Primary standards are reagents that can involve in chemical reactions. These compounds are often used to determine the unknown concentration of a solution that can undergo a chemical reaction with the primary standard. Primary standards are defined via other quantities like length, mass and time.
Primary standards are used to calibrate other standards referred to as working standards. It is a reference chemical used to measure an unknown concentration of another known chemical. It can be used directly when performing titrations or used to calibrate standard solutions.