Standard Solution
Solutions of accurately known strength are called standard solutions. A standard solution contains a known weight of reagent in a definite volume of solution.
A standard solution is prepared by dissolving an accurately weighed quantity of a highly pure substance in a definite volume of solvent.
Types of Standard Solution
1. Primary Standard
2. Secondary Standard
Primary Standard
A primary standard is a chemical or reagent which has certain properties such as
1. It is extremely pure
2. Highly stable
3. It is anhydrous
4. It is less hygroscopic
5. Has very high molecular weight
6. Can be weighed easily
7. Should be ready to use and available
8. Should be preferably non toxic
9. Should not be expensive
1. It is Extremely Pure
Primary standard material should be extremely pure which means that, it should be a chemical of high grade of purity, preferably 99.98%.
2. Highly Stable
It should be highly stable which means it usually does not react easily when kept in its pure form or in other words it should have very low reactivity.
Importance:
Because if a reagent reacts easily with atmospheric oxygen or water or changes its property over time then it is unreliable chemical to use as primary standard.
3. It is Anhydrous
It should be anhydrous which means that it does not contain any water molecule in its molecular structure.
e.g. magnesium sulphate (MgSO4), is found with formula MgSO4.7H2O. Therefore, to prepare primary standard solution of MgSO4, anhydrous MgSO4 preferably of analytical reagent grade with purity greater than 99.98% will be required.
4. It is Less Hygroscopic
The chemical preferably should be less hygroscopic that is on opening the container it should not absorb water molecules from atmosphere.
5. Has Very High Molecular Weight
It should have very high molecular weight to minimize weighing errors.
Examples of Primary Standard
These are materials which, after drying under the specified conditions, are recommended for use as primary standards in the standardization of volumetric solutions.
Acid - Base Titrations: Potassium Hydrogen Phthalate (C8H5KO4), Anhydrous Sodium Carbonate (Na2CO3)
Redox titrations: Arsenic Trioxide (As2O3), Sodium Oxalate (Na2C2O4), Potassium bromate (KBrO3)
Precipitation titrations: Sodium Chloride (NaCl)
Uses of Primary standard
1. Used to standardize a volumetric solutions.
2. They are used for standardization of titration solutions.
3. They are used for calibration of secondary standards.
4. Used as reference to determine unknown concentrations or to calibrate analytical instruments.
Secondary Standard
A secondary standard is a solution which contain exactly known amount of the substance in unit volume of the solution which is determined by titrating against a primary standard.
OR
A secondary standard solution is a solution in which the concentration of dissolved solute has been determined by reaction (titration) with a primary standard solution.
Secondary Standard Properties
A secondary standard is a chemical or reagent which has certain properties such as:
1. It has less purity than primary standard
2. Less stable and more reactive than primary standard but its solution remains stable for a long time
3. Titrated against primary standard
Uses of Secondary Standards
Secondary standards are commonly used to calibrate analytical equipment and analytical techniques.
Used in titration.
