An organic compound that may act as both a weak acid and a weak base and can change color at a specific pH value is known as what?

• A. Indicator
• B. Primary standard
• C. Titrant
• D. Secondary standard

Answer: (A)

An indicator is a substance that exists in two forms and changes color as the pH changes. Many indicators are weak acids or weak bases, so they can donate or accept a proton depending on the environment. The two forms have different colors, and at a certain pH (the indicator’s transition range) the equilibrium shifts so you see a distinct color change. This makes indicators perfect for signaling when a reaction reaches its endpoint in a titration.

Other terms describe reagents used to standardize or carry out the titration, not color-changing substances: a primary standard is a highly pure substance used to determine concentrations accurately, a titrant is the solution added during titration, and a secondary standard is a solution that has been standardized against a primary standard.