A mixture containing a weak acid and its conjugate base, such as acetic acid and sodium acetate (CH3COOH(aq) + CH3COONa(aq)), or a mixture containing a weak base and its conjugate acid, such as ammonia and ammonium chloride (NH3(aq) + NH4Cl(aq)), is a buffer solution. A buffer solution resists changes in pH when small amounts of a strong acid or a strong base are added.
A solution of equal concentrations of CH3COOH and CH3COONa is slightly acidic because Ka,acetic acid > Kb, acetate anion. When a strong base, such as NaOH, is added to this solution, the OH– anions react with the H3O+ cations present in the solution in a very product-favored reaction, decreasing concentrations of H3O+:
1) NaOH(aq) + H3O+(aq) ⇌ Na+(aq) + 2H2O(ℓ) K1 = 1.0 × 1014
In response to this decrease in [H3O+] upon the addition of NaOH, the Ka,acetic acid equilibrium reaction:
shifts towards producing more products, thereby restoring [H3O+] to almost the value it was before NaOH was added. Therefore, the net effect of the added NaOH is to convert some of the CH3COOH to CH3COO−. In other words, if you add the two above reaction equations, you would have:
Overall, there is minimal change in the solution’s H3O+ concentration. (Note that the added NaOH maybe reacting directly with H3O+ or CH3COOH, because both species are present in the buffer solution when NaOH is added. Regardless of which acidic species NaOH is reacting with directly, the final result is the same: there is minimal change in the solution’s pH.)
Similarly, when a strong acid, such as HCl, is added, the net effect of the added H3O+ is to convert some of the CH3COO− to CH3COOH:
And again, there is only a minimal change in the solution’s pH.
These concepts are illustrated in the figure below. When a small amount of a strong acid or strong base is added, a buffer solution can moderate changes to pH because it consists of a weak acid that can react with added strong base as well as a weak base that can react with added strong acid.
The weak base and weak acid in a buffer solution are typically a conjugate acid-base pair, both serving to maintain a single dynamic equilibrium that responds to additions of other acids and bases. If they are not a conjugate acid-base pair, then there would be two dynamic equilibria at play, which significantly complicates the buffering actions.