Phenolphthalein is an organic compound with the formula C20H14O4. It belongs to the phthalene family and is a weak acid that can lose H+ ions in water. It comes in the form of fine crystalline, yellowish white to light orange powder. It is used as a component of the universal indicator, along with bromothymol blue, thymol blue, and methyl red.
The pKa of phenolphthalein is 9.3, and the pH range is 8.20 to 10.00. Over a range of acidic pH, the molecule has no color. At pH 8.2, it starts to turn pink, and at pH 10.00 and above, it turns a bright magenta color.
Why is phenolphthalein used in weak acid strong base titration?
Generally, phenolphthalein is used as an indicator in acid-base titration, particularly for the titration of strong bases-weak acids and strong acid-strong base titration. Depending on the acid and the base, the equivalence point (stoichiometric point) for the titrations between strong bases and weak acids is somewhere above pH 7.
Phenolphthalein indicator changes color around a pH of 8 to 10, which is right in the range of the equivalence point of these titrations, making them useful. It shows pink in basic solutions and colorless in an acidic medium.
Therefore, phenolphthalein is used as an indicator for titration of a weak acid and strong base such as acetic acid (CH3COOH) and sodium hydroxide (NaOH).
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