Last updated
Thymolphthalein skeletal.svg
Preferred IUPAC name
3D model (JSmol)
ECHA InfoCard 100.004.300 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 204-729-7
PubChem CID
  • InChI=1S/C28H30O4/c1-15(2)20-13-23(17(5)11-25(20)29)28(22-10-8-7-9-19(22)27(31)32-28)24-14-21(16(3)4)26(30)12-18(24)6/h7-16,29-30H,1-6H3 Yes check.svgY
  • InChI=1/C28H30O4/c1-15(2)20-13-23(17(5)11-25(20)29)28(22-10-8-7-9-19(22)27(31)32-28)24-14-21(16(3)4)26(30)12-18(24)6/h7-16,29-30H,1-6H3
  • O=C1OC(c2ccccc12)(c3cc(c(O)cc3C)C(C)C)c4cc(c(O)cc4C)C(C)C
Molar mass 430.544 g·mol−1
AppearanceWhite powder
Melting point 248 to 252 °C (478 to 486 °F; 521 to 525 K) (decomposes)
Hazards [1]
GHS labelling:
H341, H350, H361
P201, P202, P210, P233, P240, P241, P242, P243, P280, P281, P303+P361+P353, P308+P313, P370+P378, P403+P235, P405, P501
NFPA 704 (fire diamond)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Thymolphthalein is a phthalein dye used as an acidbase (pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianion is 38,000 M−1 cm−1 at 595 nm. [2]

Thymolphthalein(pH indicator)
below pH 9.3above pH 10.5

Thymolphthalein is also known to have use as a laxative [3] and for disappearing ink. [4]


Thymolphthalein can be synthesized from thymol and phthalic anhydride.


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  1. "Thymolphthalein".
  2. Hahn HH; Cheuk SF; Elfenbein S; Wood WB (April 1970). "Studies on the Pathogenesis of Fever: Xix. Localization of Pyrogen in Granulocytes". J. Exp. Med. 131 (4): 701–9. doi:10.1084/jem.131.4.701. PMC   2138774 . PMID   5430784.
  3. Hubacher, MH; Doernberg, S; Horner, A (1953). "Laxatives: chemical structure and potency of phthaleins and hydroxyanthraquinones". J Am Pharm Assoc Am Pharm Assoc. 42 (1): 23–30. doi:10.1002/jps.3030420108. PMID   13034620.
  4. Katz, David A. (1982). "Disappearing Ink" (PDF). Retrieved August 14, 2017.