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.
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The above image shows the Iridium metal in pure form
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.
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.
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.
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.
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.
1. What is Iridium Used For?
Iridium has many applications due to its unique properties like high melting point, corrosion-resistant, and hardness. The deepwater pipes contain the iridium based alloys due to its corrosion-resistant property. The long-life engine parts of an aircraft also contain iridium. Devices that need to operate at high temperatures also comprise iridium. It is also used as a catalyst in the decomposition of hydrazine in low-thrust rocket engines. In particle physics, scientists use Ir in the production of unique particles called anti-protons. It is also excellent to reflect X-rays after platinum and gold. Scientists also prefer to use iridium in radioisotope thermoelectric generators of the spacecraft.
2. How is Iridium Produced Commercially?
Ir is obtained commercially as a byproduct from mining and processing of nickel and copper. Noble metals like gold and silver settle to the bottom in the form of anode mud during the electrorefining of copper and nickel. To separate all these metals, first of all, they are converted into the solution form. Chemists mostly use aqua regia and sodium peroxide to turn this mixture into the solution. The continuous liquid-liquid extraction method is best for the industrial generation of Ir to get this metal from the solution. After that, hydrogen reduces this product to obtain the metal in a powder form.