Thymol blue

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 (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
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
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-vis (λmax) 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 diamond
1
1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references

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

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

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

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

References
  1. 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". PubChem. Retrieved 2017-10-09.
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