How Do Buffers Neutralize Acids?

Buffers are solutions that resist change in pH. Add an acid to pure water and the pH changes dramatically. Add an acid to a buffered solution, on the other hand, and the change in pH is modest.

Features

A buffer is essentially a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. The pH of a weak acid buffer is given by the Henderson-Hasselbach equation, pH = pKa + log (concentration of conjugate base / concentration of weak acid). pKa stands for the negative log of the dissociation constant of an acid; the pKa is a measure of the strength of an acid, and the lower the pKa the stronger the acid.

Function

When a strong acid is added to a buffer, the conjugate base neutralizes it. Acids donate hydrogen ions while bases accept them, so the reaction between a base and an acid neutralizes both. Adding an acid to a buffered solution increases the amount of acid and decreases the amount of conjugate base.

Significance

Ordinarily an acid added to pure water would donate hydrogen ions to the water molecules; since the pH is the negative log of the hydrogen ion concentration, the pH would rapidly decrease. By neutralizing the acid or acting as a sink for the hydrogen ions released by the acid, the mixture of weak acid and conjugate base ensures the hydrogen concentration changes little if at all. All buffers, however, have a maximum capacity. If too much acid is added, it will neutralize most or all of the base in the buffer solution and the buffer solution will no longer resist change in pH.

References

• "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.