Why do some titrations require more than one indicator?

Some titrations require more than one indicator because they involve multiple stages of pH changes.

In a titration, an indicator is used to signal the point at which the reaction has been completed, known as the endpoint. This is typically marked by a colour change in the indicator. However, not all titrations involve a simple one-step reaction that results in a single, clear-cut change in pH. Some reactions occur in stages, with each stage resulting in a different pH change. In these cases, a single indicator may not be sufficient to accurately signal the endpoint of each stage.

For instance, in a titration involving a diprotic acid (an acid that can donate two protons or hydrogen ions), there are two stages of reaction. The first stage involves the donation of the first proton, and the second stage involves the donation of the second proton. Each of these stages results in a different pH change, and thus requires a different indicator to accurately signal its endpoint.

Using more than one indicator in a titration can also be useful in complexometric titrations, where the reaction involves the formation of a complex ion. In these titrations, different indicators can be used to signal the endpoint of the reaction and the formation of the complex ion.

In summary, while a single indicator is sufficient for most simple titrations, more complex reactions that involve multiple stages of pH changes or the formation of complex ions may require the use of more than one indicator. This ensures that the endpoint of each stage of the reaction is accurately determined, allowing for a more precise and accurate titration result.