Thymol blue

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Thymol blue
Thymolblau skeletal.png
Thymol Blue crystals.jpg
CAS number 76-61-9 N
PubChem 65565
ChemSpider 59008 YesY
Jmol-3D images Image 1
Molecular formula C27H30O5S
Molar mass 466.59 g mol−1
Appearance Brownish-green crystal powder
Melting point 221–224 °C (430–435 °F; 494–497 K)
Solubility in water Insoluble
λmax 594 nm (1st)
376 nm (2nd)[1]
EU classification Harmful XnDangerous for the Environment (Nature) N
Main hazards Harmful
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity (yellow): no hazard code Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Thymol blue (thymolsulphonephthalein) is a brownish-green or reddish-brown crystalline powder that is used as a pH indicator. It is insoluble in water but soluble in alcohol and dilute alkali solutions.

Thymol blue (pH indicator)
below pH 8.0 above pH 9.6
8.0 9.6
Thymol blue (pH indicator)
below pH 1.2 above pH 2.8
1.2 2.8

It transitions from red to yellow at pH 1.2–2.8 and from yellow to blue at pH 8.0–9.6. It is usually a component of Universal indicator.


Thymol blue has different structures at different pH.

Color of thymol blue solution at different acid-base conditions: left: acidic, middle: neutral, right: alkaline


It may cause irritation. Its toxicological properties have not been fully investigated.


  • Merck. "Thymol Blue." The Merck Index. 14th ed. 2006. Accessed via web on 2007-02-25.


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