Sulfurous acid

• Sulfur (IV) - acid
• Sulphurous acid
• Dihydrogensulfit (IUPAC)
• Dihydrogen sulfate (IV )

Colorless liquid with a pungent odor of sulfur dioxide

Liquid

PKS1 1.81; PKS2 6.99

Completely miscible with water, only stable in solution

Risk

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The Sulphurous acid (named after the nomenclature of the IUPAC Dihydrogensulfit ) is an unstable, existing only in aqueous solution, dibasic acid produced in water when releasing sulfur dioxide; their salts and esters are called sulphites and hydrogen sulphites.

Properties

Chemical Properties

In aqueous solution, an equilibrium between sulfur dioxide and sulphurous acid is present:

The equilibrium of the reaction lies far to the left.

If you try to evaporate the solution to obtain anhydrous sulphurous acid, so these decays in a reversal of the formation reaction. On cooling from a clathrate SO2 · H2O 5.75, which decomposes again at 7 ° C. Free Sulfurous acid H2SO3 is not isolated.

The proteolysis of sulfurous acid proceeds in two stages. In the first stage, hydrogen sulfite forms:

The acidity constant is determined KS1 (analogous to carbonic acid) formally the sum of the concentrations of dissolved sulfur dioxide and sulfur dioxide:

The actual acid strength, however, lies much higher, as well as present in aqueous solutions as no H2SO3 molecules. For the second stage of the protonolysis ( KS2) the following applies:

Puts you sulfurous acid with bases, metal oxides or carbonates, such crystallize with the evaporation of the solutions, the salts of sulphurous acid, sulphites, from.

The hydrogen sulfites can be crystallized only with large cations, otherwise arise disulfites. In the solid, the bisulfite anion is present in the constitution of a sulfonate, the residual hydrogen is bound to the sulfur.

Tautomerism

Sulfur dioxide in water forms two monovalent anions tautomeric, the bisulfite and sulfonate ion with the structures and HSO3 - SHO3, can be derived as Deprotonierungsprodukte of sulfurous acid or sulfonic acid.

Biological Significance

Sulfuric acid is also a contributor to acid rain since the sulfur dioxide from the combustion of fossil fuels (such as coal, petroleum, petroleum products) or biomass formed by the rain water from the earth's atmosphere "washed " (see also: smog). To prevent the sulfur from escaping to the environment, there are various methods for flue gas desulfurization.

Use

Sulfurous acid and their salts are a reducing agent, since they are oxidized to form sulfuric acid or sulphate through inclusion of an additional oxygen atom. In aqueous solution, sulfur dioxide is oxidized by atmospheric oxygen to form sulfuric acid.