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Phenolphthalein is an acid base indicator. It is a weak acid, which is colorless in acidic solution and turns puce pink in alkaline solution. Phenolphthalein is genotoxic.
Dissolve 10.0 g of phenolphthalein powder in 750 mL of ethanol and dilute to 1 L with distilled water. Add 0.1 N sodium hydroxide (NaOH) solution dropwise to the first permanent faint pink end point. Color and pH range: colorless 8.3-10.0 red.
Phenolphthalein is basic indicator which remain colorless in acidic and neutral solution and turns pink or magenta in basic solution. If few drops of NaOH is added again to the same solution, the solution become basic and the pink color of the phenolphthalein reappears.
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.