What happens to pH when a buffer solution is overwhelmed by a large addition of acid?

• A. The pH becomes neutral (7) immediately.
• B. The buffer continues to prevent any change in pH.
• C. The pH changes slowly despite the added acid.
• D. The pH changes rapidly because the buffer capacity is exceeded.

Answer: (D)

Buffers resist pH change by neutralizing added hydrogen ions with their conjugate base, keeping the pH fairly constant. This works only up to the buffer capacity—the amount of acid or base the mixture can absorb. When a large amount of acid is added, all the conjugate base can be used up, leaving excess H+ in the solution. With no base left to neutralize them, the hydrogen ions push the pH downward quickly. Once the buffer is overwhelmed, the pH changes rapidly because the buffering action can no longer compensate for the added acid.