Fajansmetoden

Fajansmetoden, also known as the Fajans method, is a technique used in analytical chemistry for the determination of the endpoint of a precipitation titration. It was developed by Polish chemist Kazimierz Fajans. This method utilizes adsorption indicators, which are organic dyes that change color when they become adsorbed onto the surface of a precipitate.

The principle behind the Fajans method relies on the interaction between ions in the solution and the surface of the precipitate particles. As the titration progresses, the concentration of the titrant ions increases. When the equivalence point is reached, the excess titrant ions will be adsorbed onto the precipitate. This adsorption causes a change in the charge of the precipitate particles, which in turn affects the adsorption of the indicator dye.

The indicator is typically a weak acid or base dye. Before the equivalence point, the adsorbed ions on the precipitate surface are the counter-ions of the precipitate itself, leading to a specific charge on the particles and a corresponding color of the indicator in the solution. However, at and after the equivalence point, the excess titrant ions become adsorbed. This changes the surface charge of the precipitate, leading to a different interaction with the indicator. The indicator molecules then adsorb onto the precipitate surface, causing a distinct color change that signals the endpoint of the titration.

Commonly used adsorption indicators include eosin, phenolphthalein, and rhodamine 6G. The choice of indicator depends on the specific ions being titrated and the nature of the precipitate formed. Fajans method is particularly useful for titrations involving sparingly soluble salts, such as silver halides. It offers a visual and direct way to detect the endpoint, making it a practical technique in quantitative analysis.