Sulphur

Sulphur is a chemical element that belongs to the oxygen group of the periodic table. It is denoted by the symbol ‘S,’ is non-metallic, and has the atomic number 16. Pure sulphur is a brittle solid which is tasteless and odourless. It is a poor conductor of electricity and insoluble in water. Sulphur is known to be one of the most reactive elements. As sulphur makes up 0.0384% of the earth’s crust, it is also one of the most abundant elements. 

A processed form of sulphur is used in labs which has a noticeable odour. It is produced in high amounts each year, aiding in the commercial interest of manufacturing sulphuric acid. This acid is widely used in fertilisers, insecticides, matches, and other industrial products. 

Properties of Sulphur

Sulphur exhibits the following physical properties:

• Sulphur generally exists as yellow-coloured, opaque, and brittle orthorhombic crystals. 
• Its non-metallic characteristics make it a poor conductor of electricity. 
• It has twice the density of water. 
• It is insoluble in water but soluble in carbon disulphide and toluene (methylbenzene).
• The boiling point of sulphur is 444℃. Sulphur vapours, when consolidated, result in a fine powder resembling the pattern of a flower known as the Flower of Sulphur. 
• When the solid is burned, it results in a blood-red liquid while emitting a blue flame. 
• As compared to oxygen, sulphur favours single bonds and has more tendency to catenate. 

The chemical properties of sulphur are as follows:

• Sulphur burns with blue flames leading to the formation of sulphur dioxide which has a suffocating and irritating odour. 
• Sulphur reacts with nearly all other elements except for noble gases. It even reacts with Iridium, the notoriously unreactive element, to form Iridium Disulphide. 
• Some of these reactions require elevated temperatures. 
• In the absence of air, sulphur consolidates directly with most of the metals to form sulphides on heating. On the other hand, sodium, potassium, and other similar reactive elements may react with sulphur without heating.

Allotropes of Sulphur

Sulphur tends to exist in various compounds such as polycations, polyanions, oxides, metal sulphides, halides, oxyhalides, etc. One such compound is an allotrope. Allotrope is the tendency of chemical elements to exist in two or more varying forms. Sulphur forms various allotropes. It can exist in various structures while being in the same physical state. 

Two of the important crystalline structures are rhombic/octahedral (𝛂-Sulphur) and monoclinic (𝜷-Sulphur). The essential difference between these two crystalline structures is that of temperature. Rhombic sulphur takes its shape at a temperature below 96℃ while monoclinic sulphur turns into solid over 96℃. Thus, 96℃ is known as the transitional temperature. 

Apart from the crystalline structures, there are amorphous allotropes of sulphur which include plastic, colloidal, and milk of sulphur allotropes. Plastic sulphur is a sticky substance that is dark brownish in colour and is formed by the heating of sulphur till its boiling point, and then sudden cooling through cold water, forming a thick intertwined mass. 

Colloidal sulphur utilised in medicines is formed when hydrogen sulphide mixes with a cooled and saturated solution of sulphur dioxide or by the mixing of alcohol and sulphur in the water. Milk of sulphur is white and non-crystalline and is created by the reaction of weakened hydrochloric acid on ammonium sulphide. 

Sulphur Production 

Production of sulphur has been done through various methodologies throughout the years. 

In ancient times, Sulphur was found and obtained in its elemental form. Pyrite was also a rich source of Sulphur. Sulphur was also found on the surface of the earth around volcanic regions in Sicily and was thus stored and piled with small gaps by following the Sicilian process. 

Salt domes contain almost pure forms of sulphur and until the late 20th century, elemental sulphur was extracted from them. It was also produced as a by-product of various industrial processes. 

Sulphur has become a chemical commodity due to its production in large quantities via the Frasch process. In this, the native sulphur under the salt domes is considered a mineral resource. As sulphur has a comparatively lower melting point, to remove it from salt domes, heated water is pumped into the deposits leading to its melting, and the compressed air pushes the melted product to the surface. Through this method, elemental sulphur has been widely produced in its purest form. Since such sulphur deposits are limited in number and the process is cost-ineffective, this method has not been used in recent years. 

Alternatively, it is extracted from petroleum, natural gas, and other fossil resources which undergo purification through various processes including the Claus process, leading to the final product. 

Conclusion

Sulphur is a non-metallic element that exists as a brittle yellow coloured solid. It belongs to the Chalcogen group, is a bad conductor of electricity, and reacts with almost all elements. 

Sulphur forms many allotropes. The most important amongst them are Rhombic and Monoclinic in the crystalline form and plastic, colloidal, and milk of sulphur in the Amorphous forms. Sulphur is largely produced through petroleum and natural gas today and was earlier extracted from salt domes, and from deposits of it around volcanoes through the Frasch process.