3.5:
Titration of a Weak Acid with a Weak Base
The titration of a weak acid like acetic acid with a weak base like aqueous ammonia produces a neutral ammonium acetate salt and water.
The titration curve of the weak acid versus weak base is plotted by measuring the pH during the various points of the titration.
In the titration of 100 mL of 0.1 M acetic acid solution with 100 ml of 0.1 M aqueous ammonia, the initial pH is 2.87.
If the weak acid and base possess identical Ka and Kb values, the pH at the equivalence point is 7, suggesting a neutral solution. If Ka is greater than Kb, the pH is less than 7 at the equivalence point. Alternatively, if Ka is less than Kb, the pH is greater than 7.
The pH change during the entire titration curve is gradual, with no sharp end point observed. However, a mix of neutral red and methylene blue indicators may be used to approximate the endpoint.
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.