Primary Standard
A highly pure, stable substance that can be weighed accurately and used directly to prepare a solution of known concentration.
Unit
g/mol
In depth
A primary standard must be of high purity, stable in air, non-hygroscopic, of high molar mass to minimise weighing error, and readily soluble. Very few substances qualify, which is why most reagents are standardised against one rather than prepared by direct weighing.
Examples in the real world
Potassium hydrogenphthalate for alkalis, sodium carbonate for acids, and potassium dichromate for reductants.