sulfur

In industrial countries, sulfur is a critical raw material. It is used in thousands of products and processes. Sulfur is a nonmetallic element, yellow in color and similar to oxygen in its chemical behavior. Its chemical symbol is S. Sulfur burns readily with a blue flame, which earned it the name brimstone, or “burning stone.”

Sulfur is one of the elements necessary to life. It is found in many animal and vegetable substances, especially proteins. The bad smell of a rotten egg is due to hydrogen sulfide. It is the sulfur in eggs that tarnishes silver so quickly, forming the black compound silver sulfide. Cauliflower and other members of the cabbage family are particularly rich in sulfur, as are such animal substances as hair.

The fact that sulfur burns so readily accounts for its use in matches, gunpowder, and fireworks. Sulfur candles are used to kill vermin. Bordeaux mixture, a standard insecticide, contains copper sulfate. In addition, many preparations used to prevent fungus diseases are made with sulfur. All vulcanized rubber contains sulfur, and sulfur compounds are used in the manufacture of paper.

Sulfur exists in nature both in its native, uncombined form and in compounds. A large proportion of the world’s native sulfur occurs in the salt domes of the Gulf region of the southern United States. There are also large volcanic deposits of sulfur in Sicily. Sulfur can be extracted commercially from pyrites. This is a mixture of several sulfur compounds, including the mineral pyrite, or iron sulfide.

Sulfur is also recovered commercially from various gases. “Sour” natural gas is too rich in sulfur to be used in homes and is treated to extract the sulfur. Smelter gases and other industrial fumes are also sources of sulfur.

Sulfur occurs in several forms, or allotropes, all differing in density. The commonest is rhombic sulfur, whose crystals are many-sided. Monoclinic sulfur has needlelike crystals shaped like prisms. Amorphous sulfur, a white or pale yellow powder, is noncrystalline. When sulfur is heated, it melts at a little above the boiling point of water into a watery yellow liquid. With further heating this liquid turns brown and thickens, finally bursting into flame at about 482 °F (250 °C). If air is excluded, however, the brown, plastic mass becomes thin again and boils at 832 °F (444 °C). The reddish brown vapor that is given off turns colorless if heating is continued.

Sulfur has valences of 2, 4, 6, and –2. This accounts for the many ways it combines with other elements. Some of its main compounds are the sulfides, sulfites, and sulfates. Copper sulfate, or blue vitriol (CuSO₄), is a common sulfate used as a germicide and as a fixing agent in dyeing. Sodium thiosulfate (Na₂S₂O₃) is the fixing agent used in photographic darkrooms; ferrous sulfate, or green vitriol or green copperas (FeSO₄), is used in wool dyeing, disinfectants, and ink manufacture. Other sulfur compounds are the oxides. Sulfur dioxide (SO₂) is the most useful of these, serving as a bleach and as a preservative. Sulfuric acid, a very useful industrial chemical, is produced from sulfur dioxide and oxygen.