metacresol purple metacresol purple 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. it can be used to measure the ph in subzero temperatures of saline or hypersaline media. metacresol purple identifiers cas number 2303-01-7 y 3d model (jsmol) interactive image chemspider 2006272 echa infocard 100.017.237 pubchem cid 73030 unii 85odz58k3d y comptox dashboard (epa) dtxsid3062316 smiles cc1=c(c=cc(=c1)o)c2(c3=cc=cc=c3s(=o)(=o)o2)c4=c(c=c(c=c4)o)c properties chemical formula c21h18o5s molar mass 382.43 g·mol−1 except where otherwise noted, data are given for materials in their standard state (at 25 °c , 100 kpa). infobox references in colorimetric capnography, the indicator is incorporated in an aqueous matrix that provides a ph just above the indicator's colour change. 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. in chemistry, it has two useful indicator ranges: ph 1.2–2.8: red to yellow ph 7.4–9.0: yellow to purple