Phenolphthalein is a weak organic acid. It has a colorless lactone structure in solution with pH < 8.2 and a red quinone structure when pH > 8.2.
The discoloration range of phenolphthalein is 8.2-10.0, so phenolphthalein can only test alkali but not acid. Phenolphthalein is a small white or yellowish crystal, insoluble in water and soluble in alcohol. Therefore, phenolphthalein is usually used as an alcohol solution.
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