Properties of buffer solution

The pH of a buffer solution does not change appreciably

i. by addition of small amount of either strong acid or strong base,

ii. on dilution, or

iii. when it is kept for long time.

The first property follows from buffer action (section 3.8.2): added $\mathrm{H^\oplus}$ ions are consumed by the large reserve of the conjugate base, while added $\mathrm{OH^\ominus}$ ions are neutralized by the large reserve of the weak acid — so neither addition changes $[\mathrm{H^\oplus}]$ appreciably. Dilution has no effect because the pH depends on the concentration ratio term $\log_{10}\frac{[\text{salt}]}{[\text{acid}]}$ in the Henderson–Hasselbalch equation (Eq. 3.23/3.25), and diluting the solution changes both concentrations by the same factor, leaving t …