Thymolphthalein

• 3,3- bis (4- hydroxy-5 -isopropyl- 2-methylphenyl) - 1 (3H)- isobenzofuranone
• 5 ', 5 " -diisopropyl- 2', 2 " - dimethylphenolphthalein

Colorless solid

Fixed

253 ° C

Almost insoluble in water

Template: Infobox chemical / molecular formula search available

Thymolphthalein is a triphenylmethane dye and belongs to the family of phthaleins. The name is composed of thymol and phthalic anhydride. The corresponding sulfonphthalein is the thymol blue. It is used as phenolphthalein as pH indicator.

Properties

Thymolphthalein is a white crystalline powder in water and practically insoluble. It takes place mostly in 0.1 % alcoholic solution use. It is itself a weak acid.

At a pH value of from 0 to about 9.3 dissolved thymolphthalein is colorless, at higher pH ( 10.5 ), the solution turns blue, in a strongly alkaline medium at a pH value close to 14, it becomes colorless again. It is therefore used, inter alia, in the titration of basic solutions as an indicator. Particularly it is suitable for the determination of the second equivalence point of phosphoric acid at pH ≈ 9.7.

Representation

In a Friedel -Crafts acylation of 2 equivalents of thymol and one equivalent of phthalic anhydride are reacted small amounts of concentrated sulfuric acid or in the presence of zinc chloride.