How does a redox indicator function in electrochemical sensing?

• A. A species changes color depending on its oxidation state; the electrode potential drives the redox reaction, providing a visible signal.
• B. It measures current directly without color change.
• C. It acts as a catalyst for the redox reaction.
• D. It emits light when oxidized or reduced.

Answer: (B)

Redox indicators report the redox state by a color change. They are molecules that exist in two forms, oxidized and reduced, each with a distinct color. When an electrode applies a potential, it drives the indicator between these two forms. As the balance shifts, the color shifts accordingly, providing a visible (or spectrophotometric) signal that reflects the redox conditions at the electrode. This makes the indicator a reporter of the electrochemical state rather than a direct measure of current.

The key idea is that the signal comes from the change in color, not from measuring current itself. The indicator’s job isn’t to catalyze the reaction, and it isn’t generally a light-emitting reporter like a luminescent tag.