(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

a laxative used in many preparations under various trade names; also used as an acid-base indicator in titrations involving weak acids and strong bases because it is brilliant red at high alkalinity and colorless below pH 8

a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)

an azo dye used as an acid-base indicator; used for titrations involving weak bases

a coloring material (obtained from lichens) that turns red in acid solutions and blue in alkaline solutions; used as a very rough acid-base indicator

a dye used as an acid-base indicator