When a base is added to a buffer solution, what does the weak acid in the buffer do?

In a buffer solution, the weak acid plays a crucial role in maintaining pH stability when a base is added. When a base is introduced, its presence increases the pH and can make the solution more alkaline. The weak acid in the buffer responds to this increase in pH by donating protons (H⁺ ions). This process helps neutralize the added base, which would otherwise raise the pH significantly.

For example, if sodium hydroxide (the base) is added to a buffer solution containing acetic acid, the acetic acid donates H⁺ ions, thereby reacting with the hydroxide ions from the sodium hydroxide to form water. This reaction mitigates the impact of the base on the overall pH of the solution, showcasing the buffering capacity.

This mechanism underscores why the weak acid's action of donating protons is crucial in maintaining the buffer's effectiveness, making it an essential feature of buffer systems in resisting pH changes upon the addition of small amounts of acids or bases.