Metacresol purple

Not to be confused with m-Cresol.

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]

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 [77 °F], 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.^[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

See also

• Bromocresol purple

References

1. "Capnography". LITFL • Life in the Fast Lane Medical Blog. Retrieved 2017-07-30.
2. "m-Cresol Purple 857890". Sigma-Aldrich. Retrieved 2017-07-30.
3. Loucaides, Socratis; Rèrolle, Victoire M. C; Papadimitriou, Stathys; Kennedy, Hilary; Mowlem, Matthew C; Dickson, Andrew G; Gledhill, Martha; Achterberg, Eric P (2017). "Characterization of meta-Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures". Scientific Reports. 7 (1): 2481. Bibcode:2017NatSR...7.2481L. doi:10.1038/s41598-017-02624-0. PMC 5449408. PMID 28559544.
4. Gedeon, A.; Krill, P.; Mebius, C. (1994-09-01). "A new colorimetric breath indicator (Colibri)". Anaesthesia. 49 (9): 798–803. doi:10.1111/j.1365-2044.1994.tb04457.x. ISSN 1365-2044. PMID 7978140. S2CID 44501202.
5. Hagberg, Carin A. (2007-02-23). Benumof's Airway Management. Elsevier Health Sciences. ISBN 978-0323070171.

Metacresol purple (pH indicator)
below pH 7.4		above pH 9.0
7.4	⇌	9.0

Metacresol purple (pH indicator)
below pH 1.2		above pH 2.8
1.2	⇌	2.8