Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.
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.
A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight.
Some primary standards for titration of acids:
- sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol
- tris-(hydroxymethyl)aminomethane (TRIS or THAM): (CH2OH)3CNH2, mol wt. = 121.14 g/mol
Some primary standards for titration of bases:
- potassium hydrogen phthalate (KHP): KHC8H4O4, 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
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) provide 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 fow of polyethylene pipe
- electrical resistivity of Si
for engineering materials
For more info see the NIST SRM website.
- particle sizes
- magnetic computer storage media
- surface flammability
Copyright © 2000 by Brian M. Tissue, all rights reserved.