Question

Explain why phenolphthalein is used as an indicator of acid-base titration.

Answer 1

phenolphthalein is a complex weak organic acid (HPh). Its undissociated molecule (HPh) is colourless while it's an ion (Ph-) is pink in colour. pH range of phenolphthalein is 8 - 9.8.

In acid-base titration as the medium changes from acidic to basic, the following equilibrium shifts to right.

HPh ⇌ H⁺ + Ph^- and the solution turns pink from colourless giving its endpoint. when medium changes to acidic, equilibrium shifts to left and the solution becomes colourless.