5.13: Precipitation Titration Curve: Analysis
The precipitation titration curve demonstrates the change in concentration of one reactant with the volume of titrant added. During the titration of chloride ions with silver nitrate, the precipitation titration curve is divided into three regions: before, at, and after the equivalence point. Before the equivalence point, low redissolution of the sparingly soluble silver chloride precipitate gives a low silver ion concentration. However, in the second region, representing the equivalence point, the silver ion concentration sharply increases as chloride ions are completely consumed. In the last region, beyond the equivalence point, the silver ion concentration is high, mainly due to excess titrant.
The shape of the curve is influenced by the reactant concentration and the solubility product of the precipitate, as revealed by comparing the titration curves of the three halides. Because the solubility product of AgI is smaller than those of AgBr and AgCl, the least soluble AgI precipitates first, indicating a larger break at the equivalence point.
So, if a mixture of KI and KCl is titrated against AgNO3, AgI precipitates first. After the iodide ions are entirely consumed, the silver ion concentration increases, and AgCl precipitates. After completely consuming the chloride ions, the silver ion concentration increases again, and two equivalence points are observed.
