Primary standard

Primary standard

In metrology, a primary standard is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. A primary standard in chemistry is a reliable, readily quantified substance. Features of a primary standard include:

1. High purity
2. Stability (low reactivity)
3. Low hygroscopicity and efflorescence
4. High solubility (if used in titration)
5. High equivalent weight

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

External links

* [http://www.chemistry.adelaide.edu.au/external/soc-rel/content/standard.htm Analytical Standards] Primary standard is used in titration.

A primary standard must contain the following,1. high purity2. atmospheric stability3. absence of hydrate water4. modest cost5. reasonable solubility6. reasonably high molar mass

Source : Skoog, West, Holler, Crouch "Fundamentals of Analytical Chemistry" Brooks/ Cole. California, 2004

ource

*Skoog and West. Fundamentals of Analytical Chemistry. (3rd ed.) Holt, Rinehart, and Winston, 1976.


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