The main connection between sulfur dioxide and sulfuric acid is that sulfur dioxide is a precursor in the industrial production of sulfuric acid. There are two industrial processes for the manufacture of sulfuric acid, both of which involve the oxidation of sulfur dioxide to sulfur trioxide and the subsequent combining of sulfur trioxide with water to form the acid. Sulfur dioxide and sulfuric acid are also serious pollutants. They are major contributors to acid rain.
Sulfur dioxide (SO2) — a pungent, toxic gas — is formed by the burning of sulfur through the reaction S + O2 → SO2. It is also formed by the combustion of many substances containing sulfur, such as hydrogen sulfide and various organic sulfur compounds found in fossil fuels. Heating sulfur-containing ores and minerals — for example, iron pyrites (FeS2) — is another way in which the gas can be produced: 3FeS2 + 8O2 → Fe3O4 + 6SO2. Sulfur dioxide and sulfuric acid production form one of the most important sectors of the chemical industry, and account for the bulk of the world’s sulfur output.
Sulfuric acid was produced in early times by distilling iron II sulfate, or “green vitriol,” and later, by heating a mixture of saltpeter (NaNO3) and sulfur along with steam. In the 18th century, the industrial production of sulfuric acid using sulfur dioxide began, using what was initially termed the lead chamber process. The sulfur dioxide was produced by the burning of sulfur or heating of iron pyrites and oxidized to sulfur trioxide (SO3) by nitrogen dioxide (NO2): SO2 + NO2 → SO3 + NO. The nitrogen dioxide for the reaction was initially produced by the thermal decomposition of saltpeter, but was later provided by the oxidation of ammonia using a catalyst. The reaction took place in lead-lined chambers into which a spray of water was directed, dissolving the sulfur trioxide to form sulfuric acid, which collects at the bottom of the chamber.
The lead chamber method of manufacture has now been mostly replaced by the contact process. This dispenses with the need for nitrogen dioxide by reacting sulfur dioxide with air to form sulfur trioxide. Under normal circumstances, this reaction is very slow; however, it proceeds rapidly when a suitable catalyst is used. Platinum was effective for this purpose, but the modern contact process uses vanadium pentoxide.
Sulfur dioxide and sulfuric acid, although of huge industrial importance, are also major pollutants and are the biggest sources of acid rain. In the air, sulfur dioxide is slowly oxidized to sulfur trioxide, which reacts with moisture in the atmosphere to form dilute sulfuric acid. This can fall as acid rain, which can damage plant life and stone buildings, or may react with other pollutants to form sulfate particles, which can act as condensation nuclei and may play a role in cloud formation.
