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 Ka and Kb values, the pH calculated at the equivalence point is 7. If Ka is greater than Kb, the pH is less than 7, making the endpoint solution acidic. However, if Ka is less than Kb, the calculated pH is greater than 7, resulting in a basic solution at the endpoint.
