| Noun | 1. | absorption indicator - an indicator used in reactions that involve precipitation indicator - (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction fluorescein, fluoresceine, fluorescent dye, resorcinolphthalein - a yellow dye that is visible even when highly diluted; used as an absorption indicator when silver nitrate solution is added to sodium chloride in order to precipitate silver chloride (turns pink when no chloride ions are left in solution and negative fluorescein ions are then absorbed) |