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 for a weak base-strong acid titration, we need to understand the nature of the reactants and the appropriate indicators for the resulting solution.

### Step-by-Step Solution:

1. **Identify the Nature of the Reactants**:
- A **strong acid** is an acid that completely dissociates in solution to produce hydrogen ions (H⁺). Examples include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).
- A **weak base** is a base that does not completely dissociate in solution to produce hydroxide ions (OH⁻). Examples include ammonia (NH₃) and methylamine.

2. **Reaction Between Strong Acid and Weak Base**:
- When a strong acid reacts with a weak base, the reaction produces a salt and water. The general reaction can be represented as:
\[ \text{Weak Base} + \text{Strong Acid} \rightarrow \text{Salt} + \text{Water} \]
- The resulting solution will be acidic, meaning the pH will be less than 7.

3. **Choosing the Right Indicator**:
- An indicator is a substance that changes color at a specific pH range. For titrations, it is essential to choose an indicator that changes color at the pH that corresponds to the equivalence point of the titration.
- In the case of a weak base-strong acid titration, the equivalence point will be in the acidic range (pH < 7).

4. **Evaluating Potential Indicators**:
- **Methyl Red**: Changes color from red to yellow in the pH range of 4.4 to 6.3. This range is suitable for detecting the endpoint of a weak base-strong acid titration.
- **Litmus**: Changes color around pH 4.5 to 8.3. This range is not ideal since it covers too broad a spectrum and does not specifically indicate the acidic endpoint.
- **Phenol Red**: Changes color in the pH range of 6.2 to 8.2. This is also not suitable as it does not cover the acidic range effectively.
- **Phenolphthalein**: Changes color from colorless to pink in the pH range of 8.3 to 10. This is not suitable for a weak base-strong acid titration as it indicates a basic solution.

5. **Conclusion**:
- Based on the analysis, **Methyl Red** is the best choice for a weak base-strong acid titration because it effectively indicates the acidic endpoint of the titration.

### Final Answer:
**Methyl Red is the best choice for weak base-strong acid titration.**

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