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.^[1][2] It can be used to measure the pH in subzero temperatures of saline or hypersaline media.^[3]

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Metacresol purple

Identifiers
CAS Number	2303-01-7 Y HCl salt: 67763-22-8
3D model (JSmol)	Interactive image
ChemSpider	2006272
DrugBank	DB05091
ECHA InfoCard	100.017.237
EC Number	218-960-6
PubChem CID	73030
UNII	85ODZ58K3D Y
CompTox Dashboard (EPA)	DTXSID3062316
InChI InChI=1S/C21H18O5S/c1-13-11-15(22)7-9-17(13)21(18-10-8-16(23)12-14(18)2)19-5-3-4-6-20(19)27(24,25)26-21/h3-12,22-23H,1-2H3 Key: OLQIKGSZDTXODA-UHFFFAOYSA-N
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 [77 °F], 100 kPa). Infobox references

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In colorimetric capnography, the indicator is incorporated in an aqueous matrix that provides a pH just above the indicator's colour change.^[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]

• pH 1.2–2.8: red to yellow
• pH 7.4–9.0: yellow to purple