Phenolphthalein

Definition

Phenolphthalein is a chemical compound commonly used as a pH indicator in acid-base titrations. It is a weak acid that changes color depending on the pH of the solution, making it a valuable tool for quantitative chemical analysis and acid-base titrations.

5 Must Know Facts For Your Next Test

1. Phenolphthalein is a colorless compound in acidic and neutral solutions, but turns pink or red in basic solutions with a pH above 8.2.
2. The color change of phenolphthalein is due to the protonation and deprotonation of its molecular structure, which alters the compound's ability to absorb and emit light.
3. In quantitative chemical analysis, phenolphthalein is commonly used as an indicator in acid-base titrations to determine the endpoint of the reaction.
4. Phenolphthalein is a weak acid with a pKa value of approximately 9.3, which means it will only partially dissociate in water and is sensitive to changes in pH.
5. The use of phenolphthalein as an indicator in acid-base titrations allows for the accurate determination of the concentration of the unknown acid or base solution.

Review Questions

• Explain how the color change of phenolphthalein is related to its use as a pH indicator in acid-base titrations.
  • The color change of phenolphthalein is directly related to its ability to act as a pH indicator in acid-base titrations. Phenolphthalein is a weak acid that exists in two different forms: a colorless protonated form in acidic and neutral solutions, and a pink or red deprotonated form in basic solutions with a pH above 8.2. This color change occurs due to the protonation and deprotonation of the phenolphthalein molecule, which alters its ability to absorb and emit light. By monitoring the color change of phenolphthalein during a titration, the endpoint of the reaction can be accurately determined, allowing for the quantitative analysis of the acid or base concentration.
• Describe the role of phenolphthalein in the context of quantitative chemical analysis, specifically in the topic of 4.5 Quantitative Chemical Analysis.
  • In the context of quantitative chemical analysis, as covered in topic 4.5, phenolphthalein plays a crucial role as an indicator in acid-base titrations. Acid-base titrations are a common quantitative analytical technique used to determine the concentration of an unknown acid or base solution by measuring the volume of a known concentration of a standardized solution required to neutralize the unknown solution. Phenolphthalein, with its distinctive color change around pH 8.2, allows for the precise identification of the endpoint of the titration reaction. This, in turn, enables the accurate calculation of the concentration of the unknown acid or base, making phenolphthalein an essential tool in quantitative chemical analysis.
• Analyze the relationship between the use of phenolphthalein as a pH indicator and its application in the topic of 14.7 Acid-Base Titrations.
  • The use of phenolphthalein as a pH indicator is directly linked to its application in the topic of 14.7 Acid-Base Titrations. Acid-base titrations are a quantitative analytical technique that relies on the measurement of the volume of a standardized solution required to neutralize an unknown acid or base solution. Phenolphthalein, with its distinctive color change around pH 8.2, serves as an excellent indicator in these titrations. By monitoring the color change of the phenolphthalein, the endpoint of the titration can be precisely identified, allowing for the accurate determination of the concentration of the unknown acid or base solution. This connection between phenolphthalein's pH-dependent color change and its role in acid-base titrations is a critical aspect of understanding its importance in the context of 14.7 Acid-Base Titrations.

Related terms

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

Acid-Base Titration: A quantitative analytical technique used to determine the concentration of an unknown acid or base solution by measuring the volume of a known concentration of a standardized solution required to neutralize the unknown solution.

Weak Acid:

An acid that only partially dissociates in water, producing a lower concentration of hydrogen ions compared to a strong acid.