Which statement about buffering and the Henderson-Hasselbalch concept is true?

• A. Buffers are most effective when the pH is far from the buffer’s pKa.
• B. A buffer’s capacity increases as the pH moves away from the pKa.
• C. A buffer neutralizes added acid by converting conjugate base to the acid form and added base by converting acid to conjugate base.
• D. Buffers have no effect on pH in cellular compartments.

Answer: (C)

Buffering relies on a conjugate acid–base pair and the Henderson-Hasselbalch relationship, which ties pH to the ratio of base form to acid form. The true statement captures how a buffer resists pH changes: it neutralizes added acid by converting conjugate base to the acid form and neutralizes added base by converting acid to the conjugate base. In practice, when acid is added, the base form (A−) grabs a proton to become the acid form (HA); when base is added, the acid form (HA) donates a proton to become the conjugate base (A−). This is why buffers are most effective near the pKa, where small shifts in the HA/A− ratio produce small pH changes. As you move away from pKa, buffer capacity drops, and buffers no longer hold pH as well. And buffers do affect pH in cellular compartments, so they are essential for maintaining internal pH.