25.1.6—Relevant concentrations and the pH of buffer

For a buffer, use the weak-acid equilibrium to relate [H⁺], Ka, [HA] and [A⁻]. The pH depends mainly on the ratio of conjugate base to weak acid, not their common scale alone.

Account for dilution or neutralisation before substituting. Keep concentrations in the same units and use pH = pKa + log([A⁻]/[HA]) when that form is permitted.

If [A⁻]=[HA], pH≈pKa. Adding a small amount of strong acid consumes A⁻ and forms HA, so the ratio changes only modestly.

A buffer is not strongest when one component is absent, and pH is not determined by total concentration alone.