Burning elemental sulfur (S) in the presence of air causes a combustion reaction, yielding sulfur dioxide (\(\text{SO}_2\)). This gas is colorless, pungent, and toxic. The primary purpose of burning sulfur is to generate \(\text{SO}_2\), which serves as the active agent for nearly all subsequent applications. This chemical compound is either used in its gaseous form or further processed into other sulfur-containing compounds.
Industrial Foundation: Production of Sulfuric Acid
The largest use of sulfur dioxide generated from burning sulfur is as the raw material for producing sulfuric acid (\(\text{H}_2\text{SO}_4\)). This large-scale manufacturing process is known as the Contact Process. The \(\text{SO}_2\) gas is first purified and then mixed with excess oxygen before being passed over a catalyst, typically vanadium(V) oxide (\(\text{V}_2\text{O}_5\)).
This catalytic conversion occurs at elevated temperatures, around \(450^{\circ}\text{C}\), and moderate pressure, transforming the sulfur dioxide into sulfur trioxide (\(\text{SO}_3\)). Because the direct reaction of \(\text{SO}_3\) with water is highly exothermic and creates an acid mist, the sulfur trioxide is instead absorbed into concentrated sulfuric acid. This absorption creates oleum, or fuming sulfuric acid (\(\text{H}_2\text{S}_2\text{O}_7\)), which is then safely diluted with water to produce the final sulfuric acid concentration.
Sulfuric acid is a foundational commodity chemical, with global production measured in the hundreds of millions of tons annually. Its immense scale of use makes it a primary indicator of a country’s industrial strength. The acid is indispensable in a vast array of manufacturing sectors. These include the production of phosphate and ammonium sulfate fertilizers, petroleum refining, and the synthesis of detergents and pigments.
Fumigation and Disinfectant Applications
Sulfur dioxide gas is widely utilized for its powerful antimicrobial and insecticidal properties in non-consumable and post-harvest environments. This process, termed fumigation and disinfection, sterilizes containers and storage areas against biological contamination. Historically, burning sulfur candles inside empty wine barrels or other liquid storage vessels was a common method to release \(\text{SO}_2\) for sterilization.
The gas permeates surfaces, inhibiting the growth of undesirable wild yeasts, molds, and bacteria that could spoil products. Modern practices involve introducing sulfur dioxide gas into closed environments like large silos, empty warehouses, or shipping containers to control pests. This is relevant in the post-harvest treatment of stored grains, where the gas eliminates insects such as the confused flour beetle and the rice weevil.
Sulfur dioxide fumigation is being researched as a safer alternative to older, environmentally damaging fumigants for fresh produce, controlling pests like the Western Flower Thrips. For fruits such as blueberries, a brief fumigation significantly reduces fungal decay, extending the fruit’s market life. The fumigation process must be precisely controlled, however, as the gas can cause discoloration or damage to some commodities.
Role in Food and Beverage Preservation
Sulfur dioxide derived from burning sulfur is applied directly to foods and beverages as a preservative and antioxidant. This practice has a long history, particularly in the winemaking industry, where the use of sulfur is standard in nearly all conventional production. In wine, \(\text{SO}_2\) serves a dual function: it inhibits unwanted spoilage bacteria and wild yeasts, ensuring only the desired fermentation organisms are active.
The gas acts as a potent antioxidant, scavenging oxygen molecules that would react with the wine’s components, causing a loss of fresh flavor and color. This effect maintains the quality and stability of wine during aging and storage, preventing browning from oxidation. The same principles apply to dried fruits, such as apricots and raisins, where sulfur dioxide maintains their vibrant color and texture.
Without treatment, light-colored dried fruits would rapidly darken through enzymatic browning upon exposure to air. Adding the sulfur compound stabilizes the fruit’s natural color and extends its shelf life by preventing microbial spoilage. Because sulfur dioxide and its related compounds (sulfites) can cause sensitivities in some individuals, particularly those with asthma, their presence must be clearly indicated on the product label, often as the additive E220.