Question

The best indicator for titrating 0.1 N Na₂CO₃ against 0.1 N HCl is:

• A. Methyl orange
• B. Litmus
• C. Phenolphthalein
• D. Potassium ferricyanide

Hint:

When titrating a weak base with a strong acid, the equivalence point occurs at a pH less than 7 (acidic). Therefore, an indicator that changes color in the acidic pH range, such as methyl orange, is suitable. For $\text{Na}_2\text{CO}_3$ titration, there are two distinct equivalence points; methyl orange is used for the complete neutralization.

Answer 1

The Correct Option is A

Step 1: Identify Reactants
\nWe are titrating 0.1 N Na₂CO₃ with 0.1 N HCl.
\nNa₂CO₃, a salt of a strong base (NaOH) and a weak dibasic acid (H₂CO₃), hydrolyzes to produce OH^-, making the solution basic. Thus, it acts as a weak base.
\nHCl is a strong acid.
\nThis is a weak base (Na₂CO₃) titrated with a strong acid (HCl).
\n\nStep 2: Understand Equivalence Points
\nSodium carbonate reacts with HCl in two steps:
\n1. Na₂CO₃ + HCl → NaHCO₃ + NaCl
\nCarbonate converts to bicarbonate. The equivalence point is at pH 8.3-9.0.
\n2. NaHCO₃ + HCl → H₂CO₃ + NaCl
\nBicarbonate converts to carbonic acid. Carbonic acid (H₂CO₃) decomposes into CO₂ and H₂O. The complete neutralization equivalence point is at pH 3.8-4.3.
\n\nStep 3: Choose the Indicator
\nAn ideal indicator changes color near the equivalence point.
\nPhenolphthalein changes color at pH 8.2-10.0, suitable for the first equivalence point (carbonate to bicarbonate, pH 8.3-9.0).
\nMethyl orange changes color at pH 3.1-4.4, suitable for the second (complete) equivalence point (bicarbonate to carbonic acid/CO₂, pH 3.8-4.3).
\nThe question asks for the "best indicator for titrating 0.1 N Na₂CO₃ against 0.1 N HCl". This usually implies complete neutralization. For complete neutralization, the solution is acidic at the equivalence point. Therefore, an indicator that changes color in the acidic range is required.
\n\nStep 4: Evaluate Options
\n

\n
• (A) Methyl orange: Its pH range (3.1-4.4) is ideal for the second equivalence point (complete neutralization).
\n
• (B) Litmus: Litmus changes color over a broad range, which is not sharp enough for precise titration.
\n
• (C) Phenolphthalein: Its pH range (8.2-10.0) is suitable for the first equivalence point, but not for complete neutralization.
\n
• (D) Potassium ferricyanide: This is not a common acid-base indicator for this type of titration.
\n

\n\nFor complete neutralization of Na₂CO₃ with HCl, the equivalence point pH is acidic (around 3.8-4.3). Methyl orange is the most suitable indicator for this pH range.
\n\n\[ \n\boxed{\text{Methyl orange}} \n\]\n