How do redox indicators work?

Redox indicators work by changing their colour at specific potential differences during a redox reaction.

Redox indicators are chemical compounds that undergo a change in colour when they are either oxidised or reduced. This change in colour is due to the different light absorption properties of the oxidised and reduced forms of the indicator. The specific potential difference at which the colour change occurs is known as the indicator's redox potential.

In a redox reaction, one substance is reduced (gains electrons) while another is oxidised (loses electrons). The redox indicator is chosen such that its redox potential lies within the potential difference of the redox reaction being studied. When the potential difference of the reaction reaches the redox potential of the indicator, the indicator changes colour. This colour change signals that the reaction has reached a particular point, often the endpoint of a titration.

For example, in an acid-base titration, a pH indicator is used to signal when all the acid (or base) has reacted with the base (or acid). Similarly, in a redox titration, a redox indicator is used to signal when all the reductant (or oxidant) has reacted with the oxidant (or reductant).

It's important to note that the choice of redox indicator depends on the specific reaction being studied. Different indicators have different redox potentials, and the indicator must be chosen such that its redox potential lies within the potential difference of the reaction. If the redox potential of the indicator is too high or too low, the indicator will change colour too early or too late, and the endpoint of the titration will not be accurately determined.

In summary, redox indicators are a crucial tool in analytical chemistry, allowing chemists to accurately determine the endpoint of redox reactions and titrations.