Phenolphthalein is used as a single indicator in acid-base titrations. From left to right it is shown first in a base solution, where it is a pink colour. Adding a dilute acid turns it gradually colourless. In the second flask it is at pH 9.3 and the colour is barely perceivable. At pH 8 it is colourless. In the third flask alkali has been added to reverse the reaction, and the colour starts to return again at pH9.3. In the fourth flask it has returned to full strength again at about pH 11.