5.14: Precipitation Titration: Endpoint Detection Methods
In argentometric precipitation titrations, endpoints can be detected visually by the Mohr, Volhard, and Fajans methods. In the Mohr method, adding a soluble chromate indicator gives an initial yellow color to the analyte solution. As the titrant is added, the first excess of silver ions forms a red silver chromate precipitate, marking the endpoint. The solution pH should be maintained at about 8 by adding solid CaCO3.
In the Volhard method, a standard excess of AgNO3 is first added to the chloride ions to precipitate AgCl, which is removed by filtration. Now, the filtrate containing excess Ag+ is back-titrated against thiocyanate in the presence of a ferric ion indicator. A soluble red complex is formed, signaling the endpoint of the titration. The titration must be performed in an acidic medium.
Fajans method employs an anionic adsorption indicator that exhibits different colors when it is in solution and when it is adsorbed. Before the equivalence point, when the chloride ions are in excess, they form the primary adsorbed layer on the AgCl precipitate. The negatively charged precipitate surface repels the anionic indicator, which remains in solution. However, beyond the equivalence point, silver ions are in excess, forming the primary adsorbed layer on the precipitate. The positively charged precipitate surface now attracts the anionic indicator, which is adsorbed. This results in a color change, indicating the endpoint. In this method, the pH of the solution is adjusted based on the indicator used.
