Metacresol purple or m-cresol purple, also called m-cresolsulfonphthalein, is a triarylmethane dye and a pH indicator.

It is used as a capnographic indicator for detecting detect end-tidal carbon dioxide to ensure successful tracheal intubation in an emergency.

[1][2] It can be used to measure the pH in subzero temperatures of saline or hypersaline media.

[4] When exposed to carbon dioxide (CO2), it undergoes a colour change from purple to yellow, because when CO2 dissolves in the matrix, it forms carbonic acid.

[5] In chemistry, it has two useful indicator ranges:[2] This article about an organic compound is a stub.