Friday, November 5, 2010

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.

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