Primary Standard is not possible all time to prepare a volumetric solution of desired theoretical concentration. A solution can be prepared at an approximate concentration standardized with the help of primary standard. The concentration (molarity/normality) obtained after standardization is used for further quantitative analysis. Volumetric solutions should not differ by >10% from the prescribed limit.
A primary standard is a highly pure stable compound preferably used for the standardization of less stable secondary standard compounds.
A primary standard compound should satisfy the following requirement:
- It must be readily available at less cost.
- It should be chemically stable.
- It should be almost 100% pure and in dry form.
- The total amount of impurities should not exceed 0.01-0.02%.
- It should be stable at drying temperature and should be dried before weighing.
- It should not be hygroscopic.
- It should not be oxidized in presence of air and remain stable in the air during weighing.
- It should not be affected by carbon dioxide.
- It should be readily soluble under experimental conditions.
- The reaction with a standard solution should be stoichiometric and practically instantaneous (i.e. must have a high rate of forward reaction than backward reaction).
- The titration error with it should be negligible.
In practice, it is very difficult to obtain an ideal primary standard and have to compromise with the above requirements.
Primary Standard Examples:
1. Primary standard used in neutralization titration: (a) For acidic titrant: Anhydrous sodium carbonate. (b) For basic titrant: Potassium hydrogen phthalate, benzoic acid, oxalic acid.
2. Primary standard used in redox titration: (a) As oxidants: Potassium dichromate, potassium bromated, potassium iodate. (b) As reluctant: Oxalic acid, sodium oxalate, arsenic trioxide.
3. Primary standard used in complexometric titration: Pure metallic zinc, pure metallic magnesium, zinc chloride, calcium chloride.
4. Primary standard used in precipitation titration: Sodium chloride, potassium chloride, potassium bromide.
A secondary standard is a less stable compound generally used for quantitative analysis, and its concentration can be found by comparison with the primary standard. i.e. a titration with a primary standard is carried out and the molarity or normality of the secondary standard is determined. The strength of the secondary standard must be determined before the actual quantitative analysis of any pharmaceutical compound.
Secondary Standard Examples:
1. Secondary standard used in neutralization titration: (a) Acidic Titrant: Hydrochloric acid, sulphuric acid (b) Basic Titrant: Sodium hydroxide,
2. Secondary standard used in non-aqueous titration: (a) Acidic Titrant: Perchloric acid, p-toluenesulfonic acid, 2,4-dinitrobenzene sulfonic acid. (b) Basic Titrant: Tetrabutylammonium hydroxide, sodium acetate, potassium methoxide, sodium amino ethoxide.
3. Secondary standard used in precipitation titration: Silver nitrate, Ammonium thiocyanate, Potassium thiocyanate, Potassium cyanate, Mercuric nitrate.
4. Secondary standard used in complexometric titration: Ethylenediamine tetraacetic acid, lead nitrate.
5. Secondary standards used in redox titration: Potassium permanganate, Potassium dichromate, Iodine solution, and Cerric ammonium sulfate.
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