Methyl Orange

Definition

Methyl orange is a pH indicator dye commonly used in acid-base titrations to determine the endpoint of the reaction. It is an azo dye that changes color based on the acidity or basicity of the solution it is added to.

5 Must Know Facts For Your Next Test

1. Methyl orange is yellow in basic solutions and red in acidic solutions, making it useful for determining the endpoint of an acid-base titration.
2. The color change of methyl orange occurs at a pH around 3.1-4.4, making it suitable for titrations involving strong acids and strong bases.
3. The pKa of methyl orange is 3.7, meaning it is a weak acid and can act as both an acid and a base depending on the pH of the solution.
4. Methyl orange is often used in the titration of hydrochloric acid (HCl) with sodium hydroxide (NaOH) to determine the concentration of the acid.
5. The use of methyl orange in acid-base titrations allows for the accurate determination of the equivalence point, where the moles of acid and base are equal, and the solution is neutralized.

Review Questions

• Explain the role of methyl orange as a pH indicator in acid-base titrations.
  • Methyl orange is a pH indicator that is used to determine the endpoint of an acid-base titration. It changes color from yellow in basic solutions to red in acidic solutions, with the color change occurring around a pH of 3.1-4.4. This makes methyl orange suitable for titrations involving strong acids and strong bases, as the color change indicates when the solution has reached the equivalence point, where the moles of acid and base are equal, and the solution is neutralized.
• Describe the relationship between the pKa of methyl orange and its use as a pH indicator.
  • The pKa of methyl orange is 3.7, which means it is a weak acid that can act as both an acid and a base depending on the pH of the solution. This property allows methyl orange to be a useful pH indicator, as it will change color based on the acidity or basicity of the solution. In acidic solutions with a pH below the pKa, methyl orange will be in its protonated, red form. In basic solutions with a pH above the pKa, methyl orange will be in its deprotonated, yellow form. This color change is what makes methyl orange effective for determining the endpoint of acid-base titrations.
• Analyze the use of methyl orange in the titration of hydrochloric acid (HCl) with sodium hydroxide (NaOH), and explain how it helps identify the equivalence point.
  • In the titration of hydrochloric acid (HCl) with sodium hydroxide (NaOH), methyl orange is commonly used as the pH indicator. As the NaOH is added to the HCl solution, the pH of the solution will increase. When the equivalence point is reached, where the moles of acid and base are equal, the solution will be neutralized, and the pH will be around 7. Methyl orange, with its color change occurring around a pH of 3.1-4.4, will indicate the endpoint of the titration by changing from red to yellow. This allows the experimenter to accurately determine the concentration of the HCl solution by measuring the volume of NaOH required to reach the equivalence point, as indicated by the color change of the methyl orange.

Related terms

pH Indicator: A chemical compound that changes color depending on the pH of the solution, used to determine the acidity or basicity of a substance.

Acid-Base Titration: A laboratory technique used to determine the concentration of an acid or base by precisely measuring the volume of a standard solution of a base or acid required to neutralize the acid or base.

Azo Dye: A class of synthetic dyes characterized by the presence of one or more azo groups (-N=N-), which are responsible for their vibrant colors.