Buffer Capacity

A buffer resists pH change by converting added strong acid or base into its weak acid or conjugate base partner. Add H+ and the conjugate base consumes it; add OH- and the weak acid consumes it. That only works while both partners still exist in decent supply. Once you add enough strong acid to use up essentially all the conjugate base, there is nothing left to absorb the next drop, and pH falls fast. The buffer did not get weaker, it ran out of ammunition.

The ratio of acid to base sets the pH through the Henderson-Hasselbalch equation, but the absolute moles of each set how much the buffer can take before collapsing. A 0.010 M acetic acid and acetate buffer and a 1.0 M version at the same ratio sit at the same pH, but the 1.0 M buffer has one hundred times more moles of each component in the same volume. Pour in a fixed amount of strong acid, and the dilute buffer's conjugate base runs out almost immediately while the concentrated one barely notices. Same pH, wildly different durability. This is why buffer capacity is reported separately from pH: capacity is about amount, pH is about ratio.

A buffer resists change best in both directions when the ratio of acid to base is 1 to 1, so pH equals pKa. There both components are equally abundant, so there is equal room to absorb acid or base before either runs out. Move the ratio far from 1 to 1 and one partner becomes scarce, making the buffer lopsided and vulnerable in one direction. This sets up titration curves, where the flattest, most buffered region sits right at the half-equivalence point.