Iodine (I): Properties & Uses


One of the most popular elements among all the members of the halogen family is Iodine. Many of us are familiar with it or perhaps heard of the thyroid diseases caused by its deficiency. Iodine is a micronutrient element that is fundamental to a living body and is essential for the support and growth of humans and animals. The efficacy of iodine in preventing goitre is known to humankind since 3000 BC, and the knowledge of this treatment was available in Greece by the time of Hippocrates. Nevertheless, iodine, as we know it today, did not come into the picture until the nineteenth century. Today it is used in various areas and is closely linked to our lives. Apart from its medical properties, it is also well known for its lustrous black-purple colour and extensive chemistry, which makes it employable in various industrial sectors throughout the world.

Discovery and Naming

Bernard Courtois

Translation: Bernard Courtois The Discoverer of Iodine

The brilliant purple coloured vapours of iodine were first observed in 1811 by a French chemist Bernard Courtois, son of Jean-Baptist Courtois. J B Courtis was in the business of manufacturing nitre (Pottasium nitrate, KNO3) for gunpowder during the French Revolution. Bernard Courtois joined the family business and started working with his father. The nitre was produced from the seaweed that was found along the coast of Normandy and Brittany. The process involved burning the seaweed and then extracting its ash with water to obtain a mother liquid which contained chlorides, bromides, iodides, carbonates, and sulfates of sodium, potassium, magnesium, and calcium. This liquid then goes under the process of leaching to produce sodium chloride, potassium chloride, and potassium sulfate. As the evaporation continues, the liquid is left with sodium iodide and potassium iodide, along with several other chemicals including sulfur compounds. They used sulphuric acid to get rid of these sulfur compounds. However, on one eventful day in 1811, B. Courtois accidentally added the sulphuric acid in excess. To his astonishment, violet-coloured vapours started evolving from the liquid which on condensation formed a black-purple coloured crystal. Courtois noticed that the new substance did not readily form compounds with oxygen or with carbon, that it was not decomposed at red heat, and that it was combined with hydrogen and with phosphorus. He suspected that it was a new element, but he lacked the funding to pursue his research on it. Therefore, Courtois gave the specimen to his university mates Charles Bernard Desormes and Nicolas Clement. He also gave a few units to Joseph Louis Gay-Lusac. On 29 November 1813, Desormes and Clément made Courtois’ discovery public, and it was Gay-Lusac who gave the name “iode” in reference from a Greek word for violet colour.


Louis Joseph Gay Lussac

Occurrence and Production


Around 99.6% of the total earth composition contributes to only 32 elements in the periodic table. Iodine is one of the other 64 naturally occurring elements that contribute to 0.4% of the earth. Its estimated abundance on earth places it on the 46th rank among these elements. Iodine is one of the rarest nonmetallic element to be found on the earth. Therefore, it takes sophisticated methods to detect and isolate it. It is only found in large quantities in seaweeds, sponges, and corals. During the nineteenth century, seaweed and algae were the major sources of iodine.



It is also found in the underground waters from certain deep oil-well, gas-well boring, and mineral springs, and, the most impressive of all, the vast natural deposits of sodium nitrate. The iodine-rich natural gas repositories are mainly located in Japan and the USA, whereas, the deposits of sodium nitrate known as “Caliche Ores” are mainly found in the northeastern parts of Chile. In fact, approximately 2/3 of the total iodine production in the world originates from Chile and 1/3 from Japan, together accounting for nearly 90% of the iodine globally. However, with the advancement in technology, the method of iodine synthesis has been changed a lot with time. These methods can be categorised as follow:

1. Earlier Methods of Iodine Production

1. Ash or Kelp Burning

Seaweed Kiln

A Seaweed Kiln

Most of the earlier methods for the economical extraction of iodine were majorly based on seaweed for the source. The seaweeds are dried on dunes and burned in pits or kilns to produce the ash called kelp. Seaweed kilns are trenches dug in the ground, between 5 and 10 m long and completely lined with flat stones, fixed together with clay. The kelp is then lixiviated with water to extract the soluble salts, such as alkaline chlorides, sulfates, and carbonates, by the process of crystallization. Sulphuric acid is then added to remove any remaining impurities. The liquid is then gently warmed, with manganese dioxide in small quantities being added from time to time, when the iodine distils over and is collected.

{2NaI} + {3H}_{2}{SO}_{4} + {MnO}_{2} →  {I}_{2}+ {2NaHSO}_{4}+ {MnSO}_{4} + {H}_{2}{O}

2. Precipitation

The precipitation method is used for the extraction of iodine from a brine solution. After removing the impurities from brine, the solution is passed through a stream of sulfur dioxide and then through a number of containers holding bundles of copper wire which causes the precipitation of insoluble cuprous iodides. This precipitated cuprous iodide is then stirred with water to separate the adhering iodide. The crude cuprous iodide is then oxidised to give iodine.

{2NaI} + {2CuSO}_{4} + {2FeSO}_{4}{2CuI} + {Na}_{2}{SO}_{4}+ {Fe}_{2}({SO}_{4})_{3}

{2CuI} + {O}_{2}{2CuO} + {I}_{2}

For silver iodide, silver nitrate solution is added to the brine, which results in the precipitation of silver iodide. The silver iodide is filtered and treated with scrap iron to form metallic silver and a solution of ferrous iodide. Ferrous iodide is then treated with chlorine to liberate iodine.

{NaI} + {AgNO}_{3}{AgI} + {NaNO}_{3}

{2AgI} + {Fe}{FeI}_{2} + {2Ag}

{2FeI}_{2} + {3Cl}_{2}{2FeCl}_{2} + {2I}_{2}

3. Adsorption

This process involves acidification of brine to pH 2 by adding sulfuric acid and then oxidizing it with sodium nitrite. Thus obtained brine solution, containing the free iodine, is treated directly with active charcoal. The iodine-saturated charcoal is then separated and washed. Iodine absorbed on active charcoal is recovered by the elution of sodium hydroxide or sodium carbonate. The combined eluents are acidified with sulfuric acid and then oxidized with sodium nitrite to form iodine precipitate.

{2NaI}  + {2NaNO}_{2} + {2H}_{2}{SO}_{4}{I}_{2} + {2NO} + {2H}_{2}{O} + {2Na}_{2}{SO}_{4}

2. Modern Methods of Iodine Production



Though iodine is one of the scariest non-metallic element found on the earth, in earth’s crust it can be found in the mineral form. With the advancement in technology and discovery of new sources of iodine, the methods of its recovery from the natural resources become different from the earlier ones. The mineral lautarite, {Ca}({IO}_{3})_{2}, and dietzeite, ({Ca}({IO}_{3})_{2}{CaCrO}_{4}), are the two crystalline forms in which iodine naturally occurs in “caliche,” the natural saltpetre. The only iodine obtained from minerals has been a by-product of the processing of nitrate ore in Chile. The other natural source of iodine is a brine of natural gas and oil. During the 19th century, caliche was the major source of iodine throughout the world, whereas, brines of natural gas and oils are considered a more economic source of iodine since the early 20th century. The method of iodine recovery from these resources can be categorised as follow.

1. Production from Caliche Ores


Caliche Ore

The main source of iodine in the mineral form, Lautarite ({Ca}({IO}_{3})_{2}) and dietzeite ({Ca}({IO}_{3})_{2}{CaCrO}_{4}), are obtained as the by-product of nitrate ore processing. The extraction of elemental iodine from these minerals is a two-step process altogether. The process includes extraction of iodate solutions from the caliche ore followed by reduction of iodate to iodine solution. Depending on the nitrate concentration in the ore, the extraction of iodine is done by leaching in one of the two ways:

  • For low concentrations of nitrate (<5%) and iodine content (over 500 ppm), heap leaching is preferred. It is a low-cost hydrometallurgical technology mainly used for the extraction of gold and uranium nowadays. More than one caliche heaps are arranged in parallel and then leached (a process of a solute becoming detached or extracted from its carrier substance by way of a solvent) with the help of suitable chemicals. Freshwater is then added for compensating the water in the purge and the evaporation losses. A purge is required for avoiding an excessive increase in dissolved salts, which would harm the process, mainly because of unwanted crystallization of slats contained in the caliche, such as sulfates, chlorides, and nitrates. Iodine is then recovered by introducing a reducing agent {SO}_{2} to the obtained product.

    Heap Leaching

    Heap Leaching

  • For higher concentrations of nitrate in the ore, heap leaching can be done by arranging the heaps in series for leaching, but a more effective method, vat leaching, is preferred due to its economic advantages. Vat leaching involves placing ore, usually after size reduction and classification, into large tanks or vats at ambient operating conditions containing a leaching solution and allowing the valuable material to leach from the ore into solution. The vat solution is first submitted to a temperature drop to crystallize sodium nitrate. The mother liquor is then submitted to an iodine extraction process similar to that for heap leaching, but the end product is prilled or flaked iodine. The larger fraction is then fed into an absorption tower where it is contacted with SO2, obtained by sulfur combustion. In the absorption tower, iodate is reduced all the way to iodide. The above-mentioned processes can be understood with the help of the following reaction.
    2{IO}_{3}^{-} + 5{SO}_{2} + 4{H}_{2}{O}{I}_{2} + 5{SO}_{4}^{-} + 8{H}^{-}
Vat Leaching

Vat Leaching Plant

2. Production from Natural Gas and Oil Brines

During the twentieth century, brines emerged as a new source of iodine. The Japanese Minami Kanto gas field east of Tokyo and the American Anadarko Basin gas field in northwest Oklahoma are the two largest such sources. The brine is hotter than 60 °C from the depth of the source. There are mainly three processes by which the iodine can be extracted from the brines.

  • Blown-Out Method: The blowout process (also known as the air-stripping process) is the principal method for extracting iodine from brines. Initially, the brine undergoes a process consisting of skimming and settling that removes impurities such as oil, clay, and other undesirable materials. Chlorine is then injected into the brine, where oxidation occurs. Oxidized iodine is then extracted in a countercurrent air flow process based on the distinct vapour pressure of the iodine.
    Blown-Out Method

    Blown-Out Method

    The iodine-filled air is reduced to iodide by the addition of sulfur oxide. In the finishing process, the acidic iodide solution is oxidized by the addition of chlorine to form iodine crystals.

    {I}_{2}(air) + {SO}_{2} + {2H}_{2}{O}{2HI} + {H}_{2}{SO}_{4}
    {2HI} + {Cl}_{2}{I}_{2} + {HCl}
  • Ion Exchange Resin Method: In ion exchange, the free iodine from oxidized brines is flown through resin-packed ion exchange columns. An oxidizing agent, chlorine or sodium hypochlorite, is added and the brine is oxidized. The resultant brine is passed through a column packed with an anion exchange resin that adsorbs the iodine, which is in the form of a polyiodide. Once the resin is saturated with free iodine, it is transferred to an elution column that is treated with a caustic salt solution. The eluted iodine is recovered by addition of acid and removed by solid/liquid separation.

    Ion Exchange Resin Method

    Ion Exchange Resin Method

Isotopes of Iodine

Iodine exists in 37 different isotopes ranging from _{ }^{ 108 }{ I } to _{ }^{ 144 }{ I }. The majority of iodine isotopes undergoes radioactive decay, except _{ }^{ 127 }{ I }. Therefore, iodine is also known as a monoisotopic element. Radioisotopes of iodine show extensively remarkable characteristics. For instance, one of its isotope, _{ }^{ 129 }{ I }, has the longest half-life of 15.7 million years, which is far short for it to be a primordial nuclide. However, the presence of stable _{ }^{ 129 }{ Xe } in excess amounts suggests that it is a decay product of _{ }^{ 129 }{ I } produced during several supernovae explosions. Hence it is also referred to as “extinct radionuclides”. Nevertheless, the traces of _{ }^{ 129 }{ I } are believed to be formed during the formation of our solar system, and therefore, helps in studying the corresponding properties. Another remarkable property of _{ }^{ 129 }{ I } is that it is one of the 21 chemical elements that are found naturally on earth essentially as a single nuclide, thus known as a mononuclidic element. Most _{ }^{ 129 }{ I } derived radioactivity on Earth is man-made, an unwanted long-lived byproduct of early nuclear tests and nuclear fission accidents.

Isotopes of Iodine

All other radioisotopes of iodine have a half-life of fewer than 60 days. With the certian risk of expouser, they have some remarkable properties which make them employable in various sectors of society. For instance, _{ }^{ 123 }{ I }, _{ }^{ 124 }{ I }, _{ }^{ 125 }{ I }, and _{ }^{ 131 }{ I } are of particular interest in medicinal biology, whereas, _{ }^{ 135 }{ I }  plays an important role in nuclear physics and chemistry.

Properties of Iodine

Iodine is a nonmetallic element of the seventeenth group of the periodic table of elements. It is the heaviest stable halogen element which is a is a purplish-black solid and has a glittering crystalline appearance under standard temperature and pressure conditions. Though iodine is the least reactive element in the halogen family, it is still one of the most reactive among the other elements of the periodic table.


Physical Properties

Solid Iodine Crystals

Solid Iodine Crystals

Iodine is a member of the halogen family with atomic no. 53 and atomic mass 129.9044 u. Iodine under standard conditions is a bluish-black solid and has a glittering crystalline appearance. It has a moderate vapour pressure at room temperature and in an open vessel slowly sublimes to a deep violet vapour that is irritating to the eyes, nose, and throat. Solid iodine starts melting at a relatively low temperature of 113.7°C and turns dark brown although the liquid is often disguised by a dense violet vapour of gaseous iodine.

Liquid dark brown iodine changes into a violet coloured gas once the temperature reaches is 184°C, the boiling point of iodine. At the room temperature and normal pressure conditions, the density of iodine is found to be 4.98 gram per cubic centimetre. The molecular lattice of iodine contains discrete diatomic molecule with a covalent radius of 139±3 pm that is also present in the molten and the gaseous state. Iodine dissolves easily in most organic solvents such as hexane, benzene, carbon tetrachloride, and chloroform owing to its lack of polarity, but it is only slightly soluble in water. However, the solubility of elemental iodine in water can be increased by the addition of sodium or potassium iodide. It will provide with extra iodine ion to molecular iodine and increase the solubility via triiodide formation. Because it has the largest atomic radius among the halogens, iodine has the lowest first ionisation energy, lowest electron affinity, lowest electronegativity and lowest reactivity of the halogens. It is also the least volatile element in group 17.

{I}_{2}(s) + {I}^{-}(aq) → {I}_{3}^{-}(aq)

Chemical Properties

Atomic Structure

Iodine has an electron configuration of [Kr]{4d}^{10}{5s}^{2}{5p}^{5} with the seven valance electrons in the fifth and outermost shell. As a member of the halogen family, iodine shares many of the typical characteristics of the other elements in group 17. For instance, like other halogen elements, iodine is also one electron short for an octet. As a consequence, it forms iodides with most elements, with iodine possessing the formal oxidation state −1, and act as a good oxidizing agent. It can also form compounds possessing other formal oxidation states −1, +1, +3, +5, and +7. However, since iodine has the largest electron cloud among them that can be easily polarised, its molecules have the strongest van der Waals interactions among the halogens. Bond dissociation energies of chlorine, bromine, and iodine decrease down the halogen group as the size of the atom increases. The bond energy of fluorine is, however, lower than that of chlorine, and bromine and only slightly larger than iodine because of interelectronic repulsions present in the small atom of fluorine. The oxidizing power of the halogens decreases from fluorine to iodine. Hence iodide ions are oxidized to iodine with either chlorine or bromine.

{2I}^{-}(aq) + {Cl}_{2}(g) → {I}_{2}(s) + {2Cl}^{-}(aq)

Compounds of Iodine

Iodine forms binary compounds with hydrogen, metals, other halogen elements, and oxygen. Iodides range from completely ionic to covalent structures; for example, potassium iodide is ionic and titanium tetraiodide is covalent. In general, the ionic iodides are the most soluble of the halides of a given element. Commercially, iodides are the most important class of iodine compounds. In general, they are less fusible and volatile than chlorides or bromides. At higher temperature, even the more stable iodides show dissociation. Volatile covalent iodides, such as those of aluminium and titanium, form inflammable mixtures with air. Dry nitrogen triiodide, NI3, formed by reacting iodine with excess ammonia, will explode at an ordinary temperature under the slightest shock.


Different Functions of Iodine in A chemical reaction

Generally, organic iodides can be divided into two classes of alkyl iodides and aryl iodides. Since alkyl iodides show the highest reactivity among alkyl halides, typical reactions of alkyl iodides include nucleophilic substitution, elimination, reduction, and the formation of organometallics. On the other hand, aryl halides do not undergo direct displacement by nucleophiles as observed in the case of alkyl halides, because of their low reactivity toward nucleophilic substitution. Therefore, aryl iodides undergo nucleophilic substitutions, iodine–metal exchange for organometallic compounds, and coupling reactions. The development of new reactions for the synthesis of aromatic compounds is one of the hot research fields in organic chemistry.

Uses Of Iodine

Industrial Applications

1. Disinfectant

Like chlorine, iodine also has some remarkable disinfectant properties that can be employed for the sterilization of drinking water, swimming pools, and even for wastewater management. Iodine was first considered as a water disinfectant during the first world war and it was used to meet up the hydration needs of the troops. Since then many studies have made iodine effective as a germicide to kill microorganisms that can be harmful to human health. One of the considerable importance is the work of Chang and Morris, which led to the development of tetraglycine hydroperiodide tablets (Globaline), which have been successfully used to disinfect small or individual water supplies in the U.S. Army.

Portable Aqua

Nevertheless,  there is a strict procedure to apply iodine as a disinfectant to drinking water, as the killing effect of iodine depends on the temperature.  It is recommended that a standing time of 60, 30, and 15 min should be followed at 5, 15, and 30°C, respectively (one iodine tablet for 1 l of water). Iodine is often used in conjunction with iodine-complexing nonionic surfactants or polymers (iodophors) in disinfectants that are used in dairies, laboratories, and food processing plants, and for the sanitation of dishes in restaurants. It can also be used as an effective aerial disinfectant.

2. Agrochemicals


Several halogen-based agrochemicals constituting several insecticides, pesticides and herbicides are dominant over the market for a very long time. Generally, iodine-containing compounds are in the minority and some of them are “mixed” with other halogens like bromine or chlorine. Most common iodine-based agrochemicals used by farmers to improve the productivity of their crops and their protections are Ioxynil (herbicide), Ioxynil octanoate (herbicide), triiodobenzoic acid (plant growth regulator), Iodobonil (herbicide), Benodanil  (fungicide), and Iodocarb (fungicide). However, considering the environmental effects of halogen-based chemicals, it is necessary to look for safe optimal substitutes. It has been shown that the introduction of iodine atoms into a molecule can influence the biological efficacy, depending on their mode of action, the physicochemical properties or the target interaction of the compound. In the past few years, the technical availability of active ingredients containing iodine has been improved by an increase in access to new intermediates.

3. Stabilizers

Stabilizers are the material used by industries to enhance the durability of several household products. Iodine is a major ingredient in the compounds that are used as stabilizers in the manufacturing of various daily-life objects. One of the high-end consumers of iodine-based stabilizers is the nylon industry. Nylon is an industrially important and useful material with multiple applications, including as an engineering resin and fibre. Thermoplastic forms of nylon are stabilized with copper iodide. Nylon fibre producers use potassium iodide for tire and airbag cord nylon. The potassium iodide reacts in situ with cupric acetate to form cupric iodide, which acts as a heat stabilizer.

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4. Optical Polarizing Film

Polarization | Baumer international

During the early nineteenth century, the only known tools for demonstrating the polarization of electromagnetic waves were tourmaline crystals, quinine, and Nicol prism. While investigating the effects of quinine in dogs, the toxicologist Dr William Bird Herapath and his assistant accidentally mixed tincture of iodine with the urine of a dog that had ingested large quantities of quinine. Instantly, metallic crystallites precipitated from the urine. These iodine-rich crystals made a remarkable transformation in the optical industry. Herapath was able to show that his synthetic crystals were at least five times more effective than the best tourmaline crystals in transmitting a polarized ray of light. He also showed that only iodine, sulfuric acid, and quinine were required to synthesize these crystals. Today, while both iodine and iodine‐free polarized films are widely available, it is still the case that those incorporating iodine are regarded as being superior in terms of transmittance and polarization efficiency for the visible part of the electromagnetic spectrum.

5. Etching Gas

The etching is a microfabrication process of removing layers from the surface of a silicon wafer during the manufacturing of integrated circuits. Generally, fluorocarbon gases are used to deposit fluoropolymers to the underlying layer of silicon. CF2 radicals are commonly used as the main gas precursor for polymer deposition. However, the chemistry of CF2 is not very favourable to control the balance between radical flux (CF2 radicals) and ion flux (CF3+ions) during SiO2 contact etching. Low-damage and highly selective etching could be achieved by using an environmentally harmonized gas chemistry (CF3I) plasma. This is because the CF3I plasma could reduce generating UV photons and F radicals.

Silicon dioxide etching

6. Fluorinated Repellents


Fluorinated water and oil repellents are widely used around the world by textile industries. The ability of a solid surface to repel a liquid droplet can be estimated by comparing the critical surface tension of the solid with the surface tension of the liquid. If the former is smaller than the latter, the solid can repel the liquid. Iodine and chemicals containing iodine are used as the main raw materials for the synthesis of fluorinated repellents. Perfluoroalkyl iodide is the most widely used compound for the synthesis. Chemicals with perfluoroalkyl groups can repel oils and alcohols.


7. Iodine in Polymer Synthesis

Iodine and its compounds play a very critical role in the chain-growth or addition polymerization of alkenes or vinyl monomers. The role of iodine in polymerisation reaction is as versatile as its chemistry since it is characterized as weakly electron-withdrawing, soft or readily polarizable, and resonance-inducing, and thereby they act as a Lewis acid (electron acceptor), a good leaving group, and a radical acceptor, depending on monomers and reaction conditions. Due to these multiple properties, iodine, alkyl iodide, iodide salts, and metal–iodide complexes can be used as initiators, catalysts, end-capping agents, and chain-transfer agents in various cationic and radical polymerizations.


8. Iodine in Dye-Sensitized Solar Cells

The evolution of solar cells has received great interest since the development of photoelectrochemical cell by the two Japanese pioneers in 1960, K Honda and A Fujushima. In the late 1960s, it was discovered that illuminated organic dyes can generate electricity at oxide electrodes in electrochemical cells. A large increase in the effective surface for the adsorption of the sensitizing dye molecules generated a large leap from conversion efficiencies below 1% to efficiencies above 7%. This groundbreaking discovery, although initially met with some distrust, led the foundation of an emerging solar cell technology with the potential to take up the challenge from established solar cell technologies. Since then, the low-cost manufacturing of DSSC had made an influencing impact on the energy sector.

Dye-Sensitized Solar Cells

Dye-Sensitized Solar Cells (DSSC)

Regeneration of the oxidized dye at the photoelectrode is a central process in the DSSC. In this aspect, other redox systems have been found to regenerate (reduce) the dye molecules at a higher rate than the iodide ion of the I–/I3 – system. The iodide/triiodide redox system ruled the field of DSSC devices in terms of stability and conversion efficiency for many years. However, with the availability of better alternatives, the use of the iodide/triiodide redox system is less prominent.

Medical Applications

1. Synthetic Thyroid Hormone


Thyroid Gland

Thyroxine (T4) and triiodothyronine (T3), collectively known as thyroid hormones (THs), are the two very essential hormones secreted by the thyroid gland present in mammals. It is essential for brain and physical development in infants and metabolic activity in adults. The function of the thyroid gland is to take iodine from the food intakes of humans and animals and combine it with amino acid, tyrosine, to produce T3 and T4. Any defects that affect the proper concentration, balance, or influence of thyroid hormones (THs) have the potential to cause thyroid disease and goitre (swelling of the thyroid). While the reduced formation of THs causes hypothyroidism with symptoms, such as fluid retention, weight gain, fatigue, and lethargy, and most commonly results from either insufficient dietary iodine or an autoimmune response to thyroid peroxidase (TPO), the increase in their concentration can cause hyperthyroidism with symptoms including weight loss, hyperactivity, anxiety, and trembling muscles.

Food Intake

Two glands in the brain, the hypothalamus and the pituitary, communicate to maintain T3 and T4 balance. The hypothalamus produces thyroid releasing hormone (TRH) that signals the pituitary to tell the thyroid gland to produce more or less of T3 and T4 by either increasing or decreasing the release of thyroid-stimulating hormone (TSH). Many countries supplement table salt with either sodium or potassium iodate to prevent dietary deficiency. Thyroid replacement hormones are used to treat hypothyroidism and myxedema, a condition that is caused by prolonged hypothyroidism. They also are used to manage thyrotoxicosis, a condition in which there are high levels of thyroid hormones resulting from overactive thyroid glands and too much thyroid hormone. Thyrotoxicosis may progress to hypothyroidism or cause the growth of goitres necessitating the use of thyroid replacement hormones. If TH production cannot be controlled by antithyroid drugs such as methimazole or propylthiouracil, then the thyroid is often destroyed by 131I radiation or removed surgically. This then similarly requires daily thyroid replacement therapy.

Goitre disease

Goitre disease

2. Pharmaceuticals


The reactivity of halogens plays an important role in the manufacturing of several pharmaceuticals. A significant number of drugs and drug candidates in clinical development have halogen substituents. Insertion of halogen atoms on lead compounds was predominantly performed to exploit their steric effects, through the ability of these bulk atoms to occupy the full binding site of molecular targets. A few iodine-containing drugs are known such as the thyroid hormone thyroxine, an anti-herpesvirus, antiviral drug, idoxuridine (IDU), and a class III antiarrhythmic agent, amiodarone.


Since C–I bonds are highly polarizable, the iodinated compounds are relatively unstable. Amiodarone is categorized as a class III antiarrhythmic agent and prolongs phase of the cardiac action potential, the repolarization phase in which there is normally decreased calcium permeability and increased potassium permeability. However, the use of amiodarone is associated with several side effects, including photosensitivity, corneal microdeposits, pulmonary toxicity, hepatotoxicity, peripheral neuropathy, hyperthyroidism, and hypothyroidism.

3. X-Ray Contrast Agents

X-Ray Image

A common X-ray examination does not require a contrasting agent to produce an image. However, if it is required to enhance the visibility of vascular structures and organs during radiographic procedures, intravenous radiocontrast agent (radiographic dye) containing iodine is used. An element with high atomic numbers Z can be used as a contrasting agent since the X-ray absorption coefficient μ is proportional to Z according to μ = ρZ/AE with ρ = density, A = atomic mass, and E = X-ray energy. As iodine has a high atomic number, 53, compared to most tissues in the body, the administration of iodinated material produces image contrast due to differential photoelectric absorption. By improving the visibility of specific organs, blood vessels or tissues, contrast materials help physicians diagnose a medical condition. Iodine has a particular advantage as a contrast agent because the energy at which its photoelectric absorption is more likely to occur is very similar to the energy of X-Rays, i.e., 33.3 keV. Iodine’s relative inertness also plays an important role in CT scan, angiography and other diagnostic imaging techniques.

4. Animal feed

Animal Feed

Another area of major iodine consumption is in the form of additives for animal feeds. In 1986, the recommended amount of ethylenediamine dihydroiodide  (EDDI) consumed in animal feeds was lowered from 50 to 10 mg per head per day. Cattle and sheep are fed iodine compounds for the prevention of soft tissue lumpy jaw. The use of iodized salt has significantly reduced the incidence of goitre in livestock in certain iodine-deficient geographical regions. Besides, iodized protein in fowl and cattle feed increases yields of eggs and milk. About 25% of the reported domestic consumption of iodine was in animal feeds, primarily as the compound EDDI, but also as potassium iodide, calcium iodate, and calcium periodate.

Miscellaneous Applications


The major end-use of iodine is in catalysts. Most acetic acid is produced by methanol carbonylation processes. The process involves hydrogen iodide and iodomethane as the intermediates. Two processes are known for the carbonylation of methanol: the rhodium-catalyzed Monsanto process and the iridium catalyzed Cativa process.

rhodium-catalyzed Monsanto process

Monsanto process

The latter process is greener and more efficient. Iodide catalysts, such as titanium tetraiodide and aluminium iodide, are also significant in the dehydrogenation of butane and butene to butadiene [], and in the preparation of stereoregular polymers.

Cativa process

Cativa process

2. Spectroscopy

Spectroscopy and Spectrometry

The spectrum of the iodine molecule, I2, lies within the wavelength range of 500–700 nm. It is, therefore, a commonly used wavelength reference. However, the hyperfine structure of the iodine molecule reveals itself within the wavelength range of  532-660 nm. The precision spectroscopy of molecular iodine (I2) is important for many applications, e.g., optical frequency metrology, optical communications, and studies in atomic and molecular physics.

3. Cloud-Seeding

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Controlling the weather may still be years away, but traditions and rituals to please the deities for a good monsoon and good harvest exists since the ancient time. However, since 1940, this approach has taken a technological turn. Clouds, whether in summer or winter, are not perfectly efficient at producing precipitation. There’s some part of a storm that’s much less than 100% efficient in turning clouds into precipitation. Inside the cloud, many water droplets or ice crystals are floating in random directions. These particles are not heavy enough to form precipitation by themselves, so they stick to other tiny particles floating in the air, such as smoke, dust particle, and meteoric debris, collectively known as condensation nuclei.

Cloud Seeding

Cloud seeding is the process of introducing a seeding agent to act as condensation nuclei for the water molecules. The most common seeding agent chemicals used for this purpose include silver iodide, potassium iodide and dry ice (solid carbon dioxide). This is done with the help of pyrotechnic flares attached to an aircraft. Silver Iodide has a very similar molecular structure to ice. Therefore, when silver iodide is introduced to the cloud, it triggers the ice crystallisation process. As more and more crystals start to stick together, they become heavy and precipitates. The 1932 Nobel prize-winning chemist, Irving Langmuir, and his deputy Vincent J Schaefer are credited for their pioneering work in this field.

Health Aspects

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Iodine provides us with a very essential micronutrient for our body, However, if elemental iodine is consumed without its dilution, it can cause harm to us. The lethal dose for an adult human is 30 mg/kg, which is about 2.1–2.4 grams for a human weighing 70 to 80 kg. The over-exposure of elemental iodine can also cause serious damage to the skin tissues including skin burns. It is advised to handle the solid iodine crystals with care. Many people can develop hypersensitivity to iodine as it can also cause several allergies.



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