Tag Archives: iridium

Iridium: Most Corrosive Resistant Precious Metal

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Iridium is a transition metal belonging to the platinum group. Its main characteristics are that of being hard and brittle and of being silvery-white in appearance. It ranks next to osmium in being the densest element. As to its main property, this is best expressed in its being considered the most corrosive resistant of all the precious metals. In fact, this is shown even in temperatures of as high as 2000°C (3632°F or 2273.15°K).

Iridium was discovered in 1803 by the English chemist Smithson Tennant. It was identified from the residue of platinum ore which was dissolved in nitro-hydrochloric acid (also known as aqua regia). Platinum ores are still the main sources today of iridium. The precious metal is likewise obtained as a by-product of mining nickel.

Below are some of the properties of iridium.

General:
• Chemical Symbol: Ir
• Atomic Number: 77
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 9/ 6/ d
• Atomic Weight: 192.217 g.mol-1
• Electron Configuration: [Xe] 4f14 5d7 6s2

Physical:
• Density (near room temperature): 22.56 g.cm-3
• Liquid Density (at melting point): 19 g.cm-3
• Melting Point: 2466°C, 4471°F, 2739°K
• Boiling Point: 4428°C, 8002°F, 4701°K
• Heat of Fusion: 41.12 kJ.mol-1
• Heat of Vaporization: 563 kJ.mol-1

Atomic:
• Oxidation States: -3, -1, 0, 1, 2, 3, 4, 5, 6
• Electronegativity: 2.20 (Pauling scale)
• Atomic Radius: 136 picometre
• Covalent Radius: 141±6 picometre

Because of its characteristic of being very brittle, pure iridium is quite difficult – almost impossible, in fact – to machine. Its primary use is as a hardening agent for platinum. High-temperature equipment, such as crucibles, are made from platinum-iridium alloys. Compass bearings, balances and fountain pen tips, on the other hand, are made from osmium-iridium alloys.

Again, iridium is the most corrosive resistant precious metal known. Coupled with its resistance to extremely high temperatures, this special characteristic makes iridium ideal for use in certain parts of aircraft engines. It is also alloyed with titanium to make deep-water pipes.

Other uses of iridium include the following:

1. Electrical contacts for spark plugs (due to its resistance to arc erosion);
2. Computer memory devices;
3. Direct-ignition engine (as a catalyst);
4. Radiotherapy (as a source of radiation);
5. X-ray telescopes.

In 2007, worldwide demand for iridium reached 3,701 kilograms (119,000 troy ounces). Distribution of these were as follows: electrochemical uses (1,100 kilograms); electrical uses (780 kilograms); for catalysis (750 kilograms); and other applications (1,100 kilograms).

Iridium is found at highest concentrations within the Earth’s crust in three specific types of geologic structures: in impact craters, in igneous deposits, and in deposits reworked from either of the first two. The Bushveld igneous complex in South Africa is the largest known primary reserves for iridium in the world. Other important sources of this precious metal are the Sudbury Basin in Canada and the nickel-copper-palladium deposits near Norilsk in Russia. Several smaller iridium reserves are also found in the United States.

Beginning the year 2000, the annual production of iridium is about 3 tonnes (96,500 troy ounces). Its price as of 2007 is 14,667 U.S. dollars per kilogram (440 U.S. dollars per troy ounce).

The Six Precious Metals Of The Platinum Group

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In the periodic table of elements, six metallic elements are bunched together in the d-block, specifically in groups 8 to 9, periods 5 and 6. All transition metals, these six elements are collectively referred to as the “platinum group metals”. These precious metals tend to occur with one another in mineral deposits. Likewise, they are alike in both chemical and physical properties.

In the order of their arrangement in the periodic table, the six metallic elements of the platinum group are ruthenium, rhodium, palladium, osmium, iridium, and platinum. A brief description of each of these precious metals is provided below.

1. Ruthenium – This element is represented by the symbol Ru. Its atomic number is 44. It is mostly found in platinum ores and often used in platinum alloys as a catalyst. Its two main physical characteristics refer to its hardness as a metal and to its silvery-white color. The former characteristic makes ruthenium ideal for use in making wear-resistance electrical contacts. The price of this precious metal as of January 2010 is estimated to be about USD173 per troy ounce (USD5,562 per kilogram).

2. Rhodium – This element is represented by the symbol Rh. Its atomic number is 45. Its occurrence is similar to that of ruthenium, and its primary use is as a catalytic converter. Rhodium is considered perhaps the rarest element. It is also known to be the most expensive precious metal, with a price estimated to be about USD2,750 per troy ounce (USD88,415 per kilogram) as of January 2010.

3. Palladium – This element is represented by the symbol Pd. Its atomic number is 46. It is also considered one of the rarest precious metals. Palladium closely resembles its co-member in the platinum group – platinum. It is soft and is silvery-white in color. Like rhodium, palladium is largely used as a catalytic converter. As of January 2010, the price of this metal is estimated to be approximately USD424 per troy ounce (USD13,632 per kilogram).

4. Osmium – This element is represented by the symbol Os. Its atomic number is 76. It is found in nature as an alloy in platinum ores. Osmium is considered the densest natural element. It is brittle and is blue-gray in color. Because of its hardness, osmium is alloyed with the other metals in its group and used in electrical contacts and high-quality fountain pen tips. The price of osmium, as of January 2010, is about USD32.15 per troy ounce (USD12,217 per kilogram).

5. Iridium – This element is represented by the symbol Ir. Its atomic number is 77. Like osmium, iridium is very hard and brittle; it has a different color though – silvery-white. Its principal use is for electrical purposes, mainly because of its density and its high resistance to corrosion even at extremely high temperatures. Iridium is considered the fourth least abundant element in the Earth’s crust, after rhenium, ruthenium, and rhodium. Its price is estimated to be about USD408 per troy ounce (USD13,117 per kilogram) as of January 2010.

6. Platinum – This element, after which this group of precious metals is named, is represented by the symbol Pt. Its atomic number is 78. Platinum is dense, ductile, and malleable; it is gray-white in color. Known to be highly resistant to corrosion, this precious metal is used in jewelry, electrical contacts, and laboratory equipment. Its price, as of January 2010, is USD1,555 per troy ounce (USD49,995 per kilogram), making it the second most expensive precious metal, after rhodium.

Evident from the description of each of these six precious metals is that all of them have outstanding catalytic properties and high resistance to tarnish and wear. These characteristics make any of them well suited for fine jewelry. Additionally, their excellent resistance to extremely high temperatures makes them ideal for many different industrial uses.

The Eight Precious Metals Considered Noble Metals

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Most metallic chemical elements are known to easily oxidize and corrode, especially at high temperatures. Metals that have excellent resistance to oxidation and corrosion, even at high temperatures, are called noble metals. While there is no strict definition for this group of metals, it usually includes those that are extremely rare.

Thus, metals that are labeled “precious” are also considered noble metals (note, however, that noble metals are not necessarily precious metals). There are nine known precious metals – gold, platinum, iridium, palladium, osmium, silver, rhodium, ruthenium, and rhenium. With the exception of the latter, all are considered noble metals.

Using the “aqua regia” test (aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, which is highly corrosive), here’s how the eight precious-noble metals react:

• Gold, platinum, palladium, and osmium dissolve.
• Ruthenium dissolves only in the presence of oxygen.
• Rhodium dissolves only when it is in a fine, pulverized form.
• Iridium and silver do not dissolve under any condition.

In another definition, noble metals may refer to electrically conductive elements. Here, the term “noble” serves as a modifying word, such that the electrical conductivity of materials is graded from noble to active. Using this definition for noble metals then, we will find that silver is less noble than, say, graphite (although graphite is an elemental form of carbon and, therefore, is not considered a metal).

Below is a comparison of the chemical nobility of the precious metals (again, excluding rhenium), with details of their specific electronic configurations, simplified reactions (as per the pH diagram), and electrode potentials. The list is presented in order of increasing atomic number.

1. Ruthenium – VIIIb/5; Ru -> Ru2+ + 2 e-; 0.455 V
2. Rhodium – VIIIb/5; Rh -> Rh2+ + 2 e-; 0.600 V
3. Palladium – VIIIb/5; Pd -> Pd2+ + 2 e-; 0.987 V
4. Silver – Ib/5; Ag -> Ag+ + e-; 0.7996 V
5. Osmium – VIIIb/6; Os + 4 H2O -> OsO4 + 8 H+ + 8 e-; 0.838 V
6. Iridium – VIIIb/6; Ir -> Ir3+ + 3 e-; 1.156 V
7. Platinum – VIIIb/6; Pt -> Pt2+ + 2 e-; 1.18 V
8. Gold – Ib/6; Au -> Au3+ + 3 e-; 1.498 V

The second item appearing after the name of the metal (the three items are separated by semi-colons) represents the metal’s reaction in water. In the pH diagram, the pH symbol is labeled on the horizontal axis to signify the -log function of the concentration of H+ ion. The lines, which represent equilibrium for the concentration, are drawn for ions at unit activity. Other concentrations may be represented by additional lines. The voltage potential is represented by a vertical axis, which is labeled Eh, where “h” stands for hydrogen.

Physics has an even more strict definition for noble metals. Here, it is required that the electronic structure’s d-bands are filled. If this definition is followed, only gold and silver (among the precious metals) qualify as noble metals. Also note that the varying reactivity of the precious metals can readily be observed while preparing their surfaces in the vacuum regime called ultra high vacuum.