Disulfite


A disulfite, commonly known as metabisulfite or pyrosulfite, is a chemical compound containing the ion. It is a colorless dianion that is primarily marketed in the form of sodium metabisulfite or potassium metabisulfite. When dissolved in water, these salts release the bisulfite ion. These salts act equivalently to sodium bisulfite or potassium bisulfite.

Structure

In contrast to disulfate, disulfite ion has an unsymmetrical structure with an S-S bond. The oxidation state of the sulfur atom bonded to 3 oxygen atoms is +5 while oxidation number of other sulfur atom is +3.
The anion consists of an SO2 group linked to an SO3 group, with the negative charge more localized on the SO3 end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å respectively.

Production

Salts of disulfite ion are produced by dehydration of salts of bisulfite ion. When solutions of sodium bisulfite or potassium bisulfite are evaporated, sodium metabisulfite and potassium metabisulfite result.
Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.
Dsulfite is the conjugate base of disulfurous acid, which originates from sulfurous acid in accordance with the dehydration reaction above:

The disulfite ion also arises from the addition of sulfur dioxide to the sulfite ion:
+ H+

SO32− + SO2

Use

Disulfites salts are used for preserving food and beverages.

Examples of disulphites