Iridium (Ir)

• Element Symbol: Ir
• Atomic Number: 77
• Atomic Mass: 192.217
• Group # in Periodic Table: 9
• Group Name: Transition metals
• Period in Periodic Table: 6
• Block of Periodic Table: d-block
• Discovered by: Smithson Tennant (1803)

Iridium is an exceptionally hard, brittle, silvery-white metallic element. It belongs to the platinum group, along with ruthenium, rhodium, palladium, osmium, and platinum, which are all transition metals. Of all the metals in the periodic table, iridium is the most resistant to corrosion, even at temperatures as high as 2000 degrees Celsius (°C). While only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.

Iridium was discovered in 1803 by English chemist Smithson Tennant. He found the element in the residue left over after he had dissolved crude platinum in a mixture of hydrochloric acid and nitric acid, or aqua regia. It just so happened that there was also osmium in the residue, which is why both elements were discovered at the same time. Iridium’s name is derived from the Ancient Greek word iris, which means "rainbow." This name was chosen because many of the element’s salts have vivid colors.

Interestingly, unusually high amounts of this element have been found in rocks dating to the Cretaceous-Paleogene (K-Pg) boundary between the Mesozoic and the Cenozoic eras, which occurred about sixty-six million years ago. This discovery has led to a popular theory that an iridium-containing comet struck Earth and was responsible for the K-Pg mass extinction event at that time.

Physical Properties

Iridium is white in color with a slight yellowish cast. It is very hard and brittle, which makes it difficult to use in manufacturing because it cannot be easily cut, bent, or formed, not even with machine. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C. Iridium becomes a superconductor at temperatures below 0.14 kelvins (K). Its standard state at 298 K is solid. After osmium, iridium is the second-densest element based on measured density. However, there is some contention on this point because certain calculations show that osmium may be less dense than iridium, which has a specific gravity of 22.56. The specific gravity of an element is the measurement of its density in comparison with that of water. The melting point of iridium is high (2446 (C), as is its boiling point (4428 (C). The specific heat of an element is the amount of energy required to raise the temperature by one degree Celsius. For iridium, at a temperature of 20 (C, the specific heat is 131 joules per kilogram-kelvin (J/kg·K).

Chemical Properties

Iridium is the most corrosion-resistant metal known. At high temperatures it does not react to most acids, aqua regia, molten metals, or silicates. However, at temperatures greater than 2000 °C, iridium can be corroded by some molten salts, such as sodium cyanide and potassium cyanide, as well as by oxygen and fluorine.

The most common oxidation states of this element are +4 and +3. Iridium has two naturally occurring stable isotopes, iridium-191 and iridium-193. It also has thirty-four synthesized radioisotopes, which have mass numbers ranging from 164 to 199. The most stable radioisotope of iridium is iridium-192, which has a half-life of 73.83 days. All known isotopes of iridium were discovered between 1934 and 2001, the most recent one being iridium-171.

Applications

Iridium is one of the nine least abundant stable elements in Earth’s crust. It has an average mass fraction of 0.001 parts per million in rock from Earth’s crust. In contrast, iridium can be commonly found in meteorites, with concentrations of 0.5 parts per million or more. The total concentration of this element on Earth is thought to be much higher than what is observed in rocks from the Earth’s crust. It is believed that due to the density and iron-loving character of iridium, the element fell below the crust and into Earth’s core when the planet was still molten.

In nature, iridium is found as an uncombined element or in natural alloys. Within Earth’s crust, the element is found at its highest concentrations in three types of geologic structures: igneous deposits, impact craters, and deposits retrieved from one of the former structures. The largest known primary reserves can be found in the Bushveld igneous complex in South Africa. The large copper-nickel deposits in the Sudbury Basin in Canada and near Norilsk in Russia are also noteworthy sources of iridium. Smaller reserves have been found in the United States.

Iridium is also obtained commercially as a by-product of copper and nickel mining and processing. During the electrorefining of nickel and copper, the platinum group metals settle to the bottom of the cell as anode mud, which is the starting point for their extraction. The first step in separating these metals is bringing them into solution. Several different separation methods are used depending on the nature of the mixture. The two most commonly used methods are dissolution in a mixture of chlorine with hydrochloric acid and fusion with sodium peroxide followed by dissolution in aqua regia.

After the mixture is dissolved, iridium is separated from the other platinum group metals by one of two methods. Either it is precipitated as ammonium hexachloroiridate, or it is extracted as hexachloroiridate using organic amines. Either way, the product is reduced using hydrogen, which yields the metal as a powder that can be treated using various powder metallurgy techniques.

The high melting points, hardness, and resistance to corrosion of iridium and its alloys determine most of the element’s applications. Historically, it was used with osmium as an alloy for fountain pen nibs and compass bearings. Also, it was used in the making of the International Standard Kilogram, which is an alloy consisting of 90 percent platinum and 10 percent iridium.

The element’s three main modern-day applications are in the medical, industrial and scientific fields. Medically, iridium-192 is used as a source of gamma radiation for the treatment of cancer using brachytherapy. Brachytherapy is a form of radiotherapy in which a sealed radioactive source is placed inside or next to the tumor. An example of a specific treatment is high-dose-rate prostate brachytherapy. Industrially, iridium is used to make heavy-duty electrical contacts as well as crucibles. Scientifically, iridium is used in particle physics for the production of antiprotons, a type of antimatter. Antiprotons are made by shooting a high-intensity proton beam at a conversion target, which needs to be made from a very high density material such as iridium.

Bibliography

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