pH = pKa + log([A⁻]/[HA])
pH = pKa + log([A⁻]/[HA]) predicts the pH of a buffer solution. When [A⁻] = [HA], pH = pKa exactly. When the ratio [A⁻]/[HA] = 10, pH = pKa + 1; when the ratio = 0.1, pH = pKa - 1. To prepare a buffer at a specific pH: choose a weak acid with pKa within 1 unit of your target pH, then calculate the ratio needed. For an acetate buffer at pH 5.0: pH = 4.76 + log(x), so log(x) = 0.24, x = 1.74. You need 1.74× more sodium acetate than acetic acid. Common buffer systems: acetic acid/acetate (pH 3.8-5.8), phosphate (pH 5.8-8.0), Tris (pH 7.0-9.0), carbonate (pH 9.2-10.8). The HH equation assumes dilute solutions and complete dissociation of the salt.