A typical chemical buffer system. (a) A buffer prepared with 0.1 mol of acetic acid and 0.1 mol of sodium acetate dissolved in 1 L of water has equal concentrations of the weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻). (b) When a strong acid such as hydrochloric acid (left) or a strong base such as sodium hydroxide (right) is added to the solution, an excess of H₃O⁺ or OH⁻ ions is introduced. The solution acts as a buffer by neutralizing the incoming H₃O⁺ or OH⁻ ions, keeping pH constant.