In the titration of `NH_(4)OH)` with HCl, the indicator which cannot be used is :

Options

A. Phenolphthalein
B. Methyl orange
C. Methyl red
D. Both methyl orange and methyl red

Explanation

To solve the question regarding the titration of ammonium hydroxide (NH₄OH) with hydrochloric acid (HCl) and determining which indicator cannot be used, we can follow these steps:

### Step 1: Identify the nature of the reactants
- Ammonium hydroxide (NH₄OH) is a weak base.
- Hydrochloric acid (HCl) is a strong acid.

### Step 2: Understand the reaction
- When NH₄OH is titrated with HCl, the reaction produces ammonium chloride (NH₄Cl), which is an acidic salt, and water.
- The overall reaction can be represented as:
\[ NH₄OH + HCl \rightarrow NH₄Cl + H₂O \]

### Step 3: Determine the pH at the equivalence point
- The equivalence point in this titration will occur when all the NH₄OH has reacted with HCl.
- Since NH₄Cl is formed, the solution will be acidic due to the presence of NH₄⁺ ions, which can hydrolyze to produce H⁺ ions.
- Therefore, the pH at the equivalence point will be less than 7.

### Step 4: Analyze the indicators
- We need to select an indicator that changes color in the acidic pH range (pH < 7).
- The indicators provided are:
1. **Phenolphthalein**: pH range 8.3 to 10 (basic range)
2. **Methyl Orange**: pH range 3.1 to 4.4 (acidic range)
3. **Methyl Red**: pH range 4.4 to 6.2 (slightly acidic range)

### Step 5: Determine which indicator cannot be used
- **Phenolphthalein** cannot be used because it only changes color in a basic environment (pH > 8.3) and will not show a color change in the acidic environment of the titration.
- **Methyl Orange** and **Methyl Red** can be used as they change color in the acidic range.

### Conclusion
- The indicator that cannot be used in the titration of NH₄OH with HCl is **Phenolphthalein**.

### Final Answer:
**Phenolphthalein** (Option 1) cannot be used as an indicator in this titration.

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