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The doubly deprotonated (In 2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH) 3− form, and its pink color undergoes a rather slow fading reaction [6] and becomes completely colorless when pH is greater than 13.
For optimal accuracy, the color difference between the two species should be as clear as possible, and the narrower the pH range of the color change the better. In some indicators, such as phenolphthalein, one of the species is colorless, whereas in other indicators, such as methyl red, both species confer a color. While pH indicators work ...
Universal indicator components Indicator Low pH colour Transition pH range High pH colour Thymol blue (first transition) Red 1.2 – 2.8 Yellow Methyl orange: Red 3.2 – 4.4 Yellow Methyl red: Red 4.8 – 6.0 Yellow Bromothymol blue: Yellow 6.0 – 7.6 Blue Thymol blue (second transition) Yellow 8.0 – 9.6 Blue Phenolphthalein: Colourless
A solution of phenol red is used as a pH indicator, often in cell culture. Its color exhibits a gradual transition from yellow (λ max = 443 nm [2]) to red (λ max = 570 nm [3]) over the pH range 6.8 to 8.2. Above pH 8.2, phenol red turns a bright pink color. [4] [5]
It is advised to check the references for photos of reaction results. [1] Reagent testers might show the colour of the desired substance while not showing a different colour for a more dangerous additive. [2]
Titration indicator table [11] Indicator name Indicator colour Transition interval (pH range) Color after high pH conditions Methyl Orange Orange/red 3.1 - 4.4 Yellow Methyl Red Red 4.4 - 6.3 Yellow Congo Red Blue 3.0 - 5.2 Red Phenolphthalein Colourless 8.3 - 10.0 Pink Thymolphthalein Colourless 9.3 - 10.5 Blue Bromophenol Blue Yellow 3.0 - 4.6
Chemical structure of phenolphthalein, a common phthalein dye. Phthalein dyes are a class of dyes mainly used as pH indicators, due to their ability to change colors depending on pH. [1] They are formed by the reaction of phthalic anhydride with various phenols. They are a subclass of triarylmethane dyes. Common phthalein dyes include ...
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