Florida Wastewater Practice Test

What is phenolphthalein commonly used for in titration?

1. Buffering agent

2. Indicator

3. Solvent

4. Precipitating agent

The correct answer is: Indicator

Phenolphthalein is commonly used as an indicator in titrations due to its ability to change color at a specific pH range. In acid-base titrations, it transitions from colorless in acidic solutions to pink in alkaline solutions, typically between a pH of around 8.2 and 10.0. This color change allows chemists to visually determine the endpoint of a titration, where the amount of titrant added is stoichiometrically equivalent to the amount of substance being analyzed. The use of an indicator like phenolphthalein is crucial in titrations, as it provides a clear and immediate visual cue that helps in quantifying the concentration of an unknown solution. In contrast, buffering agents are used to maintain a stable pH during a reaction and do not serve the purpose of indicating the completion of a titration. Solvents are mediums that dissolve the reactants but do not participate in the chemical reaction themselves, while precipitating agents are used to form solid compounds in a reaction, neither of which relate to the function of an indicator. Thus, phenolphthalein's role as an indicator makes it an integral part of titration procedures.