acidic buffer

Meaning: An acidic buffer contains a weak acid, HA, and its conjugate base, A⁻, in comparable amounts, for example CH₃COOH and CH₃COO⁻. Added H⁺ is consumed by the base (A⁻ + H⁺ → HA). Added OH⁻ is consumed by the acid (HA + OH⁻ → A⁻ + H₂O). Because only the ratio [A⁻]/[HA] shifts, and only slightly, the pH changes little. It is made by mixing the acid with its salt, or by partly neutralizing the weak acid with strong base.

Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are unitless, [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration, both in mol/L (or in moles, if both are in the same volume).

On the MCAT: Expect a weak acid titrated with strong base. The flat buffer region sits around the half-equivalence point, where [HA] = [A⁻] and pH = pKa. You may calculate the new pH after a given amount of NaOH or HCl, pick the best buffer for a target pH (choose the pKa closest to it), or predict that dilution barely changes the pH because the ratio is unchanged.

Common confusion: A strong acid with its conjugate base (HCl and Cl⁻) does not buffer, because Cl⁻ is far too weak a base to take up H⁺. The buffer needs a weak acid. Capacity is not the same as pH: larger concentrations of HA and A⁻ resist more added acid or base, even at the same pH.