Cresol red

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Structural formula
Structural formula of cresol red
General
Surname Cresol red
other names
  • o -Cresolsulfonphthalein
  • 4- [3- (4-hydroxy-3-methylphenyl) -1,1-dioxobenzo [ c ] oxathiol-3-yl] -2-methylphenol ( IUPAC )
Molecular formula C 21 H 18 O 5 S
Brief description

green to dark red crystalline powder

External identifiers / databases
CAS number 1733-12-6
EC number 217-064-2
ECHA InfoCard 100.015.513
PubChem 73013
Wikidata Q417497
properties
Molar mass 382.43 g mol −1
Physical state

firmly

Melting point

290  ° C (decomposition)

solubility
safety instructions
GHS labeling of hazardous substances
07 - Warning

Caution

H and P phrases H: 315-319-335
P: 261-305 + 351 + 338
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

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

properties

There are two color change points:

  • pH 0.2-1.8: color change from red to yellow
  • pH 7.0-8.8: color change from yellow to violet

Cresol red 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 <1.8) the quinoid system is protonated, which causes the solution to turn red. In a basic medium (pH = 7.0–8.8) the hydroxyl group is deprotonated and the solution turns purple.

The structures of the cresol red
species H 2 In In - In 2−
structure Cresol Red-low-pH.svg Cresol Red-mid-pH.svg Cresol Red-high-pH.svg
pH <1.8 2.0-7.0 > 8.8
colour red yellow violet

use

Cresol red is used as an indicator in acid-base titrations. In most cases, only the second transition range (pH = 7.0–8.8) is used for the indication.

Individual evidence

  1. ↑ a b c data sheet Cresol red from Sigma-Aldrich , accessed on April 7, 2011 ( PDF ).
  2. ↑ a b Data sheet cresol red (PDF) from Merck , accessed on February 20, 2010.
  3. ↑ Entry on cresol purple. In: Römpp Online . Georg Thieme Verlag, accessed on September 29, 2014.
  4. ^ I. Kolthoff, C. Rosenblum: Acid-Base Indicators , MacMillan, New York 1937, pp. 108-109.
  5. ↑ RW Sabnis: Handbook of Acid-Base Indicators , CRC Press, 2008, p. 105.
  6. ^ Udo R. Kunze: Basics of quantitative analysis , 3rd edition, Georg Thieme Verlag, Stuttgart 1990, p. 96.

Web links

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