Cresol Red

• O- Kresolsulfonphthalein
• 4 - [3 - (4- hydroxy-3- methylphenyl) - 1,1- dioxobenzo [c] oxathiol -3-yl ] - 2 -methylphenol (IUPAC)

Green to dark red crystalline powder

Fixed

290 ° C ( decomposition)

• Poorly in water
• Insoluble in diethyl ether and benzene

Attention

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Cresol is a triphenylmethane dye and is a member of the sulfonphthaleins. It is used as a pH indicator. His phthalein analogue is the o- cresolphthalein. By bromination of the bromocresol purple can be displayed.

Properties

There are two points ahead of color change:

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

Cresol contains two hydroxyl groups and a little stable sultone. In aqueous medium, this ring is cleaved, and after the rearrangement occurs quinoid yellow colored form of the dye. In a strongly acidic medium (pH <1.8 ) the quinoid system is protonated, the solution becomes red. In basic medium (pH = 7.0 to 8.8 ), the hydroxy group is deprotonated and the solution turns purple.

Use

Cresol is used in the acid-base titration as an indicator. It usually only the second envelope region ( pH = 7.0 to 8.8 ) used for indication.