Which is the best choice for weak base-strong acid titration ?

Options

A. Methyl red
B. Litmus
C. Phenol red
D. Phenolphthalein

Explanation

To determine the best choice of indicator for a weak base-strong acid titration, we can follow these steps:

### Step 1: Identify the Weak Base and Strong Acid
- **Weak Base**: Ammonium hydroxide (NH₄OH)
- **Strong Acid**: Hydrochloric acid (HCl)

### Step 2: Determine the pH Range of the Weak Base and Strong Acid
- The pH range of a weak base like ammonium hydroxide is approximately **8 to 11**.
- The pH range of a strong acid like hydrochloric acid is approximately **1 to 4**.

### Step 3: Find the pH Range at the Equivalence Point
- In a titration of a weak base with a strong acid, the equivalence point will occur at a pH that is lower than 7 due to the formation of a weak conjugate acid.
- The average pH at the equivalence point will typically be around **4 to 6**.

### Step 4: Evaluate the Indicators
Now, we will evaluate the given indicators based on their pH transition ranges:
1. **Methyl Red**: pH range 4.8 to 6.0
2. **Litmus**: pH range 4.3 to 8.3
3. **Phenol Red**: pH range 6.8 to 8.2
4. **Phenolphthalein**: pH range 8.2 to 10.0

### Step 5: Select the Appropriate Indicator
- The indicator should change color around the pH range of the equivalence point (4 to 6).
- **Methyl Red** is the best choice as it transitions in the pH range of 4.8 to 6.0, which falls within the expected pH range at the equivalence point.
- Other indicators like phenolphthalein and phenol red are not suitable as they transition at higher pH values (above 7), which is not appropriate for a weak base-strong acid titration.

### Conclusion
The best choice for a weak base-strong acid titration is **Methyl Red**.

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