فنل فتالئین CAS:77-09-8

Phenolphthalein is a chemical compound with the formula C₂₀اچ₁₄O₄ and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.

Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in experiments. It is a weak acid, which can lose H⁺ ions in solution. The nonionized phenolphthalein molecule is colorless and the double deprotonated phenolphthalein ion is fuchsia. Further loss of proton in higher pH occurs slowly and leads to a colorless form. Phenolphthalein ion in concentrated sulfuric acid is orange red due to sulfonation.

Phenolphthalein is one of the most widely used indicators. Phenolphthalein is used as a laxative to treat incomplete and infrequent bowel movement.Phenolphthalein also is used in automotive and aviation industries.