We continue to review some of the most important materials in heat treatment and metallurgy.
Rhodium (chemical symbol: Rh)
Rhodium (Fig. 1) is a rare, silvery-white, lustrous metallic element that is highly reflective and very corrosion-resistant. It is one of the six elements in the platinum group along with platinum, palladium, osmium, iridium and ruthenium. Rhodium is unaffected by both air and water and has a lower density and a higher melting point than platinum. Rhodium is considered the rarest and most valuable precious metal in the world, having a value far greater than both gold and silver. Rhodium is very difficult to obtain from its ores, but it is one of the waste products of the nuclear fission of 235-uranium. Despite rhodium’s availability in this capacity, it is an expensive and difficult process to remove it from nuclear fuel rods.
Rhodium was first discovered in 1803 by English chemist William H. Wollaston shortly after his discovery of palladium. He extracted the new element from a chunk of platinum ore after French chemist Hippolyte-Victor Collet-Descotils suggested that the red color of some platinum salts was perhaps the presence of a yet unidentified metal. Wollaston was able to remove the platinum and palladium from the ore and was left with a dark red powder, later determined to be sodium rhodium chloride. Further extraction led to the isolation of the new element in its raw state. He named the new element after the Greek word “rhodon,” meaning rose, after the rose color of the dilute solution of salts containing rhodium.
Rhodium’s main application is in the automobile industry, where it is commonly used in catalytic converters (Fig. 2) to clean vehicle emissions by reducing nitrogen oxide in exhaust gas. In the heat-treatment industry, it is one of the materials from which thermocouple wire is manufactured in Type-R and Type-S thermocouples. Due to rhodium’s brilliance and resistance to tarnishing, it also is found in electroplated finishes for jewelry (Fig. 3), searchlights and mirrors. Also, it can be found as a catalyst in the chemical industry in the creation of nitric acid, acetic acid and the hydrogenation reactions. Other applications which rhodium can be found are in the coating of optic fibers and headlight reflectors.
Here are a few important facts about rhodium.[2,5]
- Atomic number: 45
- Atomic weight: 102.90550
- Melting point: 2237 K (1964°C or 3567°F)
- Boiling point: 3968 K (3695°C or 6683°F)
- Density: 12.4 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 5
- Group number: 9
- Group name: none
- Electron configuration: [Kr] 4d85s1
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- The Atomic Archive (http://www.atomicarchive.com/Fission/Fission1.shtml)