If you want to increase buffer capacity, which action is most effective?

• A. Increasing the pH far from pKa.
• B. Increasing one component while keeping the other fixed.
• C. Increasing both HA and A− concentrations.
• D. Using a buffer with a smaller pKa.

Answer: (C)

Buffer capacity comes from how much buffering species you have—the weak acid and its conjugate base. The more of both HA and A− you have, the larger the reservoir that can neutralize added acid or base, so the buffer can resist pH changes over a wider amount of added strong acid or base. Increasing both HA and A− together boosts this total reservoir while keeping the ratio (and thus the pH) under control, so you get the greatest improvement in capacity. If you only raise one component, you either shift the pH by changing the ratio or fail to enlarge the overall buffering pool as much, so the capacity isn’t as greatly increased. Using a buffer with a smaller pKa changes the pH position but doesn’t by itself increase how much acid or base the buffer can absorb. Increasing pH far from the pKa makes the buffer less effective, since one form dominates and the system can’t buffer additions as well.