A type of indicator used in reactions that involve precipitation. The yellow dye fluorescein is a common example, used for the reaction

As silver nitrate solution is added to the sodium chloride, silver chloride precipitates. As long as Cl⁻ ions are in excess, they adsorb on the precipitate particles. At the end point, no Cl⁻ ions are left in solution and negative fluorescein ions are then adsorbed, giving a pink colour to the precipitate. The technique is sometimes known as Fajans’ method.