Phenolphthalein is a commonly used indicator in acid-base titrations. It is a weak acid that changes color from colorless to pink in the presence of a base. The exact pH at which the color change occurs depends on the concentration of the phenolphthalein and the temperature of the solution, but it is typically around pH 8.3.

In an acid-base titration, phenolphthalein is used to signal the endpoint of the titration, which is the point at which the acid and base have completely neutralized each other. At the endpoint, the pH of the solution will be equal to the equivalence point, which is the pH at which the moles of acid and base are equal.

When phenolphthalein is added to a solution containing acid, it will remain colorless. As the base is slowly added, the pH of the solution will increase. When the pH reaches 8.3, the phenolphthalein will change color to pink. This indicates that the endpoint of the titration has been reached and the acid and base have been completely neutralized.

Phenolphthalein is a versatile indicator that can be used in a variety of acid-base titrations. It is particularly useful for titrations involving strong acids and strong bases, as the color change is sharp and easy to see.