Question

What is meant by buffer solutions?

Answer 1

1. The solutions which resist the change in pH on dilution or with the addition of small amounts of acid or alkali are called buffer solutions.
2. It can be made are made in the following ways -
   I) Acidic buffer - Solution of a weak acid and its salt with a strong base.
   Example - Solution of acetic acid and its salt with a strong base i.e., Sodium acetate $\left({\mathrm{CH}}_3\mathrm{COONa}\right)$.
   II) Basic buffer - Solution of a weak base and its salt with a strong acid.
3. For example, a mixture of ${\mathrm{NH}}_4\mathrm{Cl}$ and ${\mathrm{NH}}_4\mathrm{OH}$ acts as a buffer at pH $9.25$.
   ${\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{l}\right)\rightleftharpoons {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$ (weakly ionized)
   ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)\to {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$ (highly ionized)
   a) When we add a base ($\mathrm{NaOH}$), ${\mathrm{OH}}^{-}$ ions react with ${\mathrm{NH}}_4^{+}$ ions to form feebly ionized ${\mathrm{NH}}_4\mathrm{OH}$.
   So, pH remains constant as ${\mathrm{OH}}^{-}$ ions concentration remains constant.
   ${\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{l}\right)$
   b) If we add a few drops of acid ($\mathrm{HCl}$),${\mathrm{H}}^{+}$ ions from acid combine with ${\mathrm{NH}}_4\mathrm{OH}$
   ${\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{l}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
4. Thus, the addition of acid doesn't increase the pH as ${\mathrm{H}}^{+}$ ions concentration remains constant.