Thymol blue (thymolsulfonephthalein) 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
Names
Preferred IUPAC name
3,3-Bis[4-hydroxy-2-methyl-5-(propan-2-yl)phenyl]-2,1λ6-benzoxathiole-1,1(3H)-dione
Other names
α-hydroxy-α,α-bis(5-hydroxycarvacryl)- o-toluenesulfonic acid γ-sultone; thymolsulfonephthalein
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.000.886 Edit this at Wikidata
EC Number
  • 200-973-3
UNII
  • InChI=1S/C27H30O5S/c1-15(2)19-13-22(17(5)11-24(19)28)27(21-9-7-8-10-26(21)33(30,31)32-27)23-14-20(16(3)4)25(29)12-18(23)6/h7-16,28-29H,1-6H3 checkY
    Key: PRZSXZWFJHEZBJ-UHFFFAOYSA-N checkY
  • InChI=1/C27H30O5S/c1-15(2)19-13-22(17(5)11-24(19)28)27(21-9-7-8-10-26(21)33(30,31)32-27)23-14-20(16(3)4)25(29)12-18(23)6/h7-16,28-29H,1-6H3
    Key: PRZSXZWFJHEZBJ-UHFFFAOYAE
  • O=S2(=O)OC(c1ccccc12)(c3cc(c(O)cc3C)C(C)C)c4cc(c(O)cc4C)C(C)C
Properties
C27H30O5S
Molar mass 466.59 g·mol−1
Appearance Brownish-green crystal powder
Melting point 221–224 °C (430–435 °F; 494–497 K)
decomposes[1]
Insoluble
UV-vismax) 594 nm (1st)
376 nm (2nd)[1]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
Harmful
GHS labelling:[2]
GHS07: Exclamation mark
Warning
H302
P264, P270, P301+P312, P330, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability (yellow): no hazard codeSpecial hazards (white): no code
1
1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.

At wavelength (378 - 382) nm, extinction coefficient > 8000 and at wavelength (298 - 302) nm, the extinction coefficient > 12000.[3]

Structures

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Thymol blue has different structures at different pH.

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

Safety

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It may cause irritation. Its toxicological properties have not been fully investigated.[4] Harmful if swallowed, Acute Toxicity. Only Hazardous when percent values are above 10%.[5]

Bibliography

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

References

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  1. ^ a b Thymol Blue
  2. ^ "Thymol blue". pubchem.ncbi.nlm.nih.gov. Retrieved 12 December 2021.
  3. ^ "Product Specification: Thymol Blue- ACS reagent" (PDF). Sigma-Aldrich. 9 October 2017. p. 1.
  4. ^ "Thymol Blue MSDS". Fisher Scientific. Retrieved 2024-07-23.
  5. ^ "Thymol blue". PubChem. Retrieved 2017-10-09.
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