Acids and Bases: Buffer solutions I

A buffer solution has a large reservoir of weak acid HA and its conjugate base A^–.

Buffer solutions resist changes in pH when a small amount of H⁺ is added because most of the added H⁺ reacts away with A^– forming HA.

A^– + H⁺ –> HA

Similarly, buffer solutions resist changes in pH when a small amount of OH^– is added because most of the added OH^– reacts away with HA forming H₂O and A^–.

HA + OH^– –> H₂O + A^–

Looking at this a bit more closely, we can start from the K_A expression and make [H⁺] the subject, remembering that in this situation [H⁺] does not equal [A^–].

This shows that the [H⁺] is directly proportional the HA/A^– ratio.

There is such a large reservoir of HA and A- in the buffer, that the HA/A^– ratio changes very little when a small amount of H⁺ or OH^– are added so [H⁺] changes very little, so pH changes very little too.

Also – an important but underappreciated result – if distilled water was added, both [HA] and [A^–] would decrease by exactly the same factor. This means that the [H⁺] would be unchanged and so would the pH.