Which of the following cannot be used as a test for `H_2O_2` ?

Options

A. A paper dipped in PbS (black) turns white when brought in contact with `H_2O_2`
B. It liberates iodine from Kl solution which gives blue coloure with starch solution
C. It gives blue colour with `K_4[Fe(CN)_6]`
D. It decolourises acidified `KMnO_4` solution

Answer

To determine which of the following cannot be used as a test for hydrogen peroxide (H₂O₂), we need to analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding H₂O₂**: Hydrogen peroxide is a compound that can act as both an oxidizing agent and a reducing agent. It has oxygen in the -1 oxidation state.

2. **Option A - PbS Test**:
- When lead sulfide (PbS) is treated with H₂O₂, it gets oxidized to lead sulfate (PbSO₄), which is white.
- Thus, a paper dipped in PbS that turns white upon contact with H₂O₂ indicates the presence of H₂O₂.
- **Conclusion**: This is a valid test for H₂O₂.

3. **Option B - Iodine Liberation Test**:
- H₂O₂ can oxidize potassium iodide (KI) to liberate iodine (I₂), which can then react with starch to produce a blue color.
- This indicates the presence of H₂O₂.
- **Conclusion**: This is also a valid test for H₂O₂.

4. **Option C - K₄[Fe(CN)₆] Test**:
- When H₂O₂ reacts with potassium ferrocyanide (K₄[Fe(CN)₆]), it does not produce a blue color. Instead, it oxidizes the iron from +2 to +3 oxidation state, resulting in a reddish color (K₃[Fe(CN)₆]) in acidic medium.
- There is no blue color produced in this reaction.
- **Conclusion**: This cannot be used as a test for H₂O₂.

5. **Option D - KMnO₄ Decolorization Test**:
- H₂O₂ can reduce potassium permanganate (KMnO₄), which is purple, to manganese dioxide (MnO₂) and becomes colorless in the process.
- This indicates the presence of H₂O₂.
- **Conclusion**: This is a valid test for H₂O₂.

### Final Answer:
The option that **cannot be used as a test for H₂O₂** is **C - K₄[Fe(CN)₆]**.