: 200
Buffers resist pH change when small amounts of acid or base are added. An acidic buffer contains a weak acid (HA) and its conjugate base (A- from a salt). The Henderson-Hasselbalch equation gives pH = pKa + log([A-]/[HA]). A basic buffer uses a weak base and its conjugate acid. Buffer capacity is maximum when [HA] = [A-] (pH = pKa) and increases with total concentration. Effective range: pKa +/- 1. Adding strong acid converts A- to HA; adding strong base converts HA to A-. After adding acid/base, recalculate moles of HA and A-, then apply Henderson-Hasselbalch. Dilution barely affects pH because the ratio [A-]/[HA] stays constant. For choosing a buffer, select an acid whose pKa is closest to the target pH. Blood uses the H2CO3/HCO3- system (pKa = 6.37) to maintain pH approximately 7.4 — the ratio [HCO3-]/[H2CO3] is approximately 20:1 to compensate for the pKa-pH gap.