Question

Why cant a strong acid be a buffer?

Answer 1

1. A buffer solution can resist a change in its pH with the addition of an acid or a base.
2. Buffer solutions are usually made by mixing weak acids/bases with their salt solutions in an aqueous medium. The resulting buffer solutions will be respectively either acidic or basic.
3. Consider an acidic buffer made by mixing aqueous acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) with aqueous sodium acetate (${\mathrm{CH}}_3\mathrm{COONa}$). Sodium acetate will completely dissociate into Sodium ions (${\mathrm{Na}}^{+}$) and acetate ions (${\mathrm{CH}}_3{\mathrm{COO}}^{-}$) while acetic acid will be very slightly dissociated.

${\mathrm{CH}}_3\mathrm{COONa}\left(\mathrm{s}\right)\to {\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)$

4. On adding acid to this buffer solution, the hydrogen ions (${\mathrm{H}}^{+}$) will be consumed by the acetate ions to form acetic acid. Since acetic acid is very weakly dissociated, it will not affect the pH of the solution.

$$\begin{gathered} {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right) \\ (\mathrm{weakly}\mathrm{dissociated}) \end{gathered}$$

5. On adding base to this solution, the hydroxyl ions (${\mathrm{OH}}^{-}$) added will react with the undissociated Acetic acid to form Acetate ions and water (${\mathrm{H}}_2\mathrm{O}$), neither of which will affect the pH of the solution.

$$\begin{gathered} {\mathrm{OH}}^{-}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)\to {\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O} \\ \left(\mathrm{neutral}\right) \end{gathered}$$

6. Now consider a mixture of a strong acid and its salt, say, for example, aqueous Hydrochloric acid ($\mathrm{HCl}$) mixed with aqueous Sodium chloride ($\mathrm{NaCl}$). Since Hydrochloric acid is a strong acid, it will dissociate completely in the solution.

$\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$

7. If an acid were added to this solution, the hydrogen ions would react with the chloride ions (${\mathrm{Cl}}^{-}$) to form Hydrochloric acid which is completely dissociable. This will reduce the pH of the solution.

${\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\to \mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$

8. Adding a base to this solution would increase the pH because the hydroxyl ions would react with Sodium ions to form Sodium hydroxide ($\mathrm{NaOH}$) which is completely dissociable.

${\mathrm{OH}}^{-}\left(\mathrm{aq}\right)+{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)\to \mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

9. Strong acids cannot be used as buffers because they dissociate completely in solution.