Mechanism of buffer action
A buffer is a solution containing a weak acid and its conjugate base in similar amounts. The way in which a buffer solution acts may be illustrated by considering a buffer solution made up of CH3COOH and CH3COONa. A buffer solution may be prepared by mixing comparable molar amounts of ethanoic acid and sodium ethanoate in water. In solution we have the following situation:
CH3COOH (aq) ↔ CH3COO– (aq) + H+ (aq)
CH3COONa (aq) → CH3COO– (aq) + Na+ (aq)
The salt exists completely as ions.
Thus the buffer contains both acid (CH3COOH) and its conjugate base (CH3COO–). When a small quantity of an acid is added the hydrogen ions are removed by the conjugate base (CH3COO–) as follows:
H+ (aq) + CH3COO– (aq) ↔ CH3COOH (aq)
Since ethanoic acid is only slightly dissociated and in the form CH3COOH, it does not contribute any H+ ion, the pH of the resulting solution will remain practically constant. If, however, a strong base is added, the added OH– ion is neutralized by the reaction with the acid in the buffer,
CH3COOH (aq) + OH– (aq) → CH3COO– (aq) + H2O (l)
One may also consider that the added OH- ion reacts with the H+ ion to produce water. The added OH- ions are removed and the acid equilibrium shifts to the right to replace the H+ ions used up.
In either case, the effect of the added acid or base is effectively balanced so that the pH of the solution remains approximately constant. Similarly, the mechanism of basic buffers can be explained.