Primary substance
A primary titer substance or primary standard , also called primary titer for short, is an extremely pure substance that can be weighed well and is suitable for the production of solutions with a precisely known content (primary titer solutions). These solutions then serve to determine the content of the standard solution (= standard solutions ) used in the dimensional analysis , either directly or via a secondary standard set with them .
Properties of ideal primary titre substances
Ideally, a primary substance has the following properties:
- Their composition corresponds exactly to their formula.
- It reacts stoichiometrically, i.e. H. completely according to the reaction equation.
- It can be kept indefinitely, i. H. also inert to decomposition and transformation in air.
- So it is insensitive to atmospheric oxygen.
- It does not react with the humidity and is not hygroscopic .
- However, it also does not give off any water of crystallization, if possible not even when stored above room temperature.
- It does not react with the carbon dioxide in the air.
- It has a large molar mass and also a high equivalent mass , so that the relative weighing error remains small.
- It is easily soluble in the desired solvent, mostly water , but sometimes also in acid or organic solvent.
- A long shelf life for the solutions produced is not absolutely necessary, but is desirable.
Examples of primary titer substances
The following eight substances are the basic titre substances according to the Pharmacopoeia :
- Arsenic (III) oxide As 2 O 3 for the production of sodium arsenite standard solution (addition of sodium hydroxide solution), with which sodium periodate standard solution is adjusted (up to Ph. Eur. 3, supplement 2001 also for adjusting iodine and cerium (IV) -Measured solutions that have been set with sodium thiosulphate , a secondary standard , since Ph. Eur. 4 (2002) )
- Benzoic acid C 6 H 5 COOH to adjust anhydrous bases: ethanolic sodium hydroxide solution , alkali methoxide and TBAH standard solution in isopropanol or methanol .
- Potassium bromate KBrO 3 for adjusting sodium thiosulphate standard solution
- Potassium hydrogen phthalate C 6 H 4 (COOH) (COO - K + ) for setting anhydrous perchloric acid standard solutions in glacial acetic acid
- Sodium carbonate Na 2 CO 3 to adjust hydrochloric acid , nitric acid and sulfuric acid standard solutions (but not acetic acid , which is adjusted with NaOH )
- Sodium chloride NaCl (common salt) for adjusting silver nitrate standard solution
- Sulphanilic acid H 2 N – C 6 H 4 –SO 3 H for adjusting sodium nitrite standard solution
- Zinc Zn 0 for the adjustment of EDTA standard solutions after quantitative conversion to Zn 2+ with hydrochloric acid or sulfuric acid
Depending on the purpose of use, in particular depending on the desired accuracy, the following substances can also serve as primary titers:
- Ammonium cerium (IV) nitrate for redox titrations
- Sodium hydrogen carbonate NaHCO 3 for adjusting hydrochloric acid , nitric acid and sulfuric acid
- Potassium dichromate K 2 Cr 2 O 7 in oxidimetry
- Potassium hydrogen carbonate KHCO 3 for adjusting acids
- Potassium iodate KIO 3 for adjusting sodium thiosulphate standard solution
- Sodium oxalate Na 2 C 2 O 4 to adjust potassium permanganate
- Oxalic acid C 2 H 2 O 4 for the setting of alkalis or in permanganometry. It is used as a non-hygroscopic dihydrate, but the water of crystallization content may not be exactly given.
- Mercury (II) oxide HgO, reacted with iodide (HgO + 4 I - + H 2 O → [HgI 4 ] 2− + 2 OH - ) for the adjustment of acids.
- Potassium hydrogen phthalate C 8 H 5 KO 4 in acidimetry
Examples of substances that are not suitable as primary titers
Hydroxides such as sodium hydroxide and potassium hydroxide are unsuitable as primary titers because they are hygroscopic and absorb CO 2 from the air, thereby forming hydrogen carbonates. Potassium permanganate is not suitable because it converts itself into manganese dioxide in an autocatalytic manner .
literature
- G. Jander, KF year, G. Schulze: measure analysis. 16th edition, de Gruyter, Berlin 2003, ISBN 3-11-017098-1 .
Individual evidence
- ↑ Entry on cerium compounds. In: Römpp Online . Georg Thieme Verlag, accessed on March 24, 2016.