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Buffer solutions have constant acidity and alkalinity because
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these give unionised acid or base on reaction with added acid or alkali
If small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, its $\mathrm{pH}$ remains unaffected or in other words its acidity/alkalinity remains constant. e.g.,
$$
\begin{aligned}
& \mathrm{H}_3 \mathrm{O}^{+}+A^{-} \rightleftharpoons \mathrm{H}_2 \mathrm{O}+\mathrm{H} A \\
& { }^{-} \mathrm{OH}+\mathrm{H} A \longrightarrow \mathrm{H}_2 \mathrm{O}+A^{-}
\end{aligned}
$$
If acid is added, it reacts with $A^{-}$to form undissociated HA. Similarly, if base/alkali is added, $\mathrm{OH}^{-}$combines with $\mathrm{H} A$ to give $\mathrm{H}_2 \mathrm{O}$ and $A^{-}$and thus, maintains the acidity/ alkalinity of buffer solution.
$$
\begin{aligned}
& \mathrm{H}_3 \mathrm{O}^{+}+A^{-} \rightleftharpoons \mathrm{H}_2 \mathrm{O}+\mathrm{H} A \\
& { }^{-} \mathrm{OH}+\mathrm{H} A \longrightarrow \mathrm{H}_2 \mathrm{O}+A^{-}
\end{aligned}
$$
If acid is added, it reacts with $A^{-}$to form undissociated HA. Similarly, if base/alkali is added, $\mathrm{OH}^{-}$combines with $\mathrm{H} A$ to give $\mathrm{H}_2 \mathrm{O}$ and $A^{-}$and thus, maintains the acidity/ alkalinity of buffer solution.
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