Thymol blue

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Thymol blue
Thymolblau skeletal.png
Thymol Blue crystals.jpg
IUPAC name
4-[9-(4-hydroxy-2-methyl-5-propan- 2-yl-phenyl)-7,7-dioxo-8-oxa- 7λ6-thiabicyclo[4.3.0]nona-1,3,5-trien-9-yl]- 5-methyl-2-propan-2-yl-phenol
Other names
α-hydroxy-α,α-bis(5-hydroxycarvacryl)- o-toluenesulfonic acid γ-sultone; thymolsulfonephthalein
3D model (JSmol)
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Molar mass 466.59 g·mol−1
Appearance Brownish-green crystal powder
Melting point 221–224 °C (430–435 °F; 494–497 K)
UV-vismax) 594 nm (1st)
376 nm (2nd)[1]
Main hazards Harmful
Harmful XnDangerous for the Environment (Nature) N
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentineReactivity (yellow): no hazard codeSpecial hazards (white): no codeNFPA 704 four-colored diamond
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is ☑Y☒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

Thymol blue transitions from red to yellow at pH 1.2–2.8, and from yellow to blue at pH 8.0–9.6; it is a component of Universal indicator.

At wavelength (378 - 382) nm, extinction coefficient > 8000 and at wavelength (298 - 302) nm , the extinction coefficient > 12000.[2]


Thymol blue has different molecular structures at different pH levels.

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


It may cause irritation, and its toxicological properties have not been fully investigated.[3]


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


  1. 1.0 1.1 Thymol Blue
  2. "Product Specification". Thymol Blue- ACS reagent 1: 1. 9 October 2017.
  3. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).

External links