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

Indicator selection depends on the pH at the equivalence point:

Step 1 — Equivalence point pH for each titration:

• Strong acid + Strong base: pH = 7 $\rightarrow$ both methyl orange and phenolphthalein work
• Strong acid + Weak base: pH < 7 $\rightarrow$ methyl orange (pH 3.1–4.4) is suitable
• Weak acid + Strong base: pH > 7 $\rightarrow$ phenolphthalein (pH 8.3–10) is suitable
• Weak acid + Weak base: pH ≈ 7 (depends on Ka/Kb) $\rightarrow$ no sharp endpoint, so no indicator works well

Step 2 — Evaluate each option:

• (1) Methyl orange for weak acid + weak base: ✗ Incorrect — no sharp endpoint in weak acid/weak base titration; methyl orange is NOT suitable
• (2) Phenolphthalein for strong acid + strong base: ✓ Correct (works, though not ideal)
• (3) Methyl orange for strong acid + weak base: ✓ Correct
• (4) Phenolphthalein for weak acid + strong base: ✓ Correct

Answer: Option (1) — Incorrect statement