Fajan's method of titration uses the reaction that occurs between the precipitate and the indicator during titration. The surface of colloidal AgCl absorbs fluorescein and its derivatives. The first drop of Ag+, following the use of all of the chloride, reacts with fluorescein resulting in a reddish color.
Scientists use a dye for an indicator, such as dichlorofluorescein, and it is an anion in solution. Anions are groups of atoms or single atoms with gained electrons. Chloride ions are excessive in a chloride solution, so they are responsible for the formation of the primary layer on the precipitate someone uses, with the second layer being the cations of sodium. Cations are a group of ions or individual ions that have a positive charge. During electrolysis, they always tend to move toward the negative electrode.
At the end point, or the completion of the reaction, the silver ion is excessive. This results in the primary layer now being silver ion. It attracts the indicator's ions because it now has a positive charge and forms the secondary layer. When comparing the absorbed indicator and the free indicator, the color is different. This gives scientists a visible end point so that they know when the reaction is complete.