Buffer action

Let us consider sodium acetate - acetic acid buffer. Here sodium acetate is a strong electrolyte which dissociates completely in water producing large concentration of $\mathrm{CH_3COO^{-}}$ as follows :

On the other hand since the acetic acid is a weak acid, the concentration of undissociated $\mathrm{CH_3COOH}$ molecules is usually high. If a strong acid is added to this solution the added $\mathrm{H^{+}}$ ions will be consumed by the conjugate base $\mathrm{CH_3COO^{-}}$ present in large concentration. Similarly, if small amount of base is added, the added $\mathrm{OH^{-}}$ ions will be neutralized by the large concentration of acetic acid as shown in the following reactions :

$\underset{\text{(large concentration)}}{\mathrm{CH_3COOH(aq)}} + \underset{\text{(added base)}}{\mathrm{OH^{-}(aq)}} \rightarrow \mathrm{CH_3COO^{-}(aq)} + \mathrm{H_2O}(l)$ …