Primary standard

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 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

Source

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