Phenolphthalein solution is used as an indicator during strong acid-base titartion. This indicator is added in a strong acid solution (HCl) in the conical flask and titrated with strong alkali volumetric solution (like NaOH) filled in the burette. The end point of titration is colourless to pink colour. Phenolphthalein is colourless in acid and dark pink in alkali solutions.
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