basic buffer

Meaning: A basic buffer is a weak base plus its conjugate acid, such as NH₃ with NH₄⁺ (from NH₄Cl). Added acid is consumed by the base (NH₃ + H⁺ → NH₄⁺) and added base by the conjugate acid (NH₄⁺ + OH⁻ → NH₃ + H₂O), so pH changes little. Capacity is greatest near a 1:1 ratio, and the buffer works well within about one pOH unit of pKb.

Equation: pOH = pKb + log([BH⁺]/[B]). Here pOH = −log[OH⁻] and pKb = −log Kb, both unitless; [BH⁺] is the conjugate acid and [B] the weak base, in mol/L. Then pH = 14 − pOH at 25 °C.

On the MCAT: Given Kb and the amounts of base and salt, calculate pH, or predict the direction and size of the pH change after adding strong acid or strong base. In a titration of a weak base with strong acid, the half-equivalence point has [B] = [BH⁺], so pOH = pKb.

Common confusion: Students invert the ratio or report pOH as if it were pH. A strong base with its own cation is not a buffer, since the cation is not an acid. A buffer is also not unlimited: it fails once the added acid or base exceeds the amount of base or conjugate acid present.