Primary standard

A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. 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. See Hierarchy of Standards.

In chemistry

Standards are used in analytical chemistry. In this field, primary standard is typically a reagent that can be weighed easily, and which is so pure that its weight is the number of moles of substance contained. Features of a primary standard include:

High purity
Stability
Low hygroscopicity
High equivalent weight
Long lasting molar solution i.e. concentration remains unchanged for long periods of time

Some examples of primary standards for titration of solutions, based on their high purity, are provided:

Arsenic trioxide for making sodium arsenite solution for standardisation of sodium periodate solution
Benzoic acid
Potassium bromate and potassium iodate
Potassium hydrogen phthalate for standardisation of aqueous base
Sodium carbonate for standardisation of aqueous acids: hydrochloric, sulfuric acid and nitric acid solutions
Sodium chloride for standardisation of silver nitrate solutions
Sulfanilic acid for standardisation of sodium nitrite solutions
potassium ferrocyanide, a reductant
potassium dichromate, an oxidant

Such standards are often used to make standard solutions. These primary standards are used in titration and are essential for determining unknown concentrations or preparing working standards.