How does Phenolphthalein Work?

Phenolphthalein is used for a few different things. It can be used for quickly identifying blood. It can also be used as a pH indicator. Another uses is as a component of invisible ink. As a blood identifier and pH indicator it works the same way. If the sample being tested is blood or has a pH above 10.0 the sample will turn a fuchsia color. In invisible ink, when the ink which is a mixture of phenolphthalein and sodium hydroxide comes into contact with carbon-dioxide in the air its pH falls and the ink ‘disappears.’