Which of the following is not an indicator used in acid-base titrations?

• A. Methyl orange
• B. Phenolphthalein
• C. Ferric alum
• D. Methyl red

Answer: (C)

In an acid-base titration, the endpoint is signaled by an indicator—a substance that changes color within a specific pH range so you can tell when the reaction has reached its end. The familiar indicators like methyl orange, methyl red, and phenolphthalein each have a known color change at a particular pH: methyl orange shifts from red to yellow in acidic to mildly basic conditions, methyl red goes from red to yellow around mid-range pH, and phenolphthalein changes from colorless to pink as solutions become basic. Ferric alum, on the other hand, is a salt used for purposes like coagulation and water treatment; it doesn’t have a reversible, reliable color change with pH that would indicate the titration endpoint. So it isn’t used as an indicator in acid-base titrations.