SULPHUR

Introduction

Sulphur was known in antiquity. In Latin, it was called sulpur, and in Greek, qeion. It was considered the embodiment of fire, and related to lightning. The Greek name, indeed, also means "divinity" and was derived from qeos, which referred to Zeus, who is often shown with a handful of lightning bolts. In Christian mythology, it is the fuel of Hell. A "p" in Latin was used to represent φ in words borrowed from Greek in the times when it was pronounced with a puff of air, but was not yet the "f" sound. Asparagus, in fact, should be aspharagus. Later, when the "f" sound was used, the p often changed to ph in Latin words of Greek origin. Although "sulpur" had no Greek roots, it was attracted into the form "sulphur" in late classical Latin. The spelling was altered in medieval times to "sulfur," which is the spelling that usually appears in Latin dictionaries. The English word is taken directly from Latin, traditionally in the form "sulphur." The American Chemical Society, at a time when spelling simplification was in vogue, decreed that "sulfur" was to be the accepted form in the United States. Although resisted by technical users, this form is now general in the United States, though sulphur is still occasionally seen. In the rest of the world, it is still sulphur. I shall use this spelling here in the interests of tradition and universality.

In German, sulphur is known as Schwefel, in Dutch, as zwavel. In Welsh, it is sylffur, which is simply the Welsh spelling of the sound of the English word. In French, it is soufre, in Italian zolfo, and in Spanish azufre. Sulphate is solfato in Italian, and sulfato in Spanish, where phosphorus is fósforo, since Spanish orthography tries to represent the spoken sound accurately. The old name brimstone is from "burning stone," from the Middle English brinnen, "burn." In German, however, Bernstein, with a similar etymology, means "amber," not "sulphur." "Schwefel" comes from roots expressing "slow-burning."

The chemical symbol for sulphur is S, and is the same in all languages. Dalton's symbol is shown in the heading.

Basic Information

Name: Sulfur
Symbol: S
Atomic Number: 16
Atomic Mass: 32.066 amu
Melting Point: 112.8 °C (385.95 K, 235.04001 °F)
Boiling Point: 444.6 °C (717.75 K, 832.28 °F)
Number of Protons/Electrons: 16
Number of Neutrons: 16
Classification: Non-metal
Crystal Structure: Orthorhombic
Density @ 293 K: 2.07 g/cm3
Color: yellow
British Spelling: Sulphur
IUPAC Spelling: Sulfur

Atomic Structure

Number of Energy Levels: 3 First Energy Level: 2 Second Energy Level: 8 Third Energy Level: 6

Isotopes

Isotope	Half Life
S-32	Stable
S-33	Stable
S-34	Stable
S-35	87.2 days
S-36	Stable

Facts

Date of Discovery: Known to the ancients
Discoverer: Unknown
Name Origin: From the Latin word sulfur (brimstone)
Uses: matches, gunpowder, medicines
Obtained From: naturally

Extraction of pure sulphur

Since sulphur in free state is found at depths of more than 150 to 300 meters below the earth’s surface, the method of extraction of sulphur differs from other metal or non-metal extractions. Sulphur’s relatively low melting point (115°C) is utilized in this process. This is known as the Frasch process. Here compressed super heated water (at 170°C) is pressed into a pipe which reaches upto the sulphur deposits. The sulphur here melts. Introducing hot compressed air through another pipe brings it up. The molten sulphur and water mixture is forced up and is collected in a settling tank. The sulphur is cooled and water is evaporated. The sulphur extracted in this way is more than 99% pure.

It is not normally necessary to make sulphur in the laboratory as it is so readily available. It is found as the native element in nature and extracted by the Frasch process. This is an interesting process since it means that sulphur can be extracted from underground without mining it. In the Frasch process underground deposits of sulphur are forced to the surface using superheated water and steam (160°C, 16 atmospheres, to melt the sulphur) and compressed air (25 atmospheres). This gives molten sulphur which is allowed to cool in large basins. Purity can reach 99.5%.

The process in energy intensive. Commercial success for this operation depends upon suitable geological conditions as well as access to cheap water and energy.

Hydrogen sulphide, H₂S, is an important impurrity in natural gas which must be removed before the gas is used. This is done by an absorption and regeneration process to concentrate the H₂S, followed by a catalytic oxidation (Claus process) using porous catalysts such as Al₂O3 or Fe₂O₃.

8H₂S + 4O₂ → S₈ + 8H₂O

Over the years the Claus process has been improved and a modified process can yield 98% recovery.

In the laboratory, sulphur can be purified by recrystallisation from solutions in carbon disulphide, CS₂. However the resulting crystals are contaminated with solvent, H₂S, and SO₂. One good way to purify sulphur is to use a quartz heater (700°C) immersed in liquid sulphur. Carbon impurities decompose to form volatile materials of solid carbon, which coat the heater. After a week or so, finishing with a distillation under vacuum, the result is sulphur with a carbon content of about 0.0009%.

Properties

Physical properties of sulphur :

Since S has 6 electrons in its outermost shell, it needs 2 more electrons to complete its shell. But S combines with 7 other atoms to make a sulphur molecule that has a total of 8 sulphur atoms. Thus each S atom shares 2 electrons with its neighboring atom. The bonds are covalent in nature. A molecule of sulphur is represented as S8. It is a ringed molecule. The structure is shown below.

    

Sulphur is a yellow crystalline solid. It is tasteless and odorless. The melting point of S is 115°C. Sulphur is an insulator and is a poor conductor of heat and electricity. S is insoluble in water but is soluble in CS₂. Sulphur forms covalent bonds and shows allotropic forms. The allotropes have different crystalline shapes such as rhombic and monoclinic. There is another allotrope which has no shape and is called plastic sulphur. Vapours of sulphur are pungent and although not poisonous, they can cause health problems to humans.

Chemical properties of sulphur :

1. Valence: Since S has 6 electrons in its M-shell, the M-shell is more than half filled. Hence S does not give off its electrons easily. It readily forms covalent bonds to complete its M-shell. It shows variable valence of 2 or 6. S is quite a reactive element and forms oxides, chlorides and sulphides readily.

2. Action of oxygen:  Sulphur reacts with oxygen and burns with a blue flame. It forms sulphur dioxide which is a colourless gas having a pungent smell.

3. Reaction with chlorine: S reacts with Cl to produce sulphur monochloride when Cl gas is passed through boiling S. :

S reacts with other non-metals also. In all cases sulphur has to be heated or boiled for the reaction to take place.

4. Reaction with metals: Heated S reacts with metals like Fe, Cu, Zn, Sb directly to give metal-sulphide. A few reactions are shown below.

5. Reaction with acids: S is oxidized by strong concentrated oxidizing acids such as sulphuric acid and nitric acid.

In both the reactions S acts as a reducing agent.

16	Phosphorus ← sulfur → chlorine
O ↑ S ↓ Se	Periodic table - Extended periodic table

Uses of Sulphur

Several important uses of sulphur and its compounds have already been discussed. The uses of sulphur can be classified under the oxidation number, as sulphides (-2), elemental sulphur (0), SO2 and sulphites (+4), and SO3 and sulphates (+6). Sulphuric acid (+6) has already been mentioned as the most important product of the chemical industry, the "King of Chemicals."

A very important use of sulphur, if not of large amounts, is the vulcanization of rubber. Understanding where this fits in is an excuse for reviewing the nature of the material. Rubber was found in use by the South American natives when the Spanish arrived. They played with solid rubber balls, wore clothing made impermeable by rubber, and used rubber bottles. The Spanish could not make any use of this new material, and it remained a curiosity for over 200 years. In 1770, Joseph Priestley named the substance "rubber" from its ability to rub out pencil marks. If it was dissolved in turpentine, fabrics could be impregnated with it and made waterproof (the "MacIntosh"). It also made waterproof boots and similar objects, and by 1830 was widely used. Rubber is soluble in benzene, carbon disulphide, turpentine, and similar solvents. Rubber cement (originally rubber in benzene) was popular for art paste-ups, because any excess was easily removed by rubbing with a finger.

Rubber is elastic, water repellent and a good electrical insulator. However, it became brittle at low temperatures, sticky at high temperatures, had little mechanical durability and oxidized rapidly. It was found that sulphur improved the properties somewhat, but it was reserved for Charles Goodyear (1800-1860) to find in 1839 that if rubber was not only mixed with sulphur, but cooked at 120°-160°C for a sufficient time, it became practically a new substance. It was elastic over a wide range of temperature, becoming neither brittle nor tacky, mechanically more durable, and quite impermeable. If about 5% sulphur is used, the rubber is soft and flexible. When 32% sulphur is used, the product, called ebonite or hard rubber, is an excellent, durable thermosetting plastic, once popular as a structural material in all kinds of electrical apparatus. It has now been replaced by cheaper plastics.

Rubber was obtained from the latex of Hevea brasiliensis, a tree that grew wild in South America. The latex is a colloidal suspension of rubber that comes from just under the bark of the tree (it is not the sap). In this tree it is particularly pure and free of the resins that such liquids (which are fairly common--as in the dandelion and milkweed) usually contained. The suspension is coagulated with formic acid, and the liquid pressed out between rollers, forming sheets about 1/4" thick that are often smoked to sterilize them, and packed in bales for shipment. Seeds of H. brasiliensis were smuggled out of Brazil in 1876 by Sir Henry Wickham (1846-1928), and used to establish plantations in southeast Asia, the source of most of the natural rubber since that time.

Rubber is a polymer with the empirical formula (C₅H₈) x, with one C=C double bond to each unit. Sulphur cross-links the polymer chains, taking advantage of these double bonds. The density of rubber is about 0.915 g/cc, and it is a white elastic solid. It is essentially a (1, 4) polymer of isoprene, the molecule shown in the diagram, but efforts to reproduce natural rubber exactly have been in vain. Natural latex contains about 35% rubber substance. However, many very good rubber-like materials have been produced as artificial polymers. Natural rubber will retain its market only so long as it is cost-effective. The spur to artificial rubber production was the disruption of plantation rubber during World War II. Germany had produced artificial rubbers even during World War I.

About 80% of industrial rubber is used to make pneumatic tires and tubes. Much rubber is reclaimed from used rubber products, but not enough is done along these lines even now. The pneumatic tire was invented in 1877, and the cord tire, which replaced fabric with parallel cords and made the tire much more durable, in 1910. The mix for making a tire includes, besides rubber and sulphur, accelerators (CaO, MgO, organic compounds), softeners (stearic acid, paraffin, petroleum jelly), reinforcing agents (carbon black, zinc oxide), and antioxidants. The green tire is assembled with all its fabric, cord, and wire reinforcement, and put into the vulcanizing mold. Heat and pressure then make the finished tire.

Sulphur is an important fuel in pyrotechnic mixtures, because it is cheap and stable. It occurs in match heads, the most common pyrotechnical device, and was an ingredient of black powder. Black powder is a special mixture of 75% potassium nitrate, 15% charcoal, and 10% sulphur, more or less. The sulphur and charcoal are ground together dry so that the thixotropic sulphur thoroughly coats the active surface of the charcoal. Then the nitrate is added, and the mix is wet ground until homogenized. The dried mix is formed into grains of the desired size, and the powder is ready to use. More details can be found in Bang!.

I have found an intriguing reference to philosophers' eggs, which are made from a mixture of NaCl, S and Hg. I know nothing more about them. Perhaps they are made up into a small ball, and do something strange when set on fire. They are not the Pharoah's Serpent eggs made from mercuric thiocyanate, Hg(SCN)₂ (now banned due to mercurophobia). Perhaps they are a medical treatment. Dictionaries do not stoop to define them.
Inshort:

•  S is used to make H₂SO₄ acid, which is used in the manufacture of many compounds such as detergents, plastics, explosives, etc.
• S is used for making CS₂ molecule, gun powder, matches etc.
• S is used for manufacture of fire works.
• S is used in the rubber industry for vulcanization of rubber.
• S is used for making germicides, fungicides.
• S is used in many medicines.
• S is used in photographic development (sodium thiosulphate or hypo).
• S is used for making bleaching agents.

S is used in making artificial hair colours or dyes.

Health effects of sulphur

All living things need sulphur. It is especially important for humans because it is part of the amino acid methionine, which is an absolute dietary requirement for us. The amino acid cysteine also contains sulphur. The average person takes in around 900 mg of sulphur per day, mainly in the form of protein.

Elemental sulphur is not toxic, but many simple sulphur derivates are, such as sulphur dioxide (SO₂) and hydrogen sulfide.

Sulfur can be found commonly in nature as sulphides. During several processes sulfur bonds are added to the environment that are damaging to animals, as well as humans. These damaging sulphur bonds are also shaped in nature during various reactions, mostly when substances that are not naturally present have already been added. They are unwanted because of their unpleasant smells and are often highly toxic.

Globally sulphuric substances can have the following effects on human health:

- Neurological effects and behavioural changes
- Disturbance of blood circulation
- Heart damage
- Effects on eyes and eyesight
- Reproductive failure
- Damage to immune systems
- Stomach and gastrointestinal disorder
- Damage to liver and kidney functions
- Hearing defects
- Disturbance of the hormonal metabolism
- Dermatological effects
- Suffocation and lung embolism

References:

www.lenntech.com
www.webelements.com
www.chemicalelements.com
www.du.edu
home.att.net
En.wikipedia.org
www.drbloke.co.uk
www.springerlink.com

Editorial Team, Mindfiesta