What is the substance used to detect the end-point of a titration by a color change?

• A. Indicator
• B. Buffer
• C. Catalyst
• D. Solvent

Answer: (A)

In titration, a color-changing substance called an indicator is used to signal the endpoint. An indicator is a weak acid or base that exists in two forms, each with a distinct color. As titrant is added and the reaction progresses, the solution’s pH shifts toward the endpoint, and at a particular pH the indicator switches form and colors, showing that the endpoint (and near the equivalence point) has been reached. This is why the indicator is chosen so its color change occurs close to the equivalence point, minimizing titration error. The other options don’t provide a color signal: a buffer resists pH change, a catalyst speeds up the reaction, and a solvent simply dissolves substances.