Principle of indicator selection

In acid-base titration, the indicator must change colour in the steep pH jump region near the equivalence point.

Common indicator ranges:

• Methyl orange: $\text{pH } 3.1 - 4.4$
• Phenolphthalein: $\text{pH } 8.3 - 10.0$

Examine each titration type

(A) Weak acid vs strong base

At equivalence point, solution is basic ($\text{pH} > 7$). So an indicator changing in alkaline range is suitable.

• Phenolphthalein changes in $8.3-10.0$
• Hence, it is suitable.

Therefore, A is correct.

(B) Weak acid vs weak base

For weak acid vs weak base titration, the pH change near equivalence point is not sharp. So no common visual indicator gives a reliable end point.

Therefore, methyl orange should not be used.

Hence, B is incorrect.

(C) Strong acid vs weak base

At equivalence point, solution is acidic ($\text{pH} < 7$). So an indicator with acidic transition range is suitable.

• Methyl orange changes in $3.1-4.4$
• This matches the steep region reasonably well.

Therefore, C is correct.

(D) Strong acid vs strong base

At equivalence point, $\text{pH} \approx 7$, but the pH jump is very steep and wide. So both methyl orange and phenolphthalein can generally be used.

Therefore, D is correct.

Conclusion

The incorrect statement is: $\boxed{\text{B}}$

Comparison with stored answer

Stored correct answer = B

My derived answer = B

So, they agree.