Question

Explain the action of phenolphthalein indicator in the titration of acetic acid versus sodium hydroxide.

Answer 1

Acetic acid is a weak acid having a pH value of more than $6$. Sodium hydroxide is a strong base and addition of ${OH}^{-}$ ions neutralizes the $H^{+}$ ions formed by the dissociation of acetic acid. Since $H^{+}$ ion concentration is very low, the addition of further ${OH}^{-}$ ions increases the pH value abruptly. Therefore, the titration range of this reaction lies between $6.5$ to $10.0$. The pH range of phenolphthalein is $8.2$ to $10.0$ and hence it undergoes colour change in this range. Hence it acts as a suitable indicator for this titration.