|
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 . Primary StandardsA 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/moltris-(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 Secondary StandardsA 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) provide a wide variety of standard reference materials (SRMs) for validating and calibrating analytical methods. Some examples of SRMs: for chemical compositionelements 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 propertiesstrength and melt fow of polyethylene pipe radioactivity electrical resistivity of Si for engineering materialsparticle sizes magnetic computer storage media surface flammability For more info see the NIST SRM website .
|
|
See also: |
|
|
