Phenolphthalein is a weak acid used as an acid-base indicator in chemistry. Its most striking feature is its color change, which happens over a specific pH range, making it ideal for titrations.

How It Works

Phenolphthalein’s color depends entirely on the pH of the solution it’s in.

• In acidic or neutral solutions (pH below ~8.2), phenolphthalein remains colorless.
• In basic solutions (pH above ~8.2), the molecule’s structure changes, causing the solution to turn a bright pink or fuchsia color.

This sharp transition, occurring in the pH range of 8.2 to 10.0, makes it an excellent visual signal for the endpoint of a titration, especially when a strong base is involved.

Common Uses

Phenolphthalein’s primary use is in acid-base titrations to help determine the point of neutralization. It also has other applications, such as in the Kastle-Meyer test for detecting blood in forensic investigations.

While it was once used as a laxative, it is no longer approved for this use due to health concerns. In the lab, always handle it with proper safety precautions.