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 the nature of the acid and base involved in the titration.
We are titrating 0.1 N Na₂CO₃ (sodium carbonate) against 0.1 N HCl (hydrochloric acid).
Na₂CO₃ is a salt of a strong base (NaOH) and a weak dibasic acid (H₂CO₃). In solution, it hydrolyzes to produce OH^- ions, making the solution basic. Thus, Na₂CO₃ acts as a weak base.
HCl is a strong acid.
So, this is a titration of a weak base (Na₂CO₃) with a strong acid (HCl).
Step 2: Understand the equivalence points for Na₂CO₃ titration with a strong acid.
Sodium carbonate reacts with HCl in two steps:
1. Na₂CO₃ + HCl → NaHCO₃ + NaCl
In this first step, carbonate converts to bicarbonate. The equivalence point for this step occurs at a pH of around 8.3-9.0.
2. NaHCO₃ + HCl → H₂CO₃ + NaCl
In this second step, bicarbonate converts to carbonic acid. Carbonic acid (H₂CO₃) then rapidly decomposes into CO₂ and H₂O. The equivalence point for the complete neutralization (both steps) occurs at a pH of around 3.8-4.3.
Step 3: Choose the appropriate indicator based on the equivalence point.
An ideal indicator for a titration changes color precisely at or very near the equivalence point of the titration. This means the indicator's pH range for color change should encompass the pH at the equivalence point.
Phenolphthalein changes color in the pH range of 8.2-10.0. It is suitable for the first equivalence point (carbonate to bicarbonate) where the pH is around 8.3-9.0.
Methyl orange changes color in the pH range of 3.1-4.4. It is suitable for the second (complete) equivalence point (bicarbonate to carbonic acid/CO₂), where the pH is around 3.8-4.3.
The 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 of sodium carbonate by a strong acid, the solution becomes acidic at the equivalence point due to the formation of carbonic acid (which then breaks down to CO₂ and H₂O). Therefore, an indicator that changes color in the acidic range is required.
Step 4: Evaluate the given options.

• (A) Methyl orange: Its pH range (3.1-4.4) is ideal for the second equivalence point (complete neutralization) of Na₂CO₃ with HCl.
• (B) Litmus: Litmus changes color over a broad range (red below 4.5, blue above 8.3), which is not sharp enough for precise titration, although it indicates general acidity/basicity.
• (C) Phenolphthalein: Its pH range (8.2-10.0) is suitable for the first equivalence point of Na₂CO₃ (carbonate to bicarbonate), but not for complete neutralization.
• (D) Potassium ferricyanide: This is a redox indicator or a precipitating agent in some titrations, not a common acid-base indicator for this type of titration.

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