In Henderson–Hasselbalch, increasing the ratio [A-]/[HA] has what effect on pH?

• A. Increase pH
• B. Decrease pH
• C. No change
• D. Depends on pKa

Answer: (A)

In a buffer, pH is given by pH = pKa + log([A-]/[HA]). The pKa is fixed for a given acid–base pair, so what changes pH is the ratio of conjugate base to weak acid. Increasing [A-]/[HA] raises the log term, which raises the pH. Intuitively, more conjugate base relative to the acid makes the solution more basic, so the pH goes up. The baseline pH shifts with pKa, but the direction of change—pH increasing as the ratio increases—holds regardless of the exact pKa.