Definition of Standards

What is a Standard?

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

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