U8.8 · Properties of BuffersPractice · 缓冲溶液的性质

The added H⁺ is taken up by the base form, CO₃²⁻, which becomes HCO₃⁻, so [H⁺] barely rises. The second equation is what happens when base is added, the solution has little OH⁻ to neutralize H⁺ directly, and Na⁺ and Cl⁻ are spectators (NaCl is soluble and doesn’t precipitate).

Initially pH = 3.19 + log(0.150/0.100) = 3.37. The added H⁺ reacts with F⁻: F⁻ becomes 0.130 mol and HF becomes 0.120 mol. pH = 3.19 + log(0.130/0.120) = 3.22, a drop of only 0.14.

The buffer holds plenty of F⁻ (the base form), so H⁺ + F⁻ → HF goes essentially to completion. HF is a weak acid that barely ionizes, so [H⁺] hardly changes: the pH is set by the ratio [F⁻]/[HF], which shifts only a little. HF is an acid and can’t neutralize acid, both solutions have the same volume, and Na⁺ and Cl⁻ are spectators.

The base form, HCO₃⁻, takes up the H⁺ from lactic acid to form H₂CO₃, which breaks down to CO₂ and H₂O, and the lungs breathe out the CO₂ (which is why you breathe harder). So blood pH drops only slightly. H₂CO₃, the acid form, handles bases entering the blood.

OH⁻ reacts with HF: HF + OH⁻ → F⁻ + H₂O. HF falls to 0.050 mol and F⁻ rises to 0.200 mol. pH = 3.19 + log(0.200/0.050) = 3.19 + 0.60 = 3.79. The base has used up half the HF, so the pH is starting to move more.