Allotropes Of Sulfur

Sulfur exists in several allotropes, the most critical being yellow rhombic (α-sulfur) and monoclinic (β-sulfur) forms. At room temperature, rhombic sulfur is the stable form, while it transforms into monoclinic sulfur when heated above 369 K.

1.0Allotropes of Sulfur

Some elements exist in two or more forms that differ in chemical properties while maintaining the same physical properties. These forms are known as allotropes or allotropic forms, and the phenomenon is called allotropy.

Sulfur exhibits a remarkable propensity for forming various solid allotropes, surpassing all other elements. Currently, there is a comprehensive understanding of approximately 30 distinct sulfur allotropes. Multiple sulfur allotropes can be attributed to the different arrangements of sulfur atoms within crystal structures. These differences in crystal structures result in variations in density, external crystal morphology, colouration, thermal stability, and intrinsic energy among the different forms. The following elucidations represent the various allotropic forms of sulfur:

1. Rhombic Sulfur

Rhombic Sulfur is the most common and most stable form of sulfur. It is also known as alpha (α) sulfur or octahedral sulfur.

Preparation: At room temperature, rhombic sulfur is prepared by dissolving powdered sulfur in carbon disulfide. The mixture is then filtered, and the filtrate is stored in a small beaker lined with filter paper. As the carbon disulfide evaporates, massive octahedral rhombic sulfur crystals are left behind.

Properties:

• Rhombic sulfur exists as rhombic octahedral crystals.
• It is pale yellow.
• Its density is 2.06 g/mL.
• It melts at 112.8°C.
• It is stable at room temperature. When rhombic sulfur is slowly heated to 96°C, it converts into monoclinic sulfur. Upon cooling below 96°C, it reverts to the rhombic form.
• It is insoluble in water but soluble in carbon disulfide.
• Rhombic sulfur consists of S₈ units at room temperature, where the eight sulfur atoms form a puckered ring.

2. Monoclinic Sulfur

Monoclinic sulfur, also known as β-sulfur or prismatic sulfur, is stable only above 96°C (369K) and transforms into rhombic sulfur below this temperature.

Preparation: Monoclinic sulfur is prepared by slowly heating sulfur in an evaporating dish until it melts. The molten sulfur is then allowed to cool gradually. As it cools, a solid crust forms on the surface. Two holes are made in this crust to pour out the remaining molten sulfur. The crust is removed, revealing long needle-shaped crystals of monoclinic sulfur on the underside.

Properties:

• Monoclinic sulfur forms long, needle-like prisms; hence, it is called prismatic sulfur.
• It melts at 119°C.
• Its density is 1.98 g/mL.
• It is insoluble in water but dissolves readily in carbon disulfide.
• Monoclinic sulfur exists as S₈ molecules, which have a puckered ring structure similar to rhombic sulfur but differ in symmetry.

3. Plastic Sulfur

Plastic sulfur, also known as 𝛄-sulfur, is an amorphous form.

Preparation: Plastic sulfur is prepared by suddenly cooling molten sulfur near its boiling point by pouring it into cold water. This results in the formation of yellow, rubbery ribbons of sulfur.

Properties:

• Plastic sulfur is an amorphous form of sulfur. Initially, it is soft and elastic like rubber but hardens over time, gradually transforming into rhombic sulfur.
• It has a rubber-like, transparent yellow thread appearance and is insoluble in carbon disulfide and water.
• Its density is 1.92 g/mL.
• It does not have a sharp melting point.
• Plastic sulfur comprises a zigzag chain structure, sometimes including S₈ and other rings.

4. Milk of Sulfur

Preparation: Milk of sulfur is produced when sulfur flowers are boiled with lime milk, resulting in a mixture of calcium pentasulfide (CaS₅) and calcium thiosulfate (CaS₂O₃). Treating this mixture with hydrochloric acid yields a white amorphous precipitate known as milk sulfur.

The reactions involved are: 

• 3Ca(OH)₂ + 12S → 2CaS₅ + CaS₂O₃ + 3H₂O
• 2CaS₅ + CaS₂O₃ + 8HCl → 12S + 4CaCl₂ + 4H₂O

Properties:

• Milk of sulfur is soluble in carbon disulfide.
• It tends to revert to the rhombic variety upon standing for a long time.
• It is widely used in medicines.

5. Colloidal Sulfur

Preparation: Colloidal sulfur can be obtained by various methods:

• Passing hydrogen sulfide through nitric acid:

  H₂S + 2HNO₃ → 2H₂O + 2NO₂ + S

• Treating sodium thiosulfate solution with dilute hydrochloric acid: 

    Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + H₂O + S

• Reacting hydrogen sulfide with sulfur dioxide: 2H₂S + SO₂ → 2H₂O + 3S

Properties: Colloidal sulfur consists of fine particles dispersed in a medium, often appearing as a cloudy suspension.

2.0Uses of Sulphur

Some of the uses of sulfur are as follows:

• Sulfur is used to manufacture sulfur dioxide, which is used for fumigation and producing sulfuric acid.
• It is utilised to create matches, fireworks, gunpowder, and dyes.
• Sulfur is used in the preparation of several drugs.
• It acts as a fungicide.
• Sulfur is essential in the vulcanisation of rubber.
• To make calcium hydrogen sulfite (Ca(HSO₃)₂), used as a bleaching agent in manufacturing paper.