Primary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working standards.[1][2]
[edit] In chemistry
Primary standards are used in analytical chemistry. Here, a primary standard is typically a reagent which can be weighed easily, and which is so pure that its weight is truly representative of the number of moles of substance contained. Features of a primary standard include:
- High purity
- Stability (low reactivity)
- Low hygroscopicity and efflorescence
- High solubility (if used in titration)
- High equivalent weight
- Non-toxicity
- Ready and cheap availability
- Eco-friendliness
(The last three are not as essential as the first five.)
Some examples of primary standards according to the European Pharmacopoeia 5, ch. 4.2:
- Arsenic trioxide for making sodium arsenite solution for standardisation of sodium periodate solution (until Ph. Eur. 3, Appendix 2001 also for iodine and cerium(IV) sulfate solutions, since Ph. Eur. 4, 2002 standardised by sodium thiosulfate)
- Benzoic acid for standardisation of waterless basic solutions: ethanolic sodium and potassium hydroxide, TBAH, and alkali methanolates in methanol, isopropanol, or DMF
- Potassium bromate (KBrO3) for standardisation of sodium thiosulfate solutions
- Potassium hydrogen phthalate (usually called KHP) for standardisation of aqueous base and perchloric acid in acetic acid solutions
- Sodium carbonate for standardisation of aqueous acids: hydrochloric, sulfuric acid and nitric acid solutions (but not acetic acid)
- Sodium chloride for standardisation of silver nitrate solutions
- Sulfanilic acid for standardisation of sodium nitrite solutions
- Zinc powder, after being dissolved in sulfuric or hydrochloric acid, for standardisation of EDTA solutions
Primary standards are used in titration and are essential for determining unknown concentrations.[1]
[edit] References
- ^ a b Skoog, Douglas A., Donald M. West and F. James Holler. "Fundamentals of Analytical Chemistry 8th ed." Harcourt Brace College Publishers. 1995 ISBN 0030355230
- ^ Holt Science and Technology: Physical Science. Ed. Rinehart and Winston, Inc. Holt. Holt McDougal (July 2000). ISBN 9780030519574.
[edit] External links
- Analytical Standards. Department of Chemistry, University of Adelaide, Australia.
