It is a solution having accurately known concentration. The reagent required to prepare a standard solution is called standard reagent.
Standard reagent or standard solution are of two types. They are:
1. Primary standard
A reagent which is pure, stable, non-hydroscopic has relatively high molecular mass and remains in a solution without undergoing any change in its concentration even if it is stored for a long time in the lab is called primary standard reagent.
A solution of known concentration prepared from primary standard reagent is called primary standard solution.
Eg. Anhydrous Na2CO3, H2C2O4, 2H2O, etc.
The concentration of primary standard solution is fixed directly by weighing a required amount of reagent and dissolving it in a given volume of the solution.
2. Secondary standard
A reagent which is either impure, less stable, hygroscopic has relatively low molecular mass or undergoes slow change in its concentration from the solution if it is stored in a lab for a long time is called secondary standard reagent.
A solution of known concentration prepared from secondary standard reagent is called secondary standard solution.
Eg. NaOH, HCl, KMnO4, etc.
The concentration of secondary standard is fixed indirectly with the help of suitable primary standard solution by means of simple.