A substance that indicates an excess of a reactant in a precipitation reaction.
dichlorofluorescein is added to an NaCl solution being titrated with silver nitrate. Before the endpoint, excess chloride ions make the surface of the AgCl precipitate negative, and dichlorofluorescein anions remain in solution. After the endpoint, the excess silver ions make the surface of the AgCl precipitate positive, and the dichlorofluorescein anions are adsorbed onto their surface. Adsorption changes the colour of the indicator from yellow-green to pink.
See also: Precipitate.