Sulfur

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Sulfur

Sulfur—the tenth most abundant element in the universe—is the nonmetallic chemical element of atomic number 16. It has a symbol of S, an atomic weight of 32.07, and a specific gravity of 2.07 (rhombic form) or 1.96 (monoclimic form). Sulfur boils at 832.5°F (444.7°C) and consists of four stable isotopes of mass numbers 32 (95.0%), 33 (0.75%), 34 (4.2%) and 36 (0.015%). Sulfur atoms found in different locations have slightly different percentages of the four isotopes, however.

Sulfur is a bright yellow solid that can exist in many allotropic forms with slightly different melting points, all around 239°F (115°C). The two main forms are called rhombic sulfur and monoclinic sulfur. There is also a rubbery, non-crystalline form, called plastic or amorphous—without shape—sulfur. An ancient name for sulfur is brimstone, meaning burning stone. It does indeed burn in air with a blue flame, producing sulfur dioxide:

S
sulfur

O₂
oxygengas

→

SO₂
sulfurdioxidegas

Sulfur itself has no odor at all, but it has a bad reputation because it makes many smelly compounds. Sulfur dioxide is one of them; it has a sharp, choking, suffocating effect on the unfortunate breather. The reference to fire and brimstone in the Bible was one of the worst punishments that its authors could think of. The fact that sulfur comes from deep under the ground and that sulfur dioxide can be smelled in the fumes of volcanoes further fueled people’s imaginations of what Hell must be like.

Where sulfur is found

Sulfur makes up only 0.05% of the Earth’s crust, but it is easy to get because it occurs uncombined as the element, S₈ (eight sulfur atoms tied together into each molecule of the element and joined in a ring).

Sulfur occurs in huge, deep underground deposits of almost pure element, notably along the Gulf Coast of the United States and in Poland and Sicily. However, miners do not have to go underground to get it. They make the sulfur come to them by a clever arrangement of three pipes within pipes, drilled down into the sulfur bed. This arrangement is called the Frasch process. Superheated steam is shot down through the outermost pipe to melt the sulfur. (The steam has to be super-heated because sulfur does not melt until 239°F [115°C], which is hotter than steam.) Then, compressed air is shot down the innermost pipe, which forces the liquid sulfur up and out the middle pipe.

Sulfur is also widely distributed in the form of minerals and ores. Many of these are in the form of sulfates, including gypsum (calcium sulfate, CaSO₄• H₂O), barite (barium sulfate, BaSO₄) and Epsom salts (magnesium sulfate, MgSO₄• 7H₂O). Others are metal sulfides, including iron pyrites (iron sulfide, FeS₂), galena (lead sulfide, PbS), cinnabar (mercuric sulfide, HgS), stibnite (antimony sulfide, Sb₂S₃) and zinc blende (zinc sulfide, ZnS). The sulfur is recovered from these metal ores by roasting them—heating them strongly in air, which converts the sulfur to sulfur dioxide. For example,

2Pbs
leadsulfide

3O₂
oxygen oxide

→

2PbO
leaddioxide

2So₂
sulfur

Then the sulfur dioxide can go directly into the manufacture of sulfuric acid, which is where more than 90% of the world’s mined sulfur winds up.

Compounds of sulfur

Some sulfur is used directly as a fungicide and insecticide, in matches, fireworks, and gunpowder, and in the vulcanization of natural rubber. Most, however, is converted into a multitude of useful compounds.

Sulfur is in group 16 of the periodic table, directly below oxygen. It can form compounds in any one of its three major oxidation states:−2,+4, or+6.

Sulfuric acid is the parent acid of all sulfate compounds. Among the most important sulfates is calcium sulfate (CaSO₄), which occurs naturally as gypsum, alabaster, and selenite. Copper sulfate is used as an agricultural insecticide and to kill algae in water supplies. Alums are double sulfates of aluminum and another metal such as potassium, chromium, or iron. The most common alum is potassium aluminum sulfate, KAl(SO₄)₂• 12H₂O. In water, it makes a gelatinous, goopy precipitate of aluminum hydroxide, Al(OH)₃, which, when it sinks to the bottom, carries along with it all sorts of suspended dirt, leaving behind clear water. Alum is therefore used in water purification.

Hydrogen sulfide is the parent acid of the sulfides, a family of compounds that contain nothing but sulfur and a metal. This family includes many metal ores, including iron pyrites, galena, cinnabar, stibnite, and zinc blende, as mentioned above, plus sulfides of copper and silver. Other sulfides are used in leather tanning, as pesticides, and in depilatories—creams that remove unwanted hair from the hides of cattle and from people.

Hydrogen sulfide itself is a foul-smelling gas. When eggs and certain other animal matter go rotten and putrefy, the stench is mostly hydrogen sulfide. It is often present in flatus—expelled intestinal gas. Hydrogen sulfide is extremely poisonous, but fortunately, it smells so bad that people do not hang around long enough to be overcome by it. Other very bad-smelling compounds of sulfur are the mercaptans, a family of organic sulfur-containing compounds. A mercaptan is the major ingredient in the aroma of skunks. A tiny amount of a gaseous mercaptan is deliberately added to the natural gas that is used for home heating and cooking, so that dangerous gas leaks can bedetectedby smell. Natural gas itself is odorless.

Sulfur is a macronutrient for both plants and animals. It is used primarily to make proteins and nucleic acids. Sulfur-deficiency disorders are very rare, but not unheard of. For example, abnormally low levels of sulfur can cause itchy and flaking skin and improper development of hair and nails. In plants, a deficiency of sulfur can cause the young leaves on a plant to begin turning yellow.