When is buffer capacity the greatest?

When is buffer capacity the greatest?

• A. When [HA] >> [A-].
• B. When [HA] and [A-] are both very small.
• C. When the pH is far from pKa.
• D. When [HA] ≈ [A-] and they are relatively high in concentration.

Answer: (D)

Buffer capacity is greatest when the weak acid and its conjugate base are both present in high, nearly equal amounts. That way, there is plenty of both species to neutralize added acid or added base, so the solution resists pH changes in either direction. When the two forms are about equal, the pH is near the acid’s pKa, thanks to Henderson–Hasselbalch, which is the ideal region for buffering because both species can participate actively in counteracting added species. If one form dominates, buffering becomes limited because there isn’t enough of the other form to react with the added species. Very low concentrations also yield a small buffer capacity, even if the ratio is favorable.

Buffer capacity is greatest when the weak acid and its conjugate base are both present in high, nearly equal amounts. That way, there is plenty of both species to neutralize added acid or added base, so the solution resists pH changes in either direction. When the two forms are about equal, the pH is near the acid’s pKa, thanks to Henderson–Hasselbalch, which is the ideal region for buffering because both species can participate actively in counteracting added species. If one form dominates, buffering becomes limited because there isn’t enough of the other form to react with the added species. Very low concentrations also yield a small buffer capacity, even if the ratio is favorable.