How does phenolphthalein work?

Phenolphthalein is an indicator that undergoes a colour change from colourless to pink that begins at a pH of 8.0. So in the titration your performing the phenolphthalein will start to change colour at the point when the moles of acid equal the moles of base. Although this colour change occurs at a pH of 8.0 and not at a pH of 7.0 phenolphthalein is commonly used because of the distinctive colour change that occurs. Phenolphthalein is colourless in acids and pink/purple in base. It also turns orange in very strong acids.