Sulphite

Structure of sulfite ion

The sulfites are the salts or esters of the hypothetical sulfurous acid H₂SO₃. Sulfite salts contain the anion SO₃^2-, the most important being sodium sulfite and magnesium sulfite. They are formed by contacting sulfur oxide (IV) (SO₂) with alkaline solutions. These are reducing substances passing sulfur from the oxidation state +IV to +VI. In acidic solutions they decompose, once again releasing sulfur oxide (IV).

Organic sulfites of the type R-O-S(=O)-O-R'(where R and R' are organic groups) are conveniently obtained from thionyl chloride (SOCl₂) and alcohols.

The sulfite ion is shaped like a trigonal pyramid with the sulfur atom in the center and the three oxygen atoms and one pair of electrons pointing to the corners.

Applications

Sulphites are derivatives of sulfur that are used as antioxidant additives in the food industry. They are also starting products in the chemical industry. For example, they are used to obtain some sulfonic acids.

Naturally, sulfur is a mineral found in volcanic regions.

We can find sulfites (not natural, but in the form of synthesized salts) in the list of ingredients of many food products, especially in preserves: sulfurous acid or sulfur dioxide (E220), sodium sulfite (E221), sodium hydrogen sulfite (E222), sodium disulfite (E223), potassium disulfite (E224), calcium sulfite (E225), calcium hydrogen sulfite (E226), calcium hydrogen sulfite (E227) and potassium hydrogen sulfite (E228).

However, sulfites are not just added additives, they are generated and sulfites are also found naturally in food in many of the foods we eat if they have gone through a fermentation process, in which bacteria oxidize the sulfur that naturally contain: dried ginger, dried sweet fruits, fermented vegetables (especially onion, cabbage), and of course wine, in which sulfites are also a natural by-product of yeast metabolism during fermentation.

Sulphites in wine are present given the need to rack oak barrels during the last stage of fermentation.

Reactions

With aldehydes they give a-hydroxysulfonates in basic solution. These are soluble in water and can thus be separated from any organic impurities by extraction. The aldehyde is released again acidifying the medium while generating sulfur oxide (IV). With elemental sulfur, sulfites react to form thiosulfates.

In inorganic chemistry, sulfites can be ligands in complexes with transition metals, joining the central metal or through its lone pair of electrons on the sulfur atom or through the oxygen electrons depending on the nature of the metal used, reaction conditions etc.