Analytical Standards

Introduction

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 Standards

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

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) 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
• radioactivity
• electrical resistivity of Si

for engineering materials

• particle sizes
• magnetic computer storage media
• surface flammability