Disulfuric acid
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| General | ||||||||||||||||
| Surname | Disulfuric acid | |||||||||||||||
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| Molecular formula | H 2 S 2 O 7 | |||||||||||||||
| Brief description |
colorless, hygroscopic crystals |
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| properties | ||||||||||||||||
| Molar mass | 178.14 g mol −1 | |||||||||||||||
| Physical state |
firmly |
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| density |
1.9 g cm −3 |
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| Melting point |
35 ° C |
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| solubility |
violent reaction with water |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||||||||
Disulfuric acid (also pyrosulfuric acid ) is an oxo acid of sulfur and is one of the inorganic acids . It is derived from sulfuric acid and can be represented formally as the agglomeration of two sulfuric acid molecules with the elimination of water. Disulfuric acid is highly hygroscopic and highly reactive, especially with water and organic materials, there are very violent reactions. The salts of disulphuric be disulfates called or pyrosulfate. They can be obtained by heating hydrogen sulfates with elimination of water.
Disulfuric acid is often equated with oleum, but formally this is only correct for 45% oleum. Oleum (also called fuming sulfuric acid) is a solution of sulfur trioxide (SO 3 ) in sulfuric acid (H 2 SO 4 ) with variable proportions of sulfur trioxide, which, depending on the concentration, is in the concentration range up to 45% as a mixture of sulfuric acid and disulfuric acid . The concentrations given for oleum mean the proportion of “free” SO 3 based on the amount of oleum.
However, in various concentrations, oleum is the predominant form of disulfuric acid in the trade, which is why this article deals with this mixture of substances.
Extraction and presentation
On an industrial scale, disulfuric acid is obtained in the form of oleum in the production of sulfuric acid by the contact process . In order to remove the intermediate product sulfur trioxide from the reaction equilibrium in accordance with the law of mass action , this intermediate absorption is carried out in two stages during oleum production. In the first stage, some of the SO 3 is absorbed in oleum, which is diluted to the required oleum concentration with concentrated sulfuric acid. In the second stage of the intermediate absorption, the remaining SO 3 is absorbed in concentrated sulfuric acid. Here one uses the property of SO 3 to dissolve well in concentrated sulfuric acid.
The product is a colorless, oily (actually: heavy, viscous) liquid in which sulfuric acid, disulfuric acid and trisulfuric acid molecules are present. The ratio of the components (SO 3 and H 2 SO 4 ) determines the molecular composition and the physicochemical properties:
- density
- 20% SO 3 : 1.90 g / cm 3
- 80% SO 3 : 1.94 g / cm 3
- Melting point
- 20% SO 3 : −11 ° C
- 45% SO 3 : 35 ° C
properties
Chemical properties
Disulfuric acid is highly hygroscopic and, in the presence of water, decomposes very quickly and with strong heat development to sulfuric acid ( hydrolysis ) and as such has a strong acidic reaction. This reaction already occurs with the water present in the air, as a result of which a thick mist of sulfuric acid droplets is formed. This is the case with oleum, for example, which is why it is also called “fuming sulfuric acid”.
safety instructions
Disulfuric acid and oleum are very dangerous. They are very caustic and react very violently, especially with organic substances. For example, strong dehydration occurs on contact with skin or clothing, even with the breakdown of molecules such as carbohydrates . This causes the affected area to turn yellow to black. Due to the intense heat generated during hydrolysis, organic material can easily ignite and even explode. Oleum reacts so violently with water that it can only be diluted by slowly pouring it onto ice. In addition, unlike sulfuric acid, it can even destroy plastic containers and neoprene gloves. Escaping sulfur trioxide vapors represent an additional risk .
Individual evidence
- ↑ a b c Entry on disulfuric acid. In: Römpp Online . Georg Thieme Verlag, accessed on November 10, 2014.
- ↑ Brockhaus ABC Chemie, FA Brockhausverlag Leipzig 1971, p. 306.
- ↑ a b In the absence of official safety instructions, those for oleum are given: Entry on oleum in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .