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Sagot :
To understand why oxalic acid ([tex]\(C_2H_2O_4 \cdot 2H_2O\)[/tex]) is considered a primary standard substance and potassium permanganate ([tex]\(KMnO_4\)[/tex]) is considered a secondary standard substance, it's important to grasp the characteristics that differentiate primary and secondary standard substances.
### Primary Standard Substances:
1. High Purity: Primary standard substances need to be of high purity.
2. Stability: They should remain stable over time without decomposing or absorbing moisture from the air (hygroscopicity).
3. Non-hygroscopic: They should not absorb water from the atmosphere or lose water when exposed to air.
4. Ease of Handling: They should be easy to handle and weigh accurately.
5. High Equivalence Weight: Typically, a high molar mass helps minimize weighing errors.
### Secondary Standard Substances:
1. Less Stability: Secondary standards may not be as stable as primary standards.
2. Variable Concentration: Their concentration can fluctuate over time or with environmental conditions.
3. Requires Standardization: They need to be standardized against a primary standard before use to ensure accurate concentrations.
### Why Oxalic Acid is a Primary Standard:
1. High Purity: Oxalic acid can be obtained in a very pure form.
2. Stability: It is stable at room temperature and does not react with atmospheric gases nor absorb moisture significantly.
3. Non-hygroscopic: It does not absorb water from the air, which makes it easy to weigh accurately.
4. Ease of Handling: It is easy to handle and can be weighed with a high degree of accuracy.
### Why Potassium Permanganate is a Secondary Standard:
1. Reactivity and Instability: Potassium permanganate is reactive and can decompose over time, especially in the presence of light, heat, or contaminants.
2. Hygroscopic Nature: It can absorb moisture from the air, which changes its concentration.
3. Oxidizing Agent: As a strong oxidizing agent, it can react with other substances in the environment, which can alter its concentration.
4. Requires Standardization: Because of its instability and variability in concentration, it needs to be standardized against a primary standard substance before use in titrations to ensure accurate results.
In conclusion, oxalic acid is a primary standard substance due to its high purity, stability, non-hygroscopic nature, and ease of accurate weighing and handling. On the other hand, potassium permanganate is a secondary standard substance because its concentration cannot be determined accurately by simple weighing and dissolving due to its reactivity, potential decomposition, and hygroscopic nature.
### Primary Standard Substances:
1. High Purity: Primary standard substances need to be of high purity.
2. Stability: They should remain stable over time without decomposing or absorbing moisture from the air (hygroscopicity).
3. Non-hygroscopic: They should not absorb water from the atmosphere or lose water when exposed to air.
4. Ease of Handling: They should be easy to handle and weigh accurately.
5. High Equivalence Weight: Typically, a high molar mass helps minimize weighing errors.
### Secondary Standard Substances:
1. Less Stability: Secondary standards may not be as stable as primary standards.
2. Variable Concentration: Their concentration can fluctuate over time or with environmental conditions.
3. Requires Standardization: They need to be standardized against a primary standard before use to ensure accurate concentrations.
### Why Oxalic Acid is a Primary Standard:
1. High Purity: Oxalic acid can be obtained in a very pure form.
2. Stability: It is stable at room temperature and does not react with atmospheric gases nor absorb moisture significantly.
3. Non-hygroscopic: It does not absorb water from the air, which makes it easy to weigh accurately.
4. Ease of Handling: It is easy to handle and can be weighed with a high degree of accuracy.
### Why Potassium Permanganate is a Secondary Standard:
1. Reactivity and Instability: Potassium permanganate is reactive and can decompose over time, especially in the presence of light, heat, or contaminants.
2. Hygroscopic Nature: It can absorb moisture from the air, which changes its concentration.
3. Oxidizing Agent: As a strong oxidizing agent, it can react with other substances in the environment, which can alter its concentration.
4. Requires Standardization: Because of its instability and variability in concentration, it needs to be standardized against a primary standard substance before use in titrations to ensure accurate results.
In conclusion, oxalic acid is a primary standard substance due to its high purity, stability, non-hygroscopic nature, and ease of accurate weighing and handling. On the other hand, potassium permanganate is a secondary standard substance because its concentration cannot be determined accurately by simple weighing and dissolving due to its reactivity, potential decomposition, and hygroscopic nature.
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