Buffer solutions do not have an unlimited capacity to keep the pH relatively constant (Figure 4.1.34.1.3\PageIndex{3}). If we add so much base to a buffer that the weak acid is exhausted, no more buffering action toward the base is possible.

A solution of acetic acid (CH3COOHCH3COOH\ce{CH3COOH} and sodium acetate CH3COONaCH3COONa\ce{CH3COONa}) is an example of a buffer that consists of a weak acid and its salt.