Figure 2.13 Buffer action. The addition of a strong acid—say, 1 M HCl—to pure water results in an immediate drop in pH to near 2, as the blue line shows. In contrast, the addition of acid to 0.1 mM sodium acetate (Na⁺CH₃COO⁻) results in a much more gradual change in pH until the ph drops below 3.5, as shown by the red line.