1.

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

Answer»

Phenolphthalein is a complex weak organic acid (HPh). Its undissociated molecule (HPh) is colourless while its 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 solution turns pink from colourless giving its endpoint. When medium changes to acidic, equilibrium shifts to left and the solution becomes colourless.



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