Iodine

Iodine is a fascinating element with unique properties and occurrences in nature. Bernard Courtois's discovery in 1811 was an important milestone in chemistry. Although iodine isn't naturally found in its free state, it is present in various compounds. For example, it's found in seaweeds as sodium iodide (NaI) and in caliche, or Chile saltpetre, as sodium iodate (NaIO₃). These natural sources have been historically significant for iodine extraction and utilisation.

1.0Introduction

Iodine, with the symbol "I" and atomic number 53, is a unique element known for its dark grey or purple-black appearance. It stands out as the least reactive and most electropositive halogen, yet it can still form compounds with many elements. Found primarily as water-soluble iodide in brine pools and oceans, iodine is also present in air, soil, and water, with oceans being a significant source.

When heated, iodine emits a striking purple vapour. Although not fully soluble in water, it can dissolve in solvents like carbon tetrachloride.

Iodine's importance extends to human health. It plays a crucial role in brain function and is stored mainly in the thyroid gland. The human body contains approximately 20 milligrams of iodine. This essential element is vital for producing thyroid hormones, which regulate various metabolic processes.

2.0Preparation of Iodine 

1. From Iodine-Containing Brines

This method is widely used for its efficiency in extracting iodine from natural brine sources and offers flexibility in the oxidising agents used, depending on availability and economic considerations.

Process:

• Filtration: The brine is first filtered to remove impurities such as clay, sand, and oil that could interfere with the extraction process.
• Formation of Copper Iodide: Sulfur dioxide (SO2) is introduced into the brine solution, which is then passed through containers holding bundles of copper wire. This reaction results in the formation of copper iodide (CuI).

2I−+SO₂+2Cu → 2CuI + SO₃²⁻

• Purification: The formed copper iodide (CuI) is removed by filtration, washed with water, dried, and finely ground to ensure purity.
• Conversion to Potassium Iodide: The purified copper iodide is heated with potassium carbonate (K₂CO₃), forming potassium iodide (KI). 

CuI+K₂​CO_3​→KI+CuCO₃​

• Oxidation to Free Iodine: The potassium iodide is then oxidised to free iodine (I2) using a mixture of dichromate and sulfuric acid. 

6KI+K₂​Cr₂​O_7​+7H₂​SO₄​→3I₂​+Cr_2​(SO₄​)₃​+4K₂​SO₄​+7H₂​O

• Alternative Process: Instead of dichromate, chlorine gas (Cl2) can be oxidised to convert potassium iodide into iodine. 

2KI+Cl₂​→2KCl+I₂​

2. Laboratory Method for Iodine Preparation

Procedure:

• Mixing: Combine potassium iodide, manganese dioxide, and sulfuric acid in a reaction vessel.
• Heating: Heat the mixture to initiate the reaction.
• Reaction: Manganese dioxide oxidises iodide ions to elemental iodine: 

2KI+MnO₂+4H₂SO₄ → I₂+MnSO₄+4KHSO₄+2H₂O

• Separation: Collect the iodine, which can be purified by sublimation.

This method efficiently produces iodine in a laboratory setting and provides a straightforward procedure for obtaining pure iodine from iodide salts.

3.0Physical Properties of Iodine

• Melting Point: 113.7°C (236.7°F, 386.9 K)
• Boiling Point: 184.4°C (363.9°F, 457.6 K)
• Density: 4.933 g/cm³
• Atomic Number: 53
• Relative Atomic Mass: 126.904
• State at 20°C: Solid
• Iodine is a nearly black, nonmetallic solid with a glittering crystalline structure.
• It consists of diatomic molecules (I₂) and is in molten and gaseous states.
• At temperatures above 700 °C (1,300 °F), iodine molecules dissociate into iodine atoms.
• Iodine has moderate vapour pressure and sublimes to a deep violet vapour, irritating the eyes, nose, and throat.
• Concentrated iodine is poisonous and can damage skin and tissues.

4.0Chemical Properties of Iodine

• Molecular Structure: Iodine molecules (I_2​) contain numerous lone pairs of electrons, making iodine a good Lewis base.
• Solubility: Iodine has poor solubility in water, but solubility increases when potassium iodide (KI) is added, forming the soluble triiodide complex:

    KI+I₂→KI₃

• Inter-Halogen Compounds: Due to its large size among halogens, iodine can form inter-halogen compounds with other halogens: 

   I₂  +X₂ → IX_n, where X=F, Cl, Br and n=1,3,5,7

• Solvent Dissolution: Iodine dissolves in various polar and non-polar solvents, such as alcohol, benzene, ether, ketones, and carbon tetrachloride, resulting in different coloured solutions.
• Electronegativity and Oxidizing Ability: Iodine has lower electronegativity compared to other halogens, making it a weaker oxidising agent than fluorine (F), chlorine (Cl), and bromine (Br).
• Direct Reactions with Metals: Iodine reacts directly with certain metals to form iodides: 

2Ag + I₂ → 2AgI

2Al + 3I₂ → 2AI₃

5.0Uses or Applications of Iodine

• Photography: Iodine was first commercially used in photography, where Louis Daguerre developed a technique involving iodine to produce images on metal plates.
• Pharmaceutical Industry: Iodine is utilised in various pharmaceuticals, especially for its antiseptic properties in wound care.
• Printing Industry: It is used in specific printing processes.
• Animal Feed: Iodine is incorporated into animal feed to ensure that livestock receive adequate iodine.
• Water Purification: Iodine is employed as a disinfectant for water purification.
• Medicinal Use: Iodine is included in many medicines for cleaning and disinfecting wounds.

6.0Environmental Effects of Iodine

• Air and Water Interaction: Iodine in the air can combine with water particles and dissolve into water or soil.
• Plant Absorption: Plants absorb iodine from water and soil during their growth.
• Human and Animal Intake: Iodine is ingested by humans and animals through plant sources.
• Evaporation: Dissolved iodine in water can evaporate and re-enter the atmosphere.
• Radioactivity: While iodine can be radioactive, it is generally not harmful to life in its stable forms.