By standard solution we meant a solution of known strength that is a solution which contains a known weight of the solute present in a known volume of solvent. Thus, if 53 gms of Na2CO3 is dissolved in water and the volume is made up to 1000 cc, So, we get a standard solution.
Primary Standard Solution:
If a substance is available in a sufficiently pure state and if a solution of definite normality can be prepared simply by weighting out a definite weight of it and dissolving it in a definite volume of water (solvent). The substance is called Primary Standard Solution. It is generally stable and they retain its correct strength in storage.
Example: Na2CO3, Na2C2O4 (Sodium Oxalate), etc.
Secondary Standard Solution:
When a substance is not available in a sufficiently pure state and hence its standard solution can’t be prepared by directly weighing the substance, the substance is called Secondary Standard Solution. These solutions can be unstable and lose their strength in storage.
Example: NaOH, KOH, HCl, H2SO4, etc.