What is a Primary Standard?
Primary standard is a compound of sufficient purity from which standard solutions of known normalities can be prepared by direct weighing of it and diluting to a defined volume of solution.
Sodium carbonate Na2CO3
Sodium borate Na2B4O7
Potassium hydrogen iodate KH(IO3)2
Pure metals and their salts like Zn, Mg, Cu, Mn, Ag, AgNO3 , NaCl, KCl, KBr – used in Acid base titrations
K2Cr2O7, KBrO3, KIO3, KI(IO3)2, NaC2O4, As2O3, pure iron – used in redox titrations
Eligibility criteria for a primary standard
A primary standard should satisfy the following conditions
- should be very pure
- should neither be deliquescent (absorbing moisture) nor efflorescent (losing water)
- should have high molecular weight so that weighing errors can be minimised
- should be chemically stable
- shall be readily soluble under given conditions
- it should react stoichiometrically
For more information one can read Benteley and Driver’s Textbook of Qualitative chemical analysis