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
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
- potassium hydrogen phthalate (KHP): C8H5KO4, mol wt. = 204.23 g/mol
- potassium hydrogen iodate: KH(IO3)2, mol wt. = 389.92 g/mol
- potassium dichromate: K2Cr2O7, mol wt. = 294.19 g/mol
A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is usually standardized against a primary standard.
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:
- 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)
- strength and melt flow of polyethylene pipe
- electrical resistivity of Si
- particle sizes
- magnetic computer storage media
- surface flammability