5.12: Precipitation Titration: Overview
Precipitation titration involves the reaction of a titrant and an analyte to generate an insoluble precipitate. While precipitation titration uses various precipitating agents, silver nitrate is the most common precipitating reagent; titrations involving Ag+ are called argentometric titrations. Usually, the endpoint in a precipitation titration can be detected by visual indicators.
A precipitation titration curve demonstrates the change in concentration of the titrant or analyte upon adding the titrant. Titration curves are plotted with the volume of the analyte on the x-axis and the p function of the analyte or titrant concentration on the y-axis. A titration curve has three significant regions: before, at, and after the equivalence point. Before the equivalence point, the analyte concentration is in excess. The concentration of the unreacted analyte is calculated from a ratio of the moles of excess analyte in the solution to the total volume of the solution.
At the equivalence point, a stoichiometric amount of titrant has reacted with the analyte to form the precipitate (e.g., AgCl). However, some redissolution of the precipitate gives equal concentrations of Ag+ (titrant) and Cl− (analyte). The analyte concentration can be estimated from the square root of the solubility product.
Beyond the equivalence point, the solution contains excess Ag+. Here, the analyte concentration can be estimated using the solubility expression, where the concentration of Ag+ is obtained from the ratio of moles of excess titrant to the total volume.
