Question

Using the same phosphoric buffer but in two different concentrations (80mM and 160mM),what is causing the difference between the two titration curves? Explain in terms of buffering regions and buffer capacity.

Answer 1

The amount of acid a buffer can absorb before it breaks the "buffer capacity for addition of strong acid". A solution with more weak base, [A^-], has a higher buffer capacity for addition of strong acid.

Similarly, a buffer will break when the amount of strong base added is so large it consumes all the weak acid, through the reaction HA + OH^- → A^-+ H₂O. A solution with more weak acid, [HA], has a higher buffer capacity for addition of strong base.

So although the pH of a buffer is determined by only the ratio [A^-]/[HA], the ability of the buffer to absorb strong acid or base is determined by the individual concentrations of [A^-] and [HA].