Standards are materials containing a precisely known concentration of a substance for use in quantitative analysis.
A standard provides a reference that can be used to determine unknown concentrations or to calibrate analytical instruments.
Primary Standards
A primary standard is a reagent that is extremely pure and stable; it not a hydrate/it has no water of hydration, and it has a high molecular weight.
Examples of primary standards for titration of acids are:
• sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol
• tris-(hydroxymethyl)aminomethane (TRIS or THAM): (CH2OH)3CNH2, mol wt. = 121.14 g/mol
Examples of primary standards for titration of bases are:
• potassium hydrogen phthalate (KHP): KHC8H4O4, mol wt. = 204.23 g/mol
• potassium hydrogen iodate: KH(IO3)2, mol wt. = 389.92 g/mol
Examples of primary standards for redox titrations are:
• potassium dichromate: K2Cr2O7, mol wt. = 294.19 g/mol
• sodium oxalate: Na2C2O4 mol wt. = 134.00 g/mol
Secondary Standards
A secondary standard is a standard that is prepared in the laboratory for a specific analysis.
It is usually standardized against a primary standard.
NIST Standard Reference Materials
The National Institute of Standards and Technology (NIST) provides a wide variety of standard reference materials (SRMs) for validating and calibrating analytical methods. Some examples of SRMs are:
1. For chemical composition
• elements in iron, steels, and other metal alloys
• sulfur in fossil fuels
• polychlorinated biphenyls (PCBs) in oils
• elements in foods and beverages (e.g. milk powder, wheat flour)
2. For physical properties
• strength and melt flow of polyethylene pipe
• radioactivity
• electrical resistivity of silicon
3. For engineering materials
• particle sizes
• magnetic computer storage media
• surface flammability