buffer

Meaning: A buffer contains a weak acid, HA, and its conjugate base, A⁻, in comparable amounts (or a weak base and its conjugate acid). Added H⁺ is consumed by A⁻, and added OH⁻ is consumed by HA, so the ratio A⁻/HA shifts modestly and pH changes little. Capacity rises with total concentration, and the buffer works best within about one pH unit of pKa.

Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are unitless and concentrations are in mol/L. For a base buffer, pOH = pKb + log([BH⁺]/[B]).

On the MCAT: Expect a titration curve whose flat buffer region is centered at the half-equivalence point, where [A⁻] = [HA] and pH = pKa. After adding strong acid or base, do the stoichiometry first, then use the new amounts in the equation. Diluting a buffer with water leaves the pH nearly unchanged but lowers capacity. Blood's bicarbonate system is the usual biological example.

Common confusion: A buffer resists pH change; it does not prevent it. A strong acid with its conjugate base (HCl and Cl⁻) is not a buffer, since Cl⁻ is too weakly basic to take up H⁺. A weak acid partly neutralized by strong base does form one.