What describes the function of a buffer solution?

• A. It resists changes in pH upon addition of acid or base
• B. It changes pH rapidly
• C. It is always acidic
• D. It is always basic

Answer: (A)

A buffer’s job is to keep pH from changing when small amounts of acid or base are added. It does this by having a weak acid and its conjugate base (or a weak base and its conjugate acid) in the solution. If acid is added, the conjugate base neutralizes the extra H+, forming the weak acid and water, which prevents a large drop in pH. If base is added, the weak acid donates H+ to neutralize OH−, forming water and the conjugate base, which prevents a large rise in pH. This tuning keeps the pH near the buffer’s effective range, often around its pKa, and is why a buffer is described as resisting pH changes upon addition of acid or base. Real-world examples include acetic acid with acetate and carbonic acid with bicarbonate in the blood. A buffer isn’t bound to be always acidic or always basic; its pH depends on the chosen pair, and its ability to resist changes has limits when large amounts are added.