An acidic buffer contains a weak acid (HA) and its conjugate base (A-, from a salt like NaA). pH = pKa + log([A-]/[HA]) — the Henderson-Hasselbalch equation. A basic buffer contains a weak base (B) and its conjugate acid (BH+). pOH = pKb + log([BH+]/[B]). Buffer capacity (ability to resist pH change) is maximum when [acid] = [conjugate base], i.e., pH = pKa. At this point, the buffer can neutralise equal amounts of added acid or base. Effective buffer range: pKa +/- 1 (roughly). When choosing a buffer for a target pH, select an acid whose pKa is close to the desired pH. Adding strong acid to a buffer converts A- to HA; adding strong base converts HA to A-. The ratio [A-]/[HA] changes, but the log function dampens the pH change.