In the titration of `NaHCO_(3)` with HCl, indicator cannot be used

Options

A. Methyl orange
B. Methyl red
C. Phenolphthalein
D. All of these

Answer

To determine which indicator cannot be used in the titration of sodium bicarbonate (NaHCO₃) with hydrochloric acid (HCl), we need to analyze the properties of the acid and base involved and the pH ranges of the indicators provided.

### Step-by-Step Solution:

1. **Identify the Nature of the Reactants:**
- Sodium bicarbonate (NaHCO₃) is a weak base.
- Hydrochloric acid (HCl) is a strong acid.

2. **Understand the Titration Type:**
- The titration involves a weak base (NaHCO₃) and a strong acid (HCl). This means the equivalence point of the titration will be at a pH less than 7.

3. **Examine the Indicators:**
- **Methyl Orange:** This indicator has a pH range of approximately 3.1 to 4.4. It changes color in the acidic range and can be used for titrations involving strong acids and weak bases.
- **Methyl Red:** This indicator has a pH range of about 4.4 to 6.3. It is also suitable for titrations between strong acids and weak bases.
- **Phenolphthalein:** This indicator has a pH range of approximately 8.0 to 10.0. It changes color in the basic range and is typically used for titrations involving strong acids and strong bases.

4. **Determine the Appropriate Indicator:**
- Since the titration of NaHCO₃ with HCl results in a solution that is acidic (due to the strong acid), we need an indicator that works in the acidic pH range.
- Methyl orange and methyl red are suitable for this titration as they change color in the acidic range.
- Phenolphthalein, however, is not suitable because it is used for titrations that reach a pH above 8, which will not occur in this case.

5. **Conclusion:**
- The indicator that cannot be used in the titration of NaHCO₃ with HCl is **Phenolphthalein** (Option C).

### Final Answer:
**C. Phenolphthalein**