = {\log}_{10} 1 = 0$, i.e. [HOAc]=[""^(-)OAc]. And such titrations are one way to measure $p {K}_{a}$ for a weak acid, if you take $p H$ measurements during the titration.

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