argentometric

Argentometric titration, or argentometry, is a class of volumetric analysis in which silver ions (Ag+) are used as the titrant to determine the quantity of analytes that form insoluble silver salts, most commonly halide ions such as chloride, bromide, and iodide.

The technique relies on the precipitation of silver halides and on endpoint detection that indicates either the presence of remaining free Ag+ or the formation of a distinct silver salt. In direct argentometry (the Mohr method), silver nitrate is added to a solution containing halide ions in slightly acidic medium. As long as halide ions are present, Ag+ precipitates as AgCl, AgBr, or AgI. The endpoint is reached when all halide has been precipitated and a small excess of Ag+ remains, detected by an indicator such as chromate, which forms a brick-red Ag2CrO4 at the endpoint.

Back-titration methods, notably the Volhard method, use an excess of AgNO3 to ensure complete precipitation of halides. After the reaction, the remaining Ag+ is titrated back with a thiocyanate solution (SCN−) using an appropriate indicator (often involving Fe3+/Fe(SCN)2+ chemistry). This approach is useful in samples with color, turbidity, or interfering substances that complicate direct detection.

Other argentometric approaches employ adsorption indicators (the Fajans method), in which a color change at the particle surface signals the endpoint. These variations broaden the applicability of argentometry to different sample types and sensitivities.

Applications and limitations: Argentometry is commonly used to determine halide content in water, food, and pharmaceutical preparations. Interferences arise from complexing or non-volatile species that bind Ag+, requiring careful pH control, sample preparation, and choice of indicator. The Mohr and Volhard methods remain foundational in inorganic analysis for halides and related species.