Question

Which indicator is used in the titration of sodium hydroxide against oxalic acid and what is the colour change at the end point?

• A. Phenolphthalein, pink to yellow
• B. Alkaline KMnO₄, colourless to pink
• C. Phenolphthalein, colourless to pink
• D. Methyl orange, yellow to pinkish red colour

Answer 1

The Correct Option is C

The question pertains to the selection of an appropriate indicator for the titration of sodium hydroxide (NaOH) against oxalic acid (H₂C₂O₄), as well as the observation of the color change at the endpoint. To answer this question, let's understand the role of an indicator in titration and examine the options given.

1. Understanding the Reaction:
   • Sodium hydroxide (NaOH) is a strong base, and oxalic acid (H₂C₂O₄) is a weak acid.
   • The titration involves the neutralization of oxalic acid with sodium hydroxide, producing sodium oxalate and water.
2. Use of Indicators in Titration:
   • Indicators are substances that change color at a particular pH level and are used to determine the endpoint of a titration.
   • For a strong base (NaOH) and weak acid (oxalic acid) titration, the endpoint will be in the basic range. Hence, a suitable indicator that changes color in the basic pH range is needed.
3. Evaluating the Options:
   • Phenolphthalein, pink to yellow: Phenolphthalein does not change from pink to yellow. This option is incorrect.
   • Alkaline KMnO₄, colourless to pink: Alkaline KMnO₄ is not an indicator; it's an oxidizing agent. This option is incorrect.
   • Phenolphthalein, colourless to pink: Phenolphthalein changes from colorless to pink at a pH of about 8.2 to 10, which is suitable for a strong base and weak acid titration. This is the correct answer.
   • Methyl orange, yellow to pinkish red colour: Methyl orange changes color at a lower pH (around 3.1 to 4.4), which is more suitable for strong acid-strong base titrations. This option is incorrect.
4. Conclusion:
   • The correct indicator for the titration of sodium hydroxide against oxalic acid is phenolphthalein. At the endpoint, the color changes from colorless to pink.