In a buffer solution of a weak acid and its conjugate base titrated with a strong acid, how does pH change at the start?

• A. It decreases gradually
• B. It increases rapidly
• C. It remains constant
• D. It oscillates

Answer: (A)

A buffer resists large pH changes when a small amount of strong acid is added, because the conjugate base present in the solution reacts with the incoming H+. This neutralization converts base into the weak acid, so the pH drops, but only a little at first. Conceptually, pH = pKa + log([A-]/[HA]); adding acid lowers the ratio [A-]/[HA], causing a slight decrease in pH. Since both HA and A- are still present, the system dampens the change, so the pH decreases gradually rather than abruptly. It wouldn’t stay constant, rise, or oscillate.