Phenolphthalein

Definition

Phenolphthalein is a chemical compound commonly used as a pH indicator in acid-base titrations. It changes color from colorless to pink as the pH of a solution rises above approximately 8.2, making it an effective tool for determining the endpoint of titrations, where the amount of acid equals the amount of base.

5 Must Know Facts For Your Next Test

1. Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions, making it easy to visualize pH changes.
2. It is often used in titrations involving strong acids and strong bases, as its color change occurs within the neutralization range.
3. The transition range for phenolphthalein is between pH 8.2 and 10.0, so it may not be suitable for all types of acid-base titrations.
4. In practice, phenolphthalein is added to the analyte solution before titration begins, allowing for an immediate visual cue when nearing the endpoint.
5. Phenolphthalein can also be used outside of titrations to indicate the pH of solutions in other chemical analyses.

Review Questions

• How does phenolphthalein function as an acid-base indicator during titrations, and what visual change indicates the endpoint?
  • Phenolphthalein functions as an acid-base indicator by changing color in response to pH changes in a solution. It remains colorless in acidic conditions and turns pink as the solution becomes basic when the pH exceeds approximately 8.2. During titrations, this visual change indicates that the endpoint has been reached, which occurs when equal amounts of acid and base have reacted.
• Discuss the limitations of using phenolphthalein as an indicator in various types of titrations.
  • Phenolphthalein has specific limitations as an indicator due to its pH transition range being between 8.2 and 10.0. This means it may not be effective for titrations involving weak acids or weak bases, where the endpoint may occur outside its color-changing range. In these cases, other indicators that have different transition ranges might be preferred to accurately signal the endpoint of the reaction.
• Evaluate how phenolphthalein's properties influence its selection for acid-base titrations and its overall impact on analytical chemistry.
  • The selection of phenolphthalein for acid-base titrations is heavily influenced by its clear visual transition from colorless to pink at relevant pH levels. This property allows for precise determination of the endpoint in strong acid-strong base titrations, enhancing accuracy in quantitative analysis. Its use demonstrates a broader impact on analytical chemistry, as reliable indicators like phenolphthalein are essential for accurate measurements in various applications, ranging from laboratory experiments to industrial processes.

Related terms

pH Indicator: A substance that changes color in response to changes in pH, allowing the determination of acidity or basicity in a solution.

Endpoint: The point in a titration at which the reaction is complete, often indicated by a color change of an indicator.

Titration: A laboratory method used to determine the concentration of an unknown solution by reacting it with a solution of known concentration.