Characteristics of primary standard substances:

A primary standard should satisfy the following requirements:

1. Purity:

It should be available in highly pure form. It should also preserve in a pure state.

2. Stability:

It should be stable (at 110˚c- 120˚c). There is no chemical change occurred.

3. Unaltered:

The substance should be unaltered in air during weighing, this condition implies that it should be hygroscopic, oxidized by air, or affected by CO₂.The standard should maintain an unchanged composition during storage.

4. Test procedure:

The substance should be capable of being tested for impurities by qualitative and other tests of known sensitivity. (The total amount of impurities should not exceed 0.01%-0.02%)

5. Molecular weight:

It should have a high molecular weight so that the weighing errors may be negligible.

6. Solubility:

It must be readily soluble in the solvent.

7. Titration error:

The reaction with the standard solution should be stoichiometric and practically instantaneous. The titration error should be negligible, or easy to determine accurately by experiment.