Phenolphthalein

Definition

Phenolphthalein is a chemical compound commonly used as a pH indicator in acid-base titrations. It changes color from colorless in acidic solutions to pink in basic solutions, which makes it a valuable tool for determining the pH of a solution during experiments. This distinctive color change occurs at a pH range of about 8.2 to 10, allowing scientists to easily observe the transition from acidity to alkalinity.

5 Must Know Facts For Your Next Test

1. Phenolphthalein is synthesized from phthalic anhydride and phenol through a condensation reaction.
2. In neutral solutions (pH 7), phenolphthalein remains colorless, making it easy to identify when a solution has become basic.
3. It is often used in acid-base titrations involving strong acids and strong bases because of its clear endpoint indication.
4. The color change of phenolphthalein can also be influenced by temperature, although it is primarily used under standard conditions.
5. Due to its clear visual cue for pH changes, phenolphthalein is frequently utilized in both educational and laboratory settings for demonstrating acid-base chemistry.

Review Questions

• How does phenolphthalein function as an indicator in acid-base titrations?
  • Phenolphthalein acts as an indicator by undergoing a color change in response to the pH level of the solution. In acidic environments, it remains colorless, while in basic conditions (above pH 8.2), it turns pink. This transition helps chemists visually determine the endpoint of titrations when the acidic solution has been neutralized by the base, making it easier to calculate concentrations.
• Discuss the significance of the pH range in which phenolphthalein changes color during titration.
  • The specific pH range for phenolphthalein's color change is crucial because it determines its effectiveness as an indicator in different types of titrations. It typically changes color between pH 8.2 and 10, making it ideal for strong acid-strong base reactions where the equivalence point falls within this range. Using phenolphthalein outside this range could lead to inaccurate results, emphasizing the importance of selecting appropriate indicators based on expected pH shifts during titrations.
• Evaluate the advantages and limitations of using phenolphthalein compared to other indicators in acid-base chemistry.
  • Phenolphthalein offers several advantages as an indicator, such as its clear and distinct color change at a specific pH range, which makes it easy to identify endpoints during titrations. However, its limitations include its ineffectiveness in situations where the endpoint falls outside its operational range or when working with weak acids and weak bases where no clear endpoint may be visible. In such cases, other indicators like bromothymol blue or methyl orange may be more suitable, highlighting the need for careful selection based on the experimental conditions.

Related terms

pH scale:

A logarithmic scale that measures the acidity or alkalinity of a solution, ranging from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral.

titration: A laboratory technique used to determine the concentration of a solution by reacting it with a solution of known concentration, often monitored with an indicator like phenolphthalein.

indicator: A substance that changes color in response to changes in pH, helping to visually represent the acidity or basicity of a solution.