Out of the following the one which is not a primary standard is

Options

A. `K_(2)Cr_(2)O_(7)`
B. `KMnO_(4)`
C. Oxalic acid
D. Ceric sulphate

Answer

To determine which of the given substances is not a primary standard, we need to understand the characteristics of primary standard substances.

### Step-by-Step Solution:

1. **Definition of Primary Standard**:
- A primary standard is a reagent that is highly pure, stable, and non-hygroscopic. It should not react with the atmosphere (like absorbing moisture or carbon dioxide) and should be easy to obtain in a pure form.

2. **Evaluation of Options**:
- We need to analyze each option provided to identify which one does not meet the criteria of a primary standard.

3. **Analysis of K2Cr2O7 (Potassium Dichromate)**:
- K2Cr2O7 is a highly pure substance that does not absorb moisture from the air and is stable. Therefore, it qualifies as a primary standard.

4. **Analysis of KMnO4 (Potassium Permanganate)**:
- KMnO4 is known to be less stable as it can decompose and is also not available in pure form due to the presence of MnO2 impurities. Additionally, it can react with atmospheric components. Hence, it does not qualify as a primary standard.

5. **Analysis of Oxalic Acid**:
- Oxalic acid is a pure substance and does not change its concentration over time, making it a primary standard.

6. **Analysis of Sulfuric Acid**:
- Sulfuric acid can be considered a primary standard under certain conditions, as it is also highly pure and stable.

7. **Conclusion**:
- Based on the analysis, the substance that is not a primary standard is **KMnO4 (Potassium Permanganate)**.

### Final Answer:
The one which is not a primary standard is **KMnO4**.

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