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, stable, it not a hydrate/has no water of hydration, and 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 fow of polyethylene pipe
electrical resistivity of silicon

3. For engineering materials
particle sizes
magnetic computer storage media
surface flammability

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