Cresol purple

Structural formula
General
Surname	Cresol purple
other names	m-cresol sulfonphthalein 4- [3- (4-hydroxy-2-methylphenyl) -1,1-dioxobenzo [ c ] oxathiol-3-yl] -3-methylphenol ( IUPAC )
Molecular formula	C 21 H 18 O 5 S
External identifiers / databases
CAS number 2303-01-7 EC number 218-960-6 ECHA InfoCard 100.017.237 PubChem 73030 Wikidata Q907695
properties
Molar mass	382.43 g mol −1
Physical state	firmly
solubility	almost insoluble in diethyl ether and benzene , poorly in water, soluble in ethanol
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 .

Cresol purple is a triphenylmethane dye and belongs to the group of sulfonphthaleins. It is used as a pH indicator . Its phthalein analogue is m-cresolphthalein . The bromocresol green can be represented by bromination .

properties

There are two color change points:

• pH 1.2-2.8: color change from red to yellow
• pH 7.4-9.0: color change from yellow to purple

Cresol purple contains two hydroxyl groups and a less stable 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, which causes the solution to turn red. In a basic medium (pH = 7.4-9.0) the hydroxyl group is deprotonated and the solution turns purple.

use

Cresol purple is used as an indicator in acid-base titrations. In most cases, only the second transition range (pH = 7.4–9.0) is used for the indication.

Individual evidence

1. ↑ ^{a b c d} data sheet m-Cresol Purple, indicator grade, dye content 90% from Sigma-Aldrich , accessed on December 1, 2019 ( PDF ).
2. ↑ Entry on cresol purple. In: Römpp Online . Georg Thieme Verlag, accessed on October 1, 2014.