The scenes below depict solutions of the same HA/A ^- buffer...

The scenes below depict solutions of the same HA/A $^{-}$ buffer (HA is red and blue and A $^{-}$ is red; other ions and water molecules have been omitted for clarity). (a) Which solution has the greatest buffer capacity? (b) Explain how the pH ranges of the buffers compare. (c) Which solution can react with the largest amount of added strong acid?

Key Concepts

Buffer Capacity

Buffer capacity is the measure of a buffer solution's ability to resist pH changes upon the addition of an acid or base. It depends on the absolute concentrations of the conjugate acid and conjugate base; the closer these concentrations are to each other and the higher their values, the greater the buffer capacity. This means that a buffer is most effective at neutralizing added strong acid or base when both components are present in comparable, ample amounts.

Buffer pH Range

The effective pH range of a buffer is generally considered to be within about one pH unit above or below the pKa of the weak acid involved. This is because the buffering action is most efficient when the concentrations of the acid and its conjugate base are similar, as described by the Henderson-Hasselbalch equation. Outside this range, one component dominates, and the solution becomes less effective at resisting pH changes.

Reaction with Added Strong Acid

When a strong acid is added to a buffer solution, the conjugate base present in the buffer reacts with the excess hydrogen ions to form more of the weak acid, thereby limiting the change in pH. The capacity to neutralize a strong acid depends on the amount of conjugate base available in the buffer. Thus, a buffer with a higher concentration of the conjugate base can react with a larger amount of added strong acid before its pH is significantly affected.

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