Elements

Intriguing Iodine

iodine crystals

The Element Iodine

Iodine is a non-metallic, solid element that belongs to the halogen family. This element makes a bold statement with its dark, shiny, gray-blue-black color. This eye-catching element is sure to be interesting. 

Cool Iodine Facts

  1. Iodine’s name originates from the Greek word “ioeides”, meaning violet.
  2. Like bismuth, iodine plays a crucial role in the drug industry. Halogen atoms are sometimes incorporated into lead drugs, which are the starting point of drug development. This results in having a lipophilic drug, meaning it is not very water-soluble and non-polar. Non-polar drugs are able to easily transport across lipid membranes and tissue. The addition of halogens improves this property. 
  3. More iodine is found in the ocean than the Earth’s crust. Seaweed in particular contains high concentrations of the element.  
  4. The only naturally occurring isotope of iodine is iodine-127. Unlike pure iodine, it exists by itself in nature. Other isotopes are man-made. 
  5. Iodine-131 is an important radioactive isotope. It has a high half-life of eight days. It is put into medicine to regulate the thyroid gland, treat goiter, treat thyroid cancer, and locate tumors.  
  6. Iodine-129 changes from radioactive form to nonradioactive over the span of millions of years. The other radioactive isotopes change to their non-radioactive form in just seconds to days. 
  7. From 1994 to 2000, worldwide iodine production increased from 13,726 tons to 18,993 tons. 

Iodine vs Other Halogens

The element iodine belongs to the halogen family, along with fluorine, chlorine, bromine, and astatine. All halogens are highly electronegative and very reactive. However, compared to its family members, iodine is relatively less reactive. Still, it has the greatest reducing strength and readily gains electrons from other atoms. Out of the halogen family, iodine is the least soluble in water, with a solubility value of 0.03 grams per 100 grams of water. However, it reacts oppositely towards aqueous solutions of iodide, where it is very soluble. More specifically, this element reacts well with solutions containing potassium iodide (KI + I2 → KI3). KI acts to decrease the vapor pressure and to dilute iodine to make it safer. 

Iodine in Nature

Out of the halogen family, iodine is the least abundant. In nature, pure iodine is never alone. Instead, it exists as compounds. In addition to the Earth’s crust, this element is in seawater. Furthermore, thyroxine, a secretion of the thyroid gland in humans, contains iodine.  

Iodine sublimates: Fact or fiction? 

What is sublimation? Usually, when a solid vaporizes, it first has to melt into a liquid. When a solid sublimates, it directly transforms into vapor, completely skipping the liquid phase. This is an endothermic transition and only occurs when the temperature and pressure are below the triple zero. For iodine, the triple point is 113.5˚C at 12.1 kPa. Many believe that under these conditions, solid iodine will sublimate into gas. However, this is wrong.

There is a large misconception about how iodine can not be a liquid, an idea that even some textbooks are guilty of teaching. At normal pressures and at a temperature range of about 113.6˚C to 184.4˚C (236.48˚F to 363.92˚F), iodine exists as a liquid. Two factors feed into this misconception: First, iodine is very volatile. This makes it easy to explain how solid iodine can just simply vaporize. Second, iodine’s liquid state can be difficult to see; The vapor cloud’s intense violet color completely overwhelms the liquid.

As shown in the diagram, there is a point where the liquid and vapor states exist at the same time. By carefully controlling the temperature, so that it sits just above the melting point, one can visibly see the process of iodine melting. 

Here is a quick video that demonstrates the liquid state of iodine:

Iodine’s Applications in Today’s World

Small amounts of iodine minerals are necessary for the production of vital thyroid hormones. These hormones play an essential role in humans and take part in controlling the rate of the heart, digestion, muscle contraction, and cell metabolism. Therefore, it is an important element in the drug development industry. Moreover, iodine is sometimes added to table salt. This not only ensures the incorporation of iodine in people’s diet but helps to prevent goiter. This is a disease when the thyroid gland swells and produces either a deficit or a surplus of thyroid hormones.   

Elemental iodine can serve as an antiseptic. As stated, iodine and potassium iodide (learn what else reacts with potassium iodide here) form the triiodide, KI3. Solutions containing potassium iodide in alcohol kill germs that exist on external wounds. Compounds such as iodoform (CHI3) are also antiseptics. 

Many may be familiar with Lugol’s iodine solution. This aqueous solution is almost guaranteed to be in any biology lab. This purple solution helps scientists detect the presence of starch in a substance, as the solution will turn dark blue.

History of Iodine

Bernard Courtois accidentally discovered iodine in 1811 while extracting potassium chloride from seaweed ash. After removing the compound, he added sulfuric acid to process the seaweed ash more. However, he added too much, producing a violet-colored vapor cloud. This gas condensed into crystals, which chemist Sir Humphry Davy later identified as solid iodine in 1813. 

Iodine Reactions

Iodine is a very reactive element. It reacts with water to form hypoiodite (OI). The hypoiodite ion is unstable and difficult to produce, as it only occurs in solution. The element also reacts with hot nitric acid and hot alkali to form iodic acid and iodate, respectively. Iodic acid acts as an astringent and crystallizes when cooled; Iodate is what gets added to table salt. As reactive as iodine is, it is unaffected by the oxygen or nitrogen in the air.

Additionally, iodine sublimes easily, meaning its solid-state vaporizes when heated. Upon vaporization, a beautiful, dark violet cloud produces. While this cloud is alluring to look at, it is crucial to avoid inhalation, as it will cause irritation. 

Iodine Clock Reactions

The iodine clock reaction is quite spectacular. In this phenomenon, two colorless solutions combine into another clear solution. Then, almost spontaneously it turns into a deep-blue color. It is very mysterious. In this reaction, a strong oxidizer reacts with a strong reducing agent. An example is potassium iodate (KIO3), a strong oxidizer reacting with sodium bisulfite (NaHSO3), a strong reducing agent. This forms iodine. 

To go into more detail, we first start with a potassium iodate-starch solution. Sodium bisulfite’s anions act to reduce the potassium iodate in this solution. This produces iodide anions, I, which react with the potassium iodate to form an iodine solution. This new solution then reacts with iodine and forms triiodide anions, I3. The bisulfite anions will then act on the triiodide anions, which converts them back to the iodide anions. When this process is complete, the remaining triiodide anions react with the starch molecules, which instantaneously produces the fascinating deep-blue color.

Where does the name come from? Depending on the concentration of the starting materials, the clear solution will take more or less time to turn blue- hence the name, “clock” reaction. The higher the concentration is, the faster the reaction rate will be. Secondly, the reaction rate increases as the temperature increases. Lastly, adding a catalyst will speed up the reaction as well.

Iodine Compounds

Nitrogen triiodide = DISASTER 

When iodine reacts with ammonia, it produces a solid called nitrogen triiodide. This black powder is a very indecisive, inorganic compound. When nitrogen triiodide crystals are wet, they are stable. When dry, the crystals are dangerous- they are touch-sensitive and become explosive. While a small explosion lacks the power to cause much damage, a large explosion certainly will. Stay away from dry nitrogen triiodide!  

Colorful Iodides

Iodides, where iodine is in the negative one oxidation state, are often white but can have a variety of colors. Silver iodide is more of a dull light yellow, while lead iodide looks like beautiful yellow gold platelets, as you can see in the golden rain experiment. Mercury (II) iodide and Tin (IV) iodide are both a nice looking orangish-red.

Synthesis of Iodine

A method of obtaining iodine is by combining an iodine ion with a stronger oxidizing agent. For example, bromine is a stronger oxidizing agent than iodine. Preparing two iodine ions with aqueous Br2, form I2 and two bromine ions ( 2 I + Br2 → I2 + 2 Br )

Iodine Oxidation States

Elemental iodine is neutral and possesses an oxidation state of 0. However, when present in a compound, iodine can possess different oxidation states and is considered a strongly acidic oxide. In its compounds, the most common oxidation state is -1, which is seen in iodide, I. It is also seen in states of +7, +5, +3, and +1. 

Oxyanions

An oxyanion is a polyatomic ion that contains oxygen. The four most common oxyanions of iodine are hypoiodite, IO; iodite, IO2; iodate, IO3; and periodate, IO4. Respectively, they have the oxidation states of +1, +3, +5, and +7.

Hypoiodites are the conjugate base of hypoiodous acid, as it is produced from the deprotonation of hypoiodous acid. This monovalent anion is often utilized as a reagent for direct oxidation reactions.

Iodides are very unstable and are never isolated. They are mainly used as an intermediate when synthesizing iodate from iodide.

Iodates are the conjugate base of iodic acid. They are utilized for iodine clock reactions.

Periodates are the conjugate base of periodic acid, as they are products of periodic acid deprotonation. They are commonly used in oxidation reactions that split bonds between vicinal carbons and unsubstituted hydroxyl or amino groups. 

Properties of Iodine

  • Melting point: 386.9 K; 113.7°C; 236.7°F
  • Boiling point: 457.6 K; 184.4°C; 363.9°F 
  • Density: 4.933 g/cm³
  • Atomic weight: 126.904
  • Atomic number: 53
  • Electronegativity: 2.66
  • Classification: Halogen
  • Natural abundance in the Earth’s crust: 0.000049%
  • Electron shell configuration: [Kr] 4d10 5s2 5p5
  • Isotopes: Iodine-127 is the only stable isotope
  • Found naturally in: Petroleum deposits and caliche ore deposits; Food; Seaweed
  • Toxicity: Toxic, especially in large amounts

Where can I buy Iodine?

Buying iodine is difficult because it is a classified precursor for illegal drug manufacturing.

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