8.5: Conductometric Titrations: Strong Acid-Weak Base and Weak Acid-Strong Base Titrations

When a weak acid such as acetic acid is titrated against a strong base like sodium hydroxide, the initial conductance is relatively low due to the weak dissociation of acetic acid. However, as sodium hydroxide is added to the solution, it reacts with the acetic acid to produce highly ionized sodium acetate, which causes an increase in conductance. Once all the acetic acid has been neutralized, any additional sodium hydroxide introduces fast-moving hydroxyl ions, leading to a sharper increase in conductance. The plot of conductance against the volume of the base shows two intersecting lines indicating the end point of the titration.

On the contrary, the conductance initially decreases when a strong acid such as hydrochloric acid is titrated against a weak base like ammonium hydroxide. This decrease is due to the replacement of fast-moving hydrogen ions of the hydrochloric acid by the slower-moving ammonium ions of the weak base. This trend continues until the point of neutralization, after which there is no significant change in conductance due to the weak dissociation of the base. Similar to the previous case, a plot of conductance against the volume of base added will yield two intersecting lines, signifying the end point of the titration.