Rhodium: Costliest Precious Metal
In 1803, the English physicist and chemist William Hyde Wollaston discovered a chemical element that is today considered the most expensive of the precious metals. This precious metal, which belongs to the platinum group metals, is rhodium. With its current price of around 1,000 U.S. dollars per troy ounce, rhodium is about five times costlier than platinum.
Wollaston's discovery of rhodium was made possible with the use of crude platinum ore. He first dissolved the ore in nitro-hydrochloric acid (also called aqua regia) and neutralized the acid with lye (or sodium hydroxide). By adding salmiac (ammonium chloride), he then precipitated the platinum. All other metals, including rhodium, were precipitated with zinc. Rhodium was further precipitated by the addition of sodium chloride. Finally, it was washed with ethanol and reacted with zinc, forming the precious metal that it is known today.
Since rhodium occurs in ores mixed with other metals (examples: silver, gold, and platinum), its extraction is complex. For example, it is obtained as a white inert metal from platinum ores, which is quite difficult to fuse. The main source of this precious metal is in South Africa, which accounts for about 80 percent of the total world exports. Estimates place the annual production of rhodium at only about 25 tons.
Some of the properties of rhodium are listed below.
• Chemical Symbol: Rh
• Atomic Number: 45
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 9/ 5/ d
• Atomic Weight: 102.90550 g.mol-1
• Electron Configuration: [Kr] 4d8 5s1
• Density (near room temperature): 12.41 g.cm-3
• Liquid Density (at melting point): 10.7 g.cm-3
• Melting Point: 1964°C, 3567°F, 2237°K
• Boiling Point: 3695°C, 6683°F, 3968°K
• Heat of Fusion: 26.59 kJ.mol-1
• Heat of Vaporization: 494 kJ.mol-1
• Oxidation States: 6, 5, 4, 3, 2, 1, -1
• Electronegativity: 2.28 (Pauling scale)
• Atomic Radius: 134 picometre
• Covalent Radius: 142±7 picometre
• Ionization Energies: 719.7 kJ.mol-1 (first), 1740 kJ.mol-1 (second), 2997 kJ.mol-1 (third)
The principal use of rhodium is as a catalytic converter in automotive vehicles, which reduces the toxicity of engine emissions by converting these into less harmful gases. Rhodium is likewise used in jewelry, such as when it is electroplated on platinum, sterling silver, or white gold (called rhodium flashing) to strengthen the metal or give it a reflective surface.
Rhodium has at least five other uses or applications:
1. As an alloying agent, to improve platinum's resistance to corrosion.
2. For optical instruments.
3. As an electrical contact material (because of its stable resistance to contact and low resistance to electricity).
4. As a filter in X-ray systems (such as mammography).
5. In surfaces of high quality pens (because of its high mechanical and chemical resistance).
Obviously because of its extremely high price, rhodium signifies wealth. In some cases, it has been used in awards or recognitions, such as when the Guinness Book of World Records handed out a rhodium-plated disc to Paul McCartney in 1979, in recognition of his being the all-time best-selling recording artist and songwriter in history.
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