adsorption indicator (absorption indicator) A type of indicator used in reactions that involve precipitation. The yellow dye fluorescein is a common example, used for the reaction NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq) As silver nitrate solution is added to the sodium chloride, silver chloride precipitates. As long as Cl” ions are in excess, they