Phenolphthalein as Indicator

Phenolphthalein is a indicator used for determining the quantity of base. The structure of phenolphthalein is as below

When phenolphthalein is reacted with acid or neutral solution or water there is no reaction as phenol present in the structure is acidic in nature i.e. it has tendency to release H+ ions in the solution. In presence of acid or neutral solution the H+ ion of phenol are not liberated. 

but when phenolphthalein is treated with a base. the acidic part of phenol reacts with the base to give corresponding  sodium salt of phenolphthalein which has pink color thus it imparts pink color to the solution 

Thus in acid base titration phenolphthalein is used to detect end point of base.