buffer region
Summary
The buffer region is the flat stretch of a weak acid or weak base titration curve where pH changes little because both the weak acid and its conjugate base are present. It is centered on the half-equivalence point, where pH = pKa, and spans roughly pKa ± 1.
Detail
Meaning: On a titration curve of a weak acid with a strong base (or a weak base with a strong acid), the buffer region is the gently sloping stretch before the equivalence point where pH changes only slightly as titrant is added. The weak acid HA and its conjugate base A⁻ are both present in appreciable amounts, so added OH⁻ is neutralized by HA and added H⁺ by A⁻. The region is centered on the half-equivalence point, where [HA] = [A⁻] and pH = pKa. It is usually taken as about pKa ± 1.
Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are dimensionless and [A⁻] and [HA] are the concentrations of conjugate base and weak acid in mol/L. Only their ratio matters.
On the MCAT: You may get a titration curve and be asked to find the flat region, read the pKa at its midpoint, or say which species dominates at a given point. You may also have to choose the buffer whose pKa is closest to a target pH, or calculate pH from a base-to-acid ratio. A polyprotic acid shows one buffer region for each pKa.
Common confusion: The equivalence point is not in the buffer region. It lies on the steep rise, where one buffer component is nearly used up and buffering is weak. A strong acid–strong base titration has no meaningful buffer region, since neither partner is a weak conjugate pair.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- Chemistry: Principles and Reactions
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.