Fajans method
/ FY-ahnz METH-od /
Picture a sticky surface that grabs whatever floats past it, and a dye that changes colour only at the moment it gets stuck. The Fajans method reads the end point from the surface of the precipitate itself, where a coloured dye latches on the instant the surface charge flips.
Specifically, it is an argentometric titration that uses an adsorption indicator — a dye that sticks to the surface of the silver halide precipitate. Before the end point the particle surface is negatively charged and repels the dye; just past it, excess silver makes the surface positive, the dye sticks on, and its colour visibly changes.
It matters because the colour change happens right at the particle surface, giving a sharp end point without a separate indicator complex. Its honest limits: it needs the precipitate to stay finely divided (so a protective colloid is often added), and it is sensitive to pH and to light, which can fog the silver halide.
Titrating chloride with silver nitrate and the dye fluorescein, the milky precipitate stays greenish-yellow until the end point, when adsorbed fluorescein turns the precipitate's surface a sudden pink.
The dye sticks to the precipitate and changes colour as the surface charge flips.
Adsorption indicators like fluorescein are weak acids, so each works only over a certain pH range where it carries the charge needed to be adsorbed; outside that range the colour change fails.