Primary Standard

A primary standard is a compound of sufficient purity from which a standard solution can be prepared by direct weighing of a quantity of it, followed by dilution to give a defined volume of solution. A primary standard should satisfy the following requirements.

1. It must be easy to obtain, to purify, to dry, (preferably at 110-1200C), and to preserve in a pure state.
2. It should be unaltered in air during weighing (should not be hygroscopic), oxidized by air or affected by CO2. It should maintain an unchanged composition during storage.
3. It should have a high relative molecular mass so that the weighing errors may be negligible.
4. The substance must be readily soluble under the conditions in which it is employed.
5. 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.
6. The substance should be capable to being tested for impurities by qualitative and other tests.

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