Question

Which of the following is not a primary standard?

• A. \( \text{Ammonia} \)
• B. \( \text{Na}_2\text{CO}_3 \)
• C. \( \text{Potassium hydrogen phthalate} \)
• D. \( \text{Sodium oxalate} \)

Hint:

Think of primary standards as solids you can weigh precisely. Ammonia is a gas, so weighing it accurately to make a standard solution is tricky.

Answer 1

The Correct Option is A

A primary standard is a highly purified compound that can be accurately weighed to prepare a solution of known concentration. It should have several key properties, including high purity, stability in air, absence of hydrates (or known stoichiometry of hydration), high molar mass (to minimize weighing errors), and solubility in the titration medium. - Ammonia (\( \text{NH}_3 \)): Ammonia is a gas at standard conditions, making it difficult to weigh accurately. Its solutions are also prone to changes in concentration due to volatility. Therefore, ammonia is not a primary standard. - Sodium carbonate (\( \text{Na}_2\text{CO}_3 \)): Sodium carbonate is a primary standard, although it can absorb moisture from the air. It is often dried before use. - Potassium hydrogen phthalate (KHP) (\( \text{KHC}_8\text{H}_4\text{O}_4 \)): KHP is a common primary standard for titrating bases. It is stable, non-hygroscopic, and has a high molar mass. - Sodium oxalate (\( \text{Na}_2\text{C}_2\text{O}_4 \)): Sodium oxalate is a primary standard used in redox titrations, particularly with potassium permanganate. It is stable and can be obtained in high purity. Thus, ammonia is not a primary standard due to its gaseous state and volatility.