Titrage des précipitations : méthodes de détection des points finaux

The Mohr method uses a soluble chromate indicator to detect the endpoint when titrating chloride with silver nitrate. Initially, the solution appears yellow from the chromate indicator. Once all chloride ions precipitate as AgCl, the first excess of silver ions combines with chromate ions to form a red precipitate, marking the endpoint. The solution pH should be maintained around 8 by adding solid CaCO3.

The Volhard method uses back-titration, adding a known excess of AgNO3 to precipitate chloride, then filtering and back-titrating the excess silver against thiocyanate with a ferric ion indicator. The Mohr method directly titrates chloride with silver nitrate using a chromate indicator. The Volhard method requires an acidic medium, while the Mohr method requires a pH around 8.

Anionic adsorption indicators exhibit different colors when dissolved versus adsorbed on the precipitate surface. Before the equivalence point, excess chloride ions form the primary adsorbed layer, creating a negatively charged precipitate that repels the anionic indicator, keeping it in solution. Beyond the equivalence point, silver ions dominate, creating a positively charged surface that attracts and adsorbs the indicator, causing a visible color change marking the endpoint.

pH control is critical because different endpoint detection methods require specific pH ranges. The Mohr method requires a pH around 8, maintained by adding solid CaCO3, to prevent chromate indicator interference. The Volhard method requires an acidic medium to ensure proper back-titration conditions. The Fajans method pH is adjusted based on the specific indicator used to optimize adsorption behavior.

In the Volhard method, ferric ions (Fe3+) serve as an indicator during the back-titration phase. When thiocyanate is added to the excess silver ions, it reacts with Ag+ first. Once all silver is consumed, thiocyanate reacts with Fe3+ to form a soluble red complex, signaling the endpoint of the back-titration and allowing calculation of the original chloride concentration.

During Fajans titration, the precipitate surface charge transitions at the equivalence point. Before equivalence, chloride ions in excess adsorb on AgCl, creating a negatively charged surface. At equivalence, the surface becomes neutral. Beyond equivalence, silver ions adsorb, creating a positively charged surface. This charge reversal causes the anionic indicator to shift from solution to the precipitate surface, producing the color change that marks the endpoint.

The Volhard method avoids chromate indicator interference by using a back-titration approach with a ferric ion indicator instead. After precipitating chloride with excess AgNO3 and filtering to remove the AgCl precipitate, the remaining silver in the filtrate is back-titrated against thiocyanate. This separation eliminates the need for chromate and allows the method to work in acidic conditions where chromate would be problematic.