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Phenolphthalein is used as an acid or base indicator where, in contact or presence of acid, it will turn colorless and with a base, it will turn into a fuchsia color. It is also a component in universal indicator, a solution consisting of a mixture of pH indicators (usually phenolphthalein, methyl red, bromothymol blue, and thymol blue).
In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink.
The half-way stage happens at pH 9.3. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy!
CAS Number 77-09-8
Empirical Formula (Hill Notation) C20H14O4
Molecular Weight 318.32
Phenolphthalein is another commonly used indicator for titrations, and is another weak acid.
Phenolphthalein Indicator, 1%. : Dissolve 1.0 g of phenolphthalein in 100 mL of alcohol (pH 8.0–10.0)
Phenolphthalein is a colourless, weak acid which dissociates in water forming pink anions. Under acidic conditions, the equilibrium is to the left,and the concentration of the anions is too low for the pink colour to be observed.