Primary standard
A volumetric standard or primary standard, also briefly called primary standard, is a good ponderable Reinstsubstanz that is suitable for the preparation of solutions of known content ( Urtiterlösungen ). These solutions are then used to determine the content of the standard solution used in volumetric analysis ( = standard solutions ) directly or via a set with them secondary standard.
Features an ideal primary standards
Ideally, a primary standard has the following properties:
- Their composition corresponds exactly to their formula.
- It reacts stoichiometrically, that is, completely according to the reaction equation.
- They will keep indefinitely, that is, also inert to degradation and transformation in the air. It is thus insensitive to the atmospheric oxygen.
- It does not react with the humidity and is not hygroscopic.
- But you also emits no water of crystallization, is not possible even when stored above room temperature.
- It does not react with the carbon dioxide in the air.
Examples of primary standards
The following eight substances are the primary standards according to pharmacopoeia:
- Arsenic (III ) oxide, As2O3 is set for the production of sodium arsenite solution ( addition of sodium hydroxide ), with the sodium hydroxide solution ( to Ph. Eur 3, Supplement 2001 also for adjusting iodine and cerium (IV) standard solutions that are since Ph. Eur 4 (2002) adjusted with sodium thiosulfate, a secondary standard )
- Benzoic acid C6H5COOH for setting anhydrous bases: ethanolic sodium hydroxide, TBAH Alkalimethanolat and standard solution in isopropanol or methanol.
- Potassium bromate KBrO3 for setting of sodium thiosulfate standard solution
- Potassium hydrogen phthalate C6H4 (COOH ) (COO -K ) for setting of anhydrous perchloric acid in glacial acetic acid standard solutions
- Na2CO3 sodium carbonate to adjust hydrochloric acid, nitric acid and sulfuric acid standard solutions (but no acetic acid, which is adjusted with NaOH )
- Sodium chloride NaCl ( saline ) for setting standard silver nitrate solution
- Sulfanilic acid H2N -C6H4 -SO3H for adjusting sodium nitrite standard solution
- Zinc Zn0 for the recruitment of EDTA standard solutions according to quantitative conversion to Zn2 with hydrochloric acid or sulfuric acid
Depending on the application, in particular depending on the desired accuracy, the following substances may be used as primary standard:
- Sodium bicarbonate NaHCO 3 to adjust hydrochloric acid, nitric acid and sulfuric acid
- Potassium dichromate K2Cr2O7 in the Oxidimetrie
- Potassium bicarbonate KHCO3 for setting acids
- Potassium iodate KIO3 for setting of sodium thiosulfate standard solution
- Sodium oxalate Na2C2O4 for adjustment of potassium permanganate
- Oxalic acid C2H2O4 for the setting of alkalis or in the permanganometry. It is used as a non-hygroscopic dihydrate, but the crystal water content may not be given exactly.
- Mercury (II ) oxide, HgO, is reacted with iodide ( HgO 4 I- H2O → [ HgI4 ] 2 - 2 OH ) for the setting of acids.
- Of potassium C8H5KO4 in acidimetry
Examples of suitable substances not as a primary standard
Hydroxides such as sodium hydroxide and potassium hydroxide are as primary standard is not suitable because they are hygroscopic and absorb CO2 from the air, forming bicarbonates. Potassium permanganate is not suitable because it autocatalytically converts to Braunstein.