Buffer solutions have constant acidity and alkalinity because
Buffer solutions have constant acidity and alkalinity because
these give unionised acid or base on reaction with added acid or alkali
acids and alkalies in these solutions are shielded from attack by other ions
they have large excess of $\mathrm{H}^{+}$or $\mathrm{OH}^{-}$ions
they have fixed value of $\mathrm{pH}$
Solution
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.