3.5 : Titration of a Weak Acid with a Weak Base

Weak acids and bases do not undergo dissociation completely, and titrations between these two are rarely studied. When such studies are performed, say, for the titration of a weak acid with a weak base, the titration curve plots the change in pH as a function of the volume of base added. Take the titration of acetic acid with ammonia, for instance. During the titration, these two species form ammonium acetate and water, but the pH change is slow and gradual.

As a result, there is no simple indicator that changes color sharply to help with precisely pinpointing the equivalence point. A relatively simple way to approach this is to mix two indicators – in this case, neutral red and methylene blue – such that the mixture exhibits a sharp color change at the equivalence point – violet-blue to green here.

For a weak acid and weak base that possess identical K_a and K_b values, the pH calculated at the equivalence point is 7. If K_a is greater than K_b, the pH is less than 7, making the endpoint solution acidic. However, if K_a is less than K_b, the calculated pH is greater than 7, resulting in a basic solution at the endpoint.