Question

Why do solutions that contain buffers tend to resist pH changes?

Answer 1

Because `[H_3O^+]` or `[HO^-]` is moderated by the equilibrium expression.

Explanation:

We can represent the acid base equilibrium of a weak acid as:

`HA +H_2O rightleftharpoons H_3O^+ + A^-`

`pH=pK_a + log_10(([A^-])/[[HA]])`

Because the ratio `([A^-])/[[HA]` is moderated by the logarithmic term, `pH` remains tolerably close to `pK_a`, the negative logarithm of the acid dissociation constant. Where `[A^-]=[HA]`, `pH=pK_a`, because `log_10(1)=0`.