If you double both the concentration of a buffer's components (HA and A-) while keeping their ratio constant, what happens to the buffer pH and buffer capacity?

• A. pH remains the same; buffer capacity increases.
• B. pH increases; buffer capacity increases.
• C. pH decreases; buffer capacity decreases.
• D. pH remains the same; buffer capacity decreases.

Answer: (A)

The pH of a buffer is set by the ratio of its conjugate base to its weak acid, described by pH = pKa + log([A-]/[HA]). If you double both components but keep their ratio the same, the ratio doesn't change, so the pH stays the same. Buffer capacity, the ability to resist changes in pH, depends on how much buffering species are present—the total amount of HA plus A-. Doubling both components increases this total, so the buffer can neutralize more added acid or base, meaning the capacity increases. So the pH remains constant while the buffer capacity increases.