buffer solution
Summary
A buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid) and resists large pH changes when small amounts of strong acid or base are added. Its pH depends on the pKa and the base-to-acid ratio, and it works best near pH = pKa.
Detail
Meaning: A buffer holds a weak acid (HA) and its conjugate base (A⁻) in comparable amounts, or a weak base and its conjugate acid. Added strong acid is consumed by A⁻ (H⁺ + A⁻ → HA). Added strong base is consumed by HA (OH⁻ + HA → A⁻ + H₂O). The ratio [A⁻]/[HA] shifts only slightly, so the pH does too. A buffer works best within about one pH unit of the pKa.
Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are unitless and the ratio of concentrations (mol/L) is unitless. This is the Henderson–Hasselbalch equation.
On the MCAT: Given a pKa and concentrations, do the strong acid or base stoichiometry first (one component falls, the other rises), then use the equation. On a titration curve of a weak acid with strong base, the flat buffer region surrounds the half-equivalence point, where pH = pKa. You may also be asked to choose the best buffer for a target pH: pick the pKa closest to it.
Common confusion: A buffer limits pH change; it does not prevent it, and it fails once one component is used up. A strong acid with its conjugate base (HCl and Cl⁻) is not a buffer, because Cl⁻ does not take up H⁺ appreciably.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.