Differences Between Primary and Secondary standards

Primary and Secondary standards

 Differences Between Primary and Secondary standards.

Primary Standards

The substance of high purity used in the preparation of standard solution is called as a primary standard.
Following requirements

• It must be easy to obtain, to purity, to dry, and stable under storage condition.
• It must be readily soluble in water.
• Impurities should not exceed 0.01-0.02 (99.98-99.99%pure).
• It should have high equivalent weight so that weighing error may be negligible.
• It should have a high equivalent weight.
• Titration error should be negligible.
• Uses simple indicators to determine the endpoint.
• They are analytical grade reagents.

Examples are

1. Acid-base titration

Sodium carbonate (borax: primary standard).

2. Oxidation-reduction

Potassium dichromate, Pot. bromate, Pot. iodate.

3. Reduction

Oxalic acid, Sodium oxalate.

4.Precipitation

Silver nitrate (KCl, NaCl primary standard).

5.Complexometric

ZnCl, calcium chloride.

Secondary Standards

A substance that is not a primary standard in nature is called a secondary standard.
Examples are
1.Alkali solution (NaOH, KOH)
2.Inorganic acid(H2SO4)
3.Deliquescent substance liquid.

Secondary standard reagents require standardization by using primary standards.