What is a buffer and how does it maintain pH when small amounts of acid or base are added?

• A. A weak acid and its conjugate base (or a weak base and conjugate acid) forms a buffer.
• B. A strong acid and a strong base form a buffer.
• C. A solution with pH 7 that cannot be changed by acids.
• D. A solution that resists pH change by changing temperature.

Answer: (A)

A buffer keeps the pH from changing much because it contains a weak acid and its conjugate base (or a weak base and its conjugate acid). When a little strong acid is added, the conjugate base part reacts with the added H+ to form the weak acid, removing protons from the solution and shifting the equilibrium toward more HA. This neutralizes the added acid and keeps the pH near the acid’s pKa. When a little base is added, the weak acid donates a proton to the base, forming the conjugate base and water, which reduces the impact of the added OH−. The pH of a buffer follows pH = pKa + log([A−]/[HA]); when the concentrations of the two forms are similar, the pH stays close to pKa, which is where buffering is most effective. Buffer capacity depends on how much buffering species you have—the more acid-base pair present, the more acid or base the buffer can neutralize before the pH shifts significantly.