Definition of Primary Standards

What are Primary Standards?

Standards are materials containing a known concentration of a substance. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.

In order to be used as a primary standard, a substance must meet four key criteria. It must:

  • be available with extremely high purity
  • be stable under the conditions it will be stored and used in
  • have no water of hydration and have no tendency to become hydrated - if it did, its weight could change with humidity changes
  • be of high molecular weight - this helps minimize the effect of small weighing errors

Making use of Standards

The accuracy of an analytical measurement is how close a result comes to the true value. Determining the accuracy of a measurement usually requires calibration of the analytical method with a known standard. This is often done with standards of several concentrations to make a calibration or working curve.

Some primary standards for titration of acids:

  • sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol
  • tris(hydroxymethyl)aminomethane (TRIS or THAM): (HOCH2)3CNH2, mol wt. = 121.14 g/mol
Some primary standards for titration of bases:
  • potassium hydrogen phthalate (KHP): C8H5KO4, mol wt. = 204.23 g/mol
  • potassium hydrogen iodate: KH(IO3)2, mol wt. = 389.92 g/mol
Some primary standards for redox titrations:
  • potassium dichromate: K2Cr2O7, mol wt. = 294.19 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:

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)
For physical properties
  • strength and melt flow of polyethylene pipe
  • radioactivity
  • electrical resistivity of Si
For engineering materials
  • particle sizes
  • magnetic computer storage media
  • surface flammability

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