Phenol red

• Phenolsulfonephthalein
• 4,4 - (3H -2 ,1- Benzoxathiol -3 -ylidene ) bisphenol -1 ,1-dioxide

Red solid with a faint odor

Fixed

> 300 ° C

Very poor in water ( 0.77 g · l-1 at 100 ° C)

Template: Infobox chemical / molecular formula search available

Phenol red is a triphenylmethane dye and is the base compound of the family of sulfonphthaleins. It is used as a pH indicator, and has a pair of turnup portions. At pH ≈ 0.9, the color changes from red to yellow at pH ≈ 6.4 to 8.2 then from yellow to red-violet.

Representation

In a Friedel -Crafts acylation of 2 equivalents of phenol and one equivalent of o- sulfobenzoic anhydride can be reacted small amounts of concentrated sulfuric acid or in the presence of zinc chloride.

Properties

Phenol red containing 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. This quinoid system can be protonated in strongly acidic medium (pH < 1), whereby the solution turns red. In neutral medium (pH = 6.4 to 8.3 ), the hydroxy group is deprotonated and the solution turns red-violet. In strongly basic (pH > 14) is attachment of an OH group to form a Triphenylmethanolstruktur. Phenol is then present as colorless trianion.

Use

Phenol is used in acid - base titrations as an indicator, with a transition range between pH 6.4 and 8.2. The color changes from red-purple to yellow in a basic medium under acidic conditions. Since phenol red very poorly soluble in water, the reconstituted solution is prepared by either

• Dissolving 0.1 g in 100 ml of 20 % ethanol, or
• Dissolving 0.04 g in 1.13 ml of sodium hydroxide solution ( 0.1 mol / l ) and then filling up with water to 100 ml

Phenol red is used as a pH indicator for cell culture media.