IODINE: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, RISKS, USES - CHEMISTRY - 2024

The iodine is a reactive non - metallic element belonging to group 17 of the periodic table (halogens) and is represented by the chemical symbol I. It is essentially an element quite popularly known from the iodine water until hormone tyrosine.

In solid state, iodine is dark gray with a metallic luster (lower image), capable of sublimating to produce a violet-colored vapor which, when condensed on a cold surface, leaves a dark residue. The experiments to demonstrate these characteristics have been numerous and attractive.

Robust iodine crystals. Source: BunGee

This element was isolated for the first time by Bernard Curtois in the year 1811, while obtaining compounds that served as raw material for the manufacture of nitrate. However, Curtois did not identify iodine as an element, a merit shared by Joseph Gay-Lussac and Humphry Davy. Gay-Lussac identified the element as “iode”, a term that came from the Greek word “ioides” with which the color violet was designated.

Elemental iodine, like the other halogens, is a diatomic molecule, made up of two iodine atoms linked by a covalent bond. The Van der Waals interaction between iodine molecules is the strongest among halogens. This explains why iodine is the halogen with the highest melting and boiling points. Furthermore, it is the least reactive of the halogens, and the one with the lowest electronegativity.

Iodine is an essential element that needs to be ingested, since it is necessary for body growth; brain and mental development; metabolism in general, etc., recommending a daily intake of 110 µg / day.

Iodine deficiency in the fetal state of a person is associated with the appearance of cretinism, a condition characterized by slowing down of body growth; as well as insufficient mental and intellectual development, strabismus, etc.

Meanwhile, an iodine deficiency at any age of the individual is associated with the appearance of a goiter, characterized by a hypertrophy of the thyroid. Goiter is an endemic disease, since it is confined to certain geographical areas with their own nutritional characteristics.

History

Discovery

Iodine was discovered by the French chemist Bernard Curtois, in the year 1811, while working with his father in the production of saltpeter, requiring sodium carbonate for this.

This compound was isolated from seaweed that they collected off the coasts of Normandy and Brittany. To this end, the algae were burned and the ashes were washed with water, the resulting residues being destroyed with the addition of sulfuric acid.

Once, perhaps by a fortuitous mistake, Curtois added an excess of sulfuric acid and a purple vapor formed that crystallized on the cold surfaces, settling as dark crystals. Curtois suspected he was in the presence of a new element and called it "Substance X".

Curtois discovered that this substance when mixed with ammonia formed a brown solid (nitrogen triiodide) that exploded on minimal contact.

However, Curtois was limited in continuing his research and decided to give samples of his substance to Charles Desormes, Nicolas Clément, Joseph Gay-Lussac and André-Marie Ampère, in order to obtain their collaboration.

Emergence of the name

In November 1813, Desormes and Clément made Curtois's discovery public. In December of that same year, Gay-Lussac pointed out that the new substance could be a new element, suggesting the name "iode" from the Greek word "ioides", designated for violet.

Sir Humphry Davy, who received a portion of the sample given to Ampère by Curtois, experimented with the sample and noted a similarity to chlorine. In December 1813, the Royal Society of London was involved in the identification of a new element.

Although a discussion arose between Gay-Lussac and Davy about the identification of iodine, they both acknowledged that Curtois was the first to isolate it. In 1839 Curtois finally received the Montyn Prize from the Royal Academy of Sciences in recognition of the isolation of iodine.

Historical uses

In 1839, Louis Daguerre gave iodine its first commercial use, inventing a method for producing photographic images called daguerreotypes, on thin sheets of metal.

In 1905, the North American pathologist David Marine investigated iodine deficiency in certain diseases and recommended its intake.

Physical and chemical properties

Appearance

Sublimation of iodine crystals. Source: Ershova Elizaveta

Solid dark gray with metallic luster. When sublimated, its vapors are purple in color (top image).

Standard atomic weight

126.904 u

Atomic number (Z)

53

Melting point

113.7 ºC

Boiling point

184.3 ºC

Density

Ambient temperature: 4.933 g / cm ³

Solubility

It dissolves in water to produce brown solutions with a concentration of 0.03% at 20 ºC.

This solubility is considerably increased if there are previously dissolved iodide ions, since an equilibrium is established between I ^- and I ₂ to form the anionic species I ₃ ^-, which solvates better than iodine.

In organic solvents such as chloroform, carbon tetrachloride, and carbon disulfide, iodine dissolves giving a purple tint. Also, it dissolves in nitrogenous compounds, such as pyridine, quinoline, and ammonia, to form a brownish solution, again.

The difference in the colorations lies in the fact that the iodine is dissolved as solvated I ₂ molecules, or as charge transfer complexes; the latter appear when dealing with polar solvents (water among them), which behave like Lewis bases by donating electrons to iodine.

Odor

Pungent, irritating and characteristic. Odor threshold: 90 mg / m ³ and irritating odor threshold: 20 mg / m ³.

Octanol / water partition coefficient

Log P = 2.49

Decomposition

When heated to decomposition it emits a smoke of hydrogen iodide and various iodide compounds.

Viscosity

2.27 cP at 116 ºC

Triple point

386.65 K and 121 kPa

Critical point

819 K and 11.7 MPa

Heat of fusion

15.52 kJ / mol

Heat of vaporization

41.57 kJ / mol

Molar caloric capacity

54.44 J / (mol K)

Vapor pressure

Iodine has a moderate vapor pressure and when the container is opened it slowly sublimates to a violet vapor, irritating to the eyes, nose and throat.

Oxidation numbers

The oxidation numbers for iodine are: - 1 (I ^-), +1 (I ⁺), +3 (I ³⁺), +4 (I ⁴⁺), +5 (I ⁵⁺), +6 (I ⁶⁺) and +7 (I ⁷⁺). In all iodide salts, such as KI, iodine has an oxidation number of -1, since in them we have the anion I ^-.

Iodine acquires positive oxidation numbers when it is combined with elements more electronegative than it; for example, in its oxides (I ₂ O ₅ and I ₄ O ₉) or interhalogenated compounds (IF, I-Cl and I-Br).

Electronegativity

2.66 on the Pauling scale

Ionization energy

First: 1,008.4 kJ / mol

Second: 1,845 kJ / mol

Third: 3,180 KJ / mol

Thermal conductivity

0.449 W / (m K)

Electrical resistivity

1.39 · 10 ⁷ Ω · m at 0 ºC

Magnetic order

Diamagnetic

Reactivity

Iodine combines with most metals to form iodides, as well as non-metallic elements such as phosphorus and other halogens. Iodide ion is a strong reducing agent, spontaneously releasing an electron. Oxidation of iodide produces a brownish tint of iodine.

Iodine, in contrast to iodide, is a weak oxidizing agent; weaker than bromine, chlorine and fluorine.

Iodine with oxidation number +1 can combine with other halogens with oxidation number -1, to give the halides of iodine; for example: iodine bromide, IBr. Likewise, it combines with hydrogen to give rise to hydrogen iodide, which after dissolving in water is called hydroiodic acid.

Hydroiodic acid is a very strong acid capable of forming iodides by reaction with metals or their oxides, hydroxides and carbonates. Iodine has a +5 oxidation state in iodic acid (HIO ₃), which dehydrates to produce iodine pentoxide (I ₂ O ₅).

Structure and electronic configuration

- Iodine atom and its bonds

Diatomic iodine molecule. Source: Benjah-bmm27 via Wikipedia.

Iodine in its ground state consists of an atom that has seven valence electrons, only one from being able to complete its octet and become isoelectronic with the noble gas xenon. These seven electrons are arranged in their 5s and 5p orbitals according to their electronic configuration:

4d ¹⁰ 5s ² 5p ⁵

Therefore, I atoms show a strong tendency to bond covalently so that each individually has eight electrons in its outermost shell. Thus, two I atoms come together and form bond II, which defines the diatomic molecule I ₂ (upper image); molecular unit of iodine in its three physical states under normal conditions.

The image shows the I ₂ molecule represented by a spatial filling model. It is not only a diatomic molecule, but also homonuclear and apolar; Therefore, their intermolecular interactions (I ₂ - I ₂) are governed by the London dispersion forces, which are directly proportional to their molecular mass and the size of the atoms.

This II bond, however, is weaker compared to the other halogens (FF, Cl-Cl and Br-Br). This is theoretically due to the poor overlap of their sp ³ hybrid orbitals.

- Crystals

The molecular mass of I ₂ allows its dispersive forces to be directional and strong enough to establish an orthorhombic crystal at ambient pressure. Its high electron content causes the light to promote endless energy transitions, which causes iodine crystals to stain black.

However, when iodine sublimates its vapors show a violet coloration. This is already indicative of a more specific transition within the I ₂ molecular orbitals (those of higher energy or anti-bonding).

Base-centered orthorhombic unit cell for the iodine crystal. Source: Benjah-bmm27.

Shown above are the I ₂ molecules, represented by a spheres and rods pattern, arranged within the orthorhombic unit cell.

It can be seen that there are two layers: the bottom one with five molecules, and the middle one with four. Also note that an iodine molecule sits at the base of the cell. Glass is built by periodically distributing these layers in all three dimensions.

Traveling the direction parallel to the II bonds, it is found that the iodine orbitals overlap to generate a conduction band, which makes this element a semiconductor; however, its ability to conduct electricity disappears if the direction perpendicular to the layers is followed.

Link distances

Link II appears to have expanded; and in fact it is, since the length of its bond increases from 266 pm (gaseous state), to 272 pm (solid state).

This may be due to the fact that the I ₂ molecules are very far apart in gas, their intermolecular forces being almost negligible; while in the solid, these forces (II - II) become tangible, attracting the iodine atoms of two neighboring molecules towards each other and consequently shortening the intermolecular distance (or interatomic, seen in another way).

Then, when the iodine crystal sublimates, the II bond contracts in the gas phase, since the neighboring molecules no longer exert the same attractive (dispersive) force on their surroundings. And also, logically, the distance I ₂ - I ₂ increases.

- Phases

It was mentioned earlier that the II bond is weaker compared to the other halogens. In the gas phase at a temperature of 575 ° C, 1% of the I ₂ molecules disintegrate into individual I atoms. There is so much heat energy that just two I's rejoin they separate, and so on.

Similarly this bond breaking can occur if enormous pressures are applied to the iodine crystals. By compressing it too much (under pressure hundreds of thousands of times greater than atmospheric), the I ₂ molecules rearrange themselves as a monatomic phase I, and iodine is then said to exhibit metallic characteristics.

However, there are other crystalline phases, such as: the body-centered orthorhombic (phase II), the body-centered tetragonal (phase III), and the face-centered cubic (phase IV).

Where to find and obtaining

Iodine has a weight ratio, in relation to the earth's crust, of 0.46 ppm, ranking 61st in abundance in it. Iodide minerals are scarce, and commercially exploitable iodine deposits are iodates.

Iodine minerals are found in igneous rocks with a concentration of 0.02 mg / kg to 1.2 mg / kg, and in magmatic rocks with a concentration of 0.02 mg to 1.9 mg / kg. It can also be found in the Kimmeridge shale, with a concentration of 17 mg / kg of weight.

Also, iodine minerals are found in phosphate rocks with a concentration ranging from 0.8 to 130 mg / kg. Seawater has an iodine concentration ranging from 0.1 to 18 µg / L. Seaweed, sponges, and oysters were formerly the main sources of iodine.

Currently, however, the main sources are caliche, sodium nitrate deposits in the Atacama desert (Chile), and brines, mainly from the Japanese gas field in Minami Kanto, east of Tokyo, and the Anadarko gas field. Basin in Oklahoma (USA).

The caliche

The iodine is extracted from the caliche in the form of iodate and is treated with sodium bisulfite to reduce it to iodide. The solution is then reacted with freshly extracted iodate to facilitate its filtration. Caliche was the main source of iodine in the 19th and early 20th centuries.

Brine

After purification, the brine is treated with sulfuric acid, which produces iodide.

This iodide solution is subsequently reacted with chlorine to produce a dilute solution of iodine, which is evaporated by a stream of air that is diverted to an absorbent tower of sulfur dioxide, producing the following reaction:

I ₂ + 2 H ₂ O + SO ₂ => 2 HI + H ₂ SO ₄

Subsequently, the hydrogen iodide gas reacts with chlorine to liberate the iodine in a gaseous state:

2 HI + Cl ₂ => I ₂ + 2 HCl

And finally, the iodine is filtered, purified and packaged for use.

Biological role

- Recommended diet

Iodine is an essential element, since it intervenes in numerous functions in living beings, which are especially known in humans. The only way for iodine to enter man is through the food he eats.

The recommended iodine diet varies with age. Thus, a 6-month-old child requires an intake of 110 µg / day; But from the age of 14, the recommended diet is 150 µg / day. In addition, it is stated that iodine intake should not exceed 1,100 µg / day.

- Thyroid hormones

Thyroid-stimulating hormone (TSH) is secreted by the pituitary gland and stimulates the uptake of iodine by the thyroid follicles. Iodine is carried into the thyroid follicles, known as colloids, where it binds to the amino acid tyrosine to form monoiodotyrosine and diiodotyrosine.

In the follicular colloid, a molecule of monoiodothyronine combines with a molecule of diiodothyronine to form a molecule called triiodothyronine (T ₃). On the other hand, two molecules of diiodotyrosine can join together, forming tetraiodothyronine (T ₄). T ₃ and T ₄ are called thyroid hormones.

The hormones T ₃ and T ₄ are secreted into plasma where they bind to plasma proteins; including the thyroid hormone transporter protein (TBG). Most of the thyroid hormones are transported in plasma as T ₄.

However, the active form of thyroid hormones is T ₃, so T ₄ in the "white organs" of thyroid hormones, undergoes deiodination and is transformed into T ₃ to exert its hormonal action.

Effects edit

The effects of the action of thyroid hormones are multiple, the following being possible: increased metabolism and protein synthesis; promotion of body growth and brain development; increased blood pressure and heart rate, etc.

- Deficiency

The deficiency of iodine and, therefore, of the thyroid hormones, known as hypothyroidism, has numerous consequences that are influenced by the age of the person.

If iodine deficiency occurs during a person's fetal state, the most relevant consequence is cretinism. This condition is characterized by signs such as impaired mental function, delayed physical development, strabismus, and delayed sexual maturation.

An iodine deficiency can induce a goiter, regardless of the age at which the deficiency occurs. A goiter is an overdevelopment of the thyroid, caused by excessive stimulation of the gland by the hormone TSH, released from the pituitary as a result of iodine deficiency.

The excessive size of the thyroid (goiter) can compress the trachea, limiting the passage of air through it. In addition, it can cause damage to the laryngeal nerves that can lead to hoarseness.

Risks

Poisoning from an excessive intake of iodine can cause burns to the mouth, throat and fever. Also abdominal pain, nausea, vomiting, diarrhea, weak pulse, and coma.

An excess of iodine produces some of the symptoms observed in a deficiency: there is an inhibition of the synthesis of thyroid hormones, thus increasing the release of TSH, which results in a hypertrophy of the thyroid; that is, a goiter.

Studies have shown that excessive iodine intake can cause thyroiditis and papillary thyroid cancer. In addition, an excessive intake of iodine can interact with medications, limiting their action.

Excessive intake of iodine in conjunction with antithyroid medications, such as methimazole, used to treat hyperthyroidism, can have an additive effect and cause hypothyroidism.

Angiotensin-converting enzyme (ACE) inhibitors, such as benazepril, are used to treat hypertension. Taking an excessive amount of potassium iodide increases the risk of hyperkalemia and hypertension.

Applications

Doctors

Iodine acts as a skin or wound disinfectant. It has almost instantaneous antimicrobial action, penetrating inside microorganisms and interacting with sulfur amino acids, nucleotides and fatty acids, which causes cell death.

It exerts its antiviral action mainly on the covered viruses, postulating that it attacks the proteins on the surface of the covered viruses.

Potassium iodide in the form of a concentrated solution is used in the treatment of thyrotoxicosis. It is also used to control the effects of ¹³¹ I radiation by blocking the binding of the radioactive isotope to the thyroid.

Iodine is used in the treatment of dendritic keratitis. To do this, the cornea is exposed to water vapors saturated with iodine, temporarily losing the epithelium of the cornea; but there is a complete recovery from it in two or three days.

Also iodine has beneficial effects in the treatment of cystic fibrosis of the human breast. Likewise, it has been suggested that ¹³¹ I could be an optional treatment for thyroid cancer.

Reactions and catalytic action

Iodine is used to detect the presence of starch, giving a blue tint. The reaction of iodine with starch is also used to detect the presence of counterfeit banknotes printed on paper containing starch.

Potassium (II) tetraiodomercurate, also known as Nessler's reagent, is used in the detection of ammonia. Also, an alkaline iodine solution is used in the iodoform test, to show the presence of methyl ketones.

Inorganic iodides are used in the purification of metals, such as titanium, zirconium, hafnium, and thorium. In one stage of the process, the tetraiodides of these metals must be formed.

Iodine serves as a stabilizer for rosin, oil, and other wood products.

Iodine is used as a catalyst in the organic synthesis reactions of methylation, isomerization and dehydrogenation. Meanwhile, hydroiodic acid is used as a catalyst for the production of acetic acid in the Monsanto and Cativa processes.

Iodine acts as a catalyst in the condensation and alkylation of aromatic amines, as well as in sulphation and sulphation processes, and for the production of synthetic rubbers.

Photography and optics

Silver iodide is an essential component of traditional photographic film. Iodine is used in the manufacture of electronic instruments such as single crystal prisms, polarizing optical instruments, and glass capable of transmitting infrared rays.

Other uses

Iodine is used in the manufacture of pesticides, aniline dyes, and phthalein. In addition, it is used in the synthesis of dyes, and is a smoke extinguishing agent. And finally, the silver iodide serves as a condensation nucleus for the water vapor in the clouds, in order to cause rain.

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