Sulfur Dioxide

Sulfur dioxide, SO₂, is a colorless gas with an irritating, choking odor. It is produced naturally in volcanoes and in geysers, and is produced in ever-increasing amounts by the burning of sulfur-containing coal. Sulfur dioxide is a key contributor to acid rain. It is used in the manufacturing of sulfuric acid, in preserving many foods, as a disinfectant, and in the wine industry.

Sulfur dioxide is produced naturally inside the earth by the reaction of sulfur with oxygen gas. This accounts for the "sulfur" smell around active volcanoes and geysers. Sulfur is a natural component of most coal, and it is the burning of sulfur-containing coal that has contributed the most to the acid rain problem in the world. When burned, the sulfur in coal reacts with oxygen and makes sulfur dioxide gas. This gas may react with water to make sulfurous acid (H₂SO₃, a moderately strong acid), or it may react with oxygen to form sulfur trioxide, SO₃, which then reacts with water in the atmosphere to produce sulfuric acid, H₂SO₄, the major contributor to acid rain. This same series of reactions is used to make sulfuric acid, the most widely produced chemical in the world. Efforts to reduce the amounts of sulfur dioxide in the atmosphere use two approaches. Low sulfur coal can be used, reducing emissions. However, as we use up more and more of the low sulfur coal, we are forced to use the remaining higher sulfur content coal. Many factories now use scrubbers, which put sulfur dioxide gas into contact with calcium oxide (CaO) to produce solid calcium sulfite, thus preventing the sulfur dioxide from reaching the atmosphere. The reaction is shown below:

One drawback to this approach is that for every ton of sulfur dioxide that reacts, two tons of solid calcium sulfite are produced, and they must be disposed of.

Sulfur dioxide is used widely to prevent dried fruits (especially apricots and peaches) from becoming discolored. Sulfur dioxide is a strong reducing agent, and prevents the formation of discolored oxidation products in the fruits. It is also used to bleach vegetables and in the wine industry is used to prevent wines from discoloring and turning brown. In the food industry, sulfur dioxide is used as a disinfectant during the manufacturing process.