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Thymol Blue Chemical Structure

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What is Thymol Blue

Thymol blue, also known as thymolsulfonphthalein, is a type of sulfomephthalein dye. It is a crystalline powder that is brownish-green or reddish-brown in colour and is insoluble in water. However, it is soluble in alcohol and in solutions of dilute alkali.

Macsen Laboratories is the leading manufacturer & supplier of Thymol Blue with ISO certification.
Name of Product Thymol Blue
IUPAC Name 4-[3-(4-hydroxy-2-methyl-5-propan-2-ylphenyl)-1,1-dioxo-2,1λ6-benzoxathiol-3-yl]-5-methyl-2-propan-2-ylphenol
Synonyms Thymolsulfonephthalein; Thymolsulfophthalein; Sodium thymol blue; Azul Timol; Timolo blu; Thymol Bleu; Thymolblau
CAS No 76-61-9
Molecular Formula C27H30O5S
Molecular Weight 466.6 g/mol
Pubchem CID 65565
Pubchem SID 469688484

Chemical Properties

SR. No Criteria Limit/Specification
1 Appearance (Form) Solid powder
2 Appearance (Color) Brownish-Green crystals
3 Melting point 221-224 °C (dec.) (lit.)
4 Density 0.979 g/cm3
5 Solubility Insoluble in water;
Gives clear solution in 0.1% ethanol;
6 UV-vis (λmax) 594 nm (1st)
376 nm (2nd)
7 Storage temp. Room temp.


  • Thymol Blue is useful as a pH indicator. It changes it’s colour from red to yellow at a pH range of 1.2–2.8, and then from yellow to blue at a pH range of 8.0–9.6, thus been used for the in situ spectrophotometric pH measurement.
  • It is also included as a component of the Universal indicator.
  • It also finds some applications in the field of hematology and histology.


Q. What is the purpose of Thymol Blue?

Thymol Blue is used as a pH indicator to detect the acidity or basicity of solutions.

Q. Is Thymol blue the same as Bromothymol blue?

Bromothymol blue is produced by adding elemental bromine to Thymol blue in glacial acetic acid solution.

Q. How does thymol blue indicator works?

An aqueous solution of thymol blue undergoes the color changes from red to yellow in an acid solution (pKa ≈ 1.6) and from yellow to blue in an alkaline solution (pKa ≈ 8.9), thus functioning as a pH indicator.

Q. What colour is thymol blue in neutral?

Thymol Blue exhibits yellow colour in neutral form. A solution of thymol blue also shows red colour and blue colour (in basic form).
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