Question

Incorrect statement for the use of indicators in acid-base titration is :

• A. Methyl orange may be used for a weak acid vs weak base titration.
• B. Phenolphthalein may be used for a strong acid vs strong base titration.
• C. Methyl orange is a suitable indicator for a strong acid vs weak base titration.
• D. Phenolphthalein is a suitable indicator for a weak acid vs strong base titration.

Hint:

Always choose the indicator based on the pH range at the equivalence point of the titration.

Answer 1

The Correct Option is A

To determine which statement is incorrect about the use of indicators in acid-base titrations, we need to understand how indicators function based on the pH range of the titration. 

Indicators in Acid-Base Titrations:

• Methyl Orange changes color in the pH range of about 3.1 to 4.4. It is best suited for titrations involving strong acids and weak bases. This is because the equivalence point for such titrations is below pH 7.
• Phenolphthalein changes color in the pH range of about 8.2 to 10.0. It is ideal for titrations involving strong bases and weak acids, or strong acids and strong bases, where the equivalence point is near pH 7 or higher.

Option Analysis:

1. Methyl orange may be used for a weak acid vs weak base titration.
   • This statement is incorrect. In a weak acid vs weak base titration, the pH change at the equivalence point is very gradual, and no sharp pH change occurs to facilitate the use of any typical indicator effectively. Hence, methyl orange is not suitable for this type of titration.
2. Phenolphthalein may be used for a strong acid vs strong base titration.
   • This statement is correct. The titration of strong acids with strong bases typically has an equivalence point around pH 7, thus phenolphthalein can be used effectively as its pH range (8.2 to 10.0) allows for a clear color change.
3. Methyl orange is a suitable indicator for a strong acid vs weak base titration.
   • This statement is correct. As mentioned, methyl orange's suitable pH range aligns with the acidic equivalence point typically observed in strong acid vs weak base titrations.
4. Phenolphthalein is a suitable indicator for a weak acid vs strong base titration.
   • This statement is correct. The equivalence point in such titrations usually falls into the basic region, making phenolphthalein an appropriate choice.

Conclusion:

The incorrect statement regarding the use of indicators in acid-base titration is: "Methyl orange may be used for a weak acid vs weak base titration." This statement is incorrect because a weak acid vs weak base titration does not produce a sufficiently sharp pH change for an indicator like methyl orange to be effective.