Thymol blue

from Wikipedia, the free encyclopedia
Structural formula
Structural formula of thymol blue
General
Surname Thymol blue
other names
  • Thymolsulfonphthalein
  • 4- [3- (4-hydroxy-2-methyl-5-propan-2-ylphenyl) -1,1-dioxobenzo [ c ] oxathiol-3-yl] -5-methyl-2-propan-2-ylphenol ( IUPAC )
Molecular formula C 27 H 30 O 5 S
Brief description

green solid with a phenolic odor

External identifiers / databases
CAS number 76-61-9
EC number 200-973-3
ECHA InfoCard 100,000,886
PubChem 65565
Wikidata Q420396
properties
Molar mass 466.59 g mol −1
Physical state

firmly

Melting point

221 ° C (decomposition)

solubility

almost insoluble in water (0.11 g l −1 at 25 ° C)

safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Thymol blue is a triphenylmethane dye and belongs to the group of sulfonphthaleins. It is used as a pH indicator . Its phthalein analogue is thymolphthalein . The bromothymol blue can be represented by bromination .

properties

Physical Properties

Thymol blue is a green solid with a phenol-like odor and melts at 221 ° C.

Green crystals of thymol blue

Chemical properties

There are two color change points:

  • pH 1.2-2.8: color change from violet-red to brownish-yellow
  • pH 8.0-9.6: color change from yellow-greenish to blue

Thymol blue contains two hydroxyl groups and an unstable sultone ring . This ring is split in an aqueous medium, and after a rearrangement the quinoid yellow colored form of the dye is formed. In a strongly acidic environment (pH <1.2) the quinoid system is protonated, the solution becomes violet-red. In a basic environment (pH = 8.0–9.6) the hydroxyl group is deprotonated and the solution turns blue.

The structures of thymol blue
species H 2 In In - In 2−
structure Structures of thymol blue at different pH values
pH <1.2 2.8-8.0 > 9.6
colour purple red yellow blue
Color change of thymol blue at different pH values

use

Thymol blue is used as an indicator in acid-base titrations. In most cases, only the second transition range (pH = 8.0–9.6) is used for the indication.

Individual evidence

  1. ↑ a b c d e f data sheet thymol blue (PDF) from Merck , accessed on February 20, 2010.
  2. ↑ Thymol Blue data sheet from Sigma-Aldrich , accessed on May 12, 2017 ( PDF ).

Web links

Commons : Thymol Blue  - Collection of images, videos and audio files