pH = pKa + log([A⁻] / [HA])
The Henderson-Hasselbalch equation pH = pKa + log([A⁻]/[HA]) relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to weak acid concentrations. When [A⁻] = [HA], the ratio is 1, log(1) = 0, and pH = pKa — this is the optimal buffering point. A buffer works effectively within ±1 pH unit of its pKa. Buffer capacity depends on the total concentration of acid + conjugate base. Common biological buffers: phosphate buffer (pKa 7.20) for neutral pH, Tris (pKa 8.06) for slightly basic conditions, and acetate buffer (pKa 4.76) for acidic conditions. Blood pH (7.35-7.45) is maintained by the bicarbonate buffer system: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pKa₁ = 6.35).