Carbon dioxide from the air is often cited as a reason for the change in color of phenolphthalein from pink to colorless. But the catch is that the solution is often quite basic. For the color to change due to a change in pH, the solution would need a lot of CO2 dissolved in it.
Reason for the change in phenolpthalein from pink to colorless in solutions with high pH. And, form of molecule which is colorless in solutions with high pH. The transition from pink to colorless is slow, and accounts to one's observation.
Chemical equation involved: The net ionic equation of the acid-base reaction.
HC2H3O2(aq) + OH-(aq) ⇒ C2H3O2-(aq) + H2O
