How Was Iridium Discovered and Why Is It Important in Chemistry?
What is Iridium?
Iridium is a rare and dense transition metal with atomic number 77. It belongs to the platinum family and is denoted by the symbol Ir. The Ir element is generally hard and brittle; still, it can become ductile at a very high temperature around 1200 to 1500 oC. It is one of the rarest elements in the earth with an annual production of only 3 tonnes. This element doesn't occur in nature in pure form. Iridium is one of the densest metals found in nature with a density of about 22.56g/cm3.
Iridium Properties
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The above image shows the Iridium metal in pure form
Discovery
When the chemists dissolved platinum in aqua regia to study its properties, they observed a small amount of insoluble residue, which is dark in color. Some thought that it is graphite, while others cannot make any conclusion. In 1803, a British scientist Smithson Tennant concluded that this residue is any new metal after analyzing it properly. He conducted various experiments and found that the salts that he obtained with this metal were strongly colored. Hence, he named it iridium after the Greek winged goddess of rainbow Iris.
Natural Occurrence
Iridium is one of the rarest metals on the earth due to its less abundance. Even the platinum metal is ten times more abundant than the Ir element. However, scientists believe that it has a higher concentration in the earth's core because of its siderophile (iron-loving) character. Generally, this element is present in nature in the form of natural alloys. In all the platinum group metal, iridium is present naturally in alloys with raw copper or nickel. Very few minerals in the earth's crust contain this element in the dominant form. Some of the rare examples are cuproiridsite and irarsite.
Iridium is present in the earth's crust in higher concentrations in three kinds of structure. They are impact craters, igneous deposits, and deposits reworked. The most popular primary reserves of iridium on earth are Sudbury basin, Norilsk in Russia, Bushveld igneous complex in South Africa, etc.
Physical Properties of Iridium
Iridium is a member of platinum group metal and hence resembles the characteristics of platinum in many ways. It is white but with a slight yellowish shade. It is hard, brittle, and has a very high melting point as compared to other metals. Hence, it is not easy to work with this transition metal. The boiling point of iridium is 4428°C, the tenth highest among all the elements. At the temperature below 0.14K, this element becomes a superconductor.
The density of iridium is 22.56g/cm3, which makes it the densest metal after osmium. The modulus of elasticity of this metal is also very high as compared to other elements. Moreover, it has a very low Poisson's ratio along with high shear modulus. All these Iridium properties make it resistant to deformation.
Chemical Properties of Iridium
Among all the known metals, Ir is one of the most corrosion-resistant elements found in nature. Even the harshest of acids like hydrochloric acid, sulfuric, and aqua regia does not affect this element. Silicates or molten metals at high temperatures cannot attack iridium. Very few molten salts and halogens at a high temperature can attack iridium. It can react with sulfur to form iridium disulfide at atmospheric pressure.
Ir element can form compounds in several oxidation states from -3 to +9. However, the most common ones are +3 and +4. The only well-characterized oxide of this transition metal is IrO2. It is a blue-black solid which can react with HNO3 to give Ir2O3. Iridium can also form iridates like K2IrO3 and KIrO3 after reacting with potassium oxide at high temperatures. Ir can react with halides, but it cannot create any mono-halides or dihalides. It can react with almost every halogen to yield trihalides. However, it can also react with fluoride to give tetrafluoride, pentafluoride at the desired conditions.
Iridium generally forms very few complexes. The complexes of this element have octahedral molecular geometry and are diamagnetic. In Organo-iridium compounds, the element is present in lower oxidation states. For example, Ir4(CO)12 is one of the most stable binary carbonyls of this metal in which Ir has the oxidation state zero.
Isotopes
Two naturally occurring isotopes of Ir are 191Ir and 193Ir. Today, there are around 37 known radioisotopes of this metal, ranging from mass number 164 to 202. The most stable known radioisotope of this transition metal is 192Ir, which has a half-life of approx 73.82 days. This isotope has many applications in industrial radiography and other industries. It is beneficial for testing welds in steel in gas and oil industries without any destruction. The scientists discovered all the isotopes of this element between the years 1934 to 2003. No isotope of iridium was found after 2008.
FAQs on Iridium: Essential Facts, Properties, and Uses
1. What is iridium and where is it located in the periodic table?
Iridium is a chemical element with the symbol Ir and atomic number 77. It is a very hard, brittle, silvery-white transition metal belonging to the platinum group. In the periodic table, iridium is located in Group 9 and Period 6, placing it within the d-block elements known for their high density and melting points.
2. What are the main physical and chemical properties of iridium?
Iridium is known for several exceptional properties that make it highly valuable in specific applications. Its key characteristics include:
High Density: It is the second-densest element known, just slightly less dense than osmium.
Corrosion Resistance: Iridium is the most corrosion-resistant metal known. It is extremely resistant to attack by air, water, salts, and even the most potent acids like aqua regia.
High Melting Point: It has a very high melting point of approximately 2466 °C (4471 °F), allowing it to be used in high-temperature environments.
Hardness and Brittleness: It is extremely hard but also brittle, which makes it difficult to machine or work with in its pure form.
3. What are the most important applications of iridium in industry and technology?
Due to its unique properties, iridium has several critical uses, particularly in applications requiring high durability and heat resistance. Common applications include:
Alloys: It is used to harden platinum, creating alloys that are used for making high-performance spark plugs, crucibles for growing crystals at high temperatures, and electrical contacts.
Fountain Pen Nibs: An alloy of iridium and osmium is famously used for the tips of fountain pens due to its extreme hardness and resistance to wear.
Aerospace: Iridium alloys are used in long-life aircraft engine parts and deep-water pipes that must withstand corrosive environments.
Catalysts: It serves as a catalyst in various industrial chemical processes, such as in the Cativa process for producing acetic acid.
4. Where is iridium naturally found and how is it extracted for commercial use?
Iridium is one of the rarest elements in the Earth's crust. It is typically not found in its own mines but is obtained commercially as a byproduct of nickel and copper mining and processing. During the electrorefining of these metals, iridium and other platinum-group metals settle at the bottom as anode mud. This mixture is then treated with chemicals like aqua regia to dissolve the metals. Iridium is then separated from the solution through a complex chemical process, such as solvent extraction, and finally reduced to a pure metal powder using hydrogen.
5. What makes iridium the most corrosion-resistant metal known?
Iridium's extraordinary resistance to corrosion and chemical attack stems from its stable electron configuration and strong metallic bonds. Its electrons are held very tightly, making it chemically inert and energetically unfavourable for it to react with other substances, including powerful acids and oxidising agents that can dissolve other noble metals like gold and platinum. This inherent stability means it does not easily form compounds, allowing it to maintain its metallic integrity even under extreme chemical conditions and high temperatures.
6. How is the rarity of iridium on Earth linked to the extinction of the dinosaurs?
The link between iridium and the extinction of dinosaurs is a key piece of scientific evidence. Iridium is very rare in Earth's crust but is significantly more abundant in asteroids and meteorites. Scientists discovered a thin, worldwide geological layer of sediment, known as the K-Pg boundary, that dates back 66 million years. This layer contains abnormally high concentrations of iridium. This discovery led to the Alvarez hypothesis, which proposes that a massive asteroid impact caused the mass extinction event that wiped out the dinosaurs. The iridium layer is considered the global fingerprint of this catastrophic impact.
7. Why is iridium often alloyed with other metals like platinum instead of being used in its pure form?
While pure iridium is incredibly hard and corrosion-resistant, it is also very brittle and difficult to work with. Alloying iridium with a more ductile (workable) metal like platinum creates a material that combines the best qualities of both. The platinum lends its workability, while the iridium imparts superior hardness and durability to the alloy. This synergy is why platinum-iridium alloys are used for crucial applications like the former international standard for the metre and kilogram, as well as for durable medical implants and scientific instruments that require both strength and stability.
8. What are some other interesting facts about the element iridium?
Beyond its primary properties, iridium has several fascinating characteristics:
Its name comes from the Latin word "iris," meaning rainbow, because its salts are known to form compounds of many different, vibrant colours.
The discovery of iridium is closely linked with the discovery of osmium, as they were both found in the black residue left after dissolving crude platinum in aqua regia in 1803.
Due to its high melting point and resistance to oxidation, radioactive isotopes of iridium are used in the thermoelectric generators of some unmanned spacecraft.
