Sulfur

Sulfur is a chemical element. Its official symbol is S and its atomic number is 16, which means that each sulfur atom has 16 protons in its nucleus. Elemental sulfur is a pale yellow colour. At room temperature, it is a soft powder that crumbles when touched. Elemental sulfur does not have a smell when it is on its own, but when it forms a compound – with hydrogen, for example – it can stink!

Elemental sulfur

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On the periodic table of elements, sulfur is represented by the symbol S. It has an atomic number of 16 because it has 16 protons in its nucleus. 

Rights: The University of Waikato Te Whare Wānanga o Waikato

Sulfur deposits are found naturally in volcanic areas like Rotorua and Whakaari White Island and in large quantities deep underground in the United States, Poland and Sicily. By mass, sulfur is the fifth most common element on Earth.

Sulfur deposits on White Island

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The gases escaping from fumaroles can be rich in hydrogen sulfide. When the hot gases come into contact with the atmosphere, they cool and oxidise and can form yellow crystals of pure sulfur around fumarole vents (inset). The extensive sulfur deposits of Whakaari White Island were mined until the 1930s.

Text courtesy of Te Ara and Manatū Taonga Ministry for Culture and Heritage, CC BY-NC 3.0 NZ.

Rights: Public domain

Black powder, brimstone and multiple spellings

Sulfur is one of the few elements that occurs in a pure form, so humans have known about and used ‘pure’ elemental sulfur for a very long time. Records show the Chinese were using sulfur in traditional medicines over 2,600 years ago. By the 7th century AD, they had learned to mix sulfur with charcoal and potassium nitrate to create black powder (gunpowder). Ancient Indian, Greek and Egyptian cultures also used sulfur in medicines, for fumigation and for bleaching fabrics.

Sulfur also features in religious writings that date back 2,600 years. English translations of The Bible refer to sulfur as ‘brimstone’ to describe destruction (as volcanic activity is capable of doing) and the unpleasant smell associated with sulfur compounds.

Fast forward 2,300 years to 1777. After extensive experimentation, Antoine Lavoisier found he could not break sulfur down into simpler substances so he declared it to be an element.

The Devil’s Bath at Wai-O-Tapu geothermal area

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This pool in Rotorua’s Wai-O-Tapu area gets its colour from a combination of hydrogen sulfide gases and ferrous (iron) salts. Rotorua, nicknamed ‘Sulfur City’, has sulfur deposits in many of its geothermal fields. The city is also known for its hydrogen sulfide smell.

Why do you think the pool has been nicknamed the Devil’s Bath? 

Rights: Takver, CC BY-SA 2.0

In recent times, the arguments were not about chemistry but spelling. The original Latin term was ‘sulpur’. With time, it became ‘sulphur’ and in the 3rd century changed to become ‘sulfur’. English kept the ‘ph’ spelling while other languages used ‘f’ – zolfo in Italian and schwefel in German, for example. The International Union of Pure and Applied Chemistry formally adopted the word ‘sulfur’ in 1990.

Sulfur – essential for life (and smelly farts)

Sulfur is present in all living plant and animal tissues. It makes up about 0.3% of the human body. It is a component of some proteins. It also helps our bodies resist bacteria, aids insulin production and helps keep our joints, skin, hair, nails and connective tissues healthy.

Dietary sulfur

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Sulfur is an essential part of the human diet. One 55 g boiled egg contains about 180 mg of sulfur.

Rights: Amin, CC BY-SA 4.0  

A balanced diet should meet the body’s sulfur requirements. Protein-rich foods – eggs, meat, poultry, fish, dairy and soy products – are good sources of dietary sulfur. Brassicas (broccoli and cabbage), onions, garlic and asparagus also provide dietary sulfur.

When the body digests sulfur-rich foods, hydrogen sulfide is produced as a byproduct. This is the gas that makes farts smell like rotten eggs – and gives Rotorua its famous smell.

Sulfur is essential for plant growth. As well as being important for some plant proteins, sulfur aids chlorophyll development and nitrogen fixation in legumes. Most of the sulfur in soil comes from organic matter and from weathered rock. We use fertilisers to supplement the soil with important elements.

Fertilisers for plant growth

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Various fertilisers contribute different nutrients for plant growth. Potash contains potassium, urea contains nitrogen, elemental sulfur contains sulfur, reactive rock phosphate contains phosphorus and superphosphate contains phosphorus, sulfur and calcium.

Rights: Massey University

On the other hand, sulfur can stop life from happening. Humans have long used sulfur to control insects, mildew and bacteria. High concentrations of hydrogen sulfide (much higher than what is produced in our gut) can cause respiratory paralysis, leading to death. It just goes to show that it is all in the dose.

Other uses

Sulfur is most widely used to produce sulfuric acid to make phosphates for fertilisers. It is also used to harden rubber to make products like tyres, shoes and hoses more durable.

Another common use of sulfur is in matches. Many matches are made up of potassium chlorate, sulfur, powdered glass and gelatine (which holds all of the materials on the wood). Swiping a match head along a striking surface causes friction and heat and then a flame. The potassium chlorate releases oxygen, and the sulfur combines with the oxygen and keeps the fire burning. The wooden match stick also acts as a fuel source. The distinctive smell of a burning match comes from the sulfur.

Natural gas, used for heating and cooking, is odourless, colourless and tasteless. Sulfur compounds are added to natural gas so we can detect even the smallest leak. Skunks also use sulfur for protection. They use a sulfur-based spray that is strong enough to drive off bears. It is so strong that, downwind, humans can smell the spray several kilometres away!