1. Properties of a primary standard

A substance used as a primary standard in titrimetric analysis should have these qualities:

• it should be available in pure form,
• it should be stable in air,
• it should be non-hygroscopic,
• it should have known composition,
• it should be readily soluble in the chosen solvent,
• it should react rapidly and stoichiometrically.

2. Check each statement

(A) It should be purely available in dry form.

This is correct. A primary standard should be pure and obtainable in dry form so that its mass can be measured accurately.

(B) It should not undergo chemical change in air.

This is correct. It must be stable in air and should not react with oxygen, carbon dioxide, or moisture.

(C) It should be hygroscopic and should react with another chemical instantaneously and stoichiometrically.

This is incorrect. Although it should react stoichiometrically and preferably quickly, a primary standard should not be hygroscopic. Hygroscopic substances absorb moisture from air, making accurate weighing difficult.

(D) It should be readily soluble in water.

This is correct. A primary standard should generally be readily soluble in water if aqueous titration is being carried out.

(E) $\mathrm{KMnO_4}$ and $\mathrm{NaOH}$ can be used as primary standard.

This is incorrect.

• $\mathrm{KMnO_4}$ is not a primary standard because it is usually contaminated with $\mathrm{MnO_2}$ and its solutions decompose on standing.
• $\mathrm{NaOH}$ is not a primary standard because it is hygroscopic and absorbs $\mathrm{CO_2}$ from air.

3. Incorrect statements

Thus, the incorrect statements are:

4. Match with options

Option A says: (C) and (E) only.

So the correct option is: