Basic analytical chemistry: precipitation volumetry

Abstract

Precipitation volumetry is an analytical method based on the titration of a species in aqueous solution which, during the process, results in the formation of a poorly soluble compound. The precipitation titrations based on the use of silver nitrate as a precipitating agent are called argentometric titrations which, among others, are used in the determination of halides in solution. In addition to direct potentiometry, the end point of these titrations can also be established by other methods: the Mohr Method, a direct titration that uses a potassium chromate solution as an indicator, the Volhard Method, whose most important application is the indirect determination of chloride ions in an acid medium using ferric ions in an acid medium as an indicator, and the Fajans Method, which employs an adsorption indicator, an organic compound that adsorbs or desorbs from the surface of the solid formed during the titration, with a change in color. Like all others, adsorption indicators must also show a noticeable variation in their organoleptic properties around the titration equivalence point. Also in these cases, it is interesting to construct a theoretical titration curve so that the most suitable conditions for observation of the end point can be established, with an acceptable titration error. Despite the existence of other argentometric analysis methods involving precipitation, the main emphasis in this article is on the three methods mentioned above, where halide ions are titrated with Ag+ ions, or vice versa.