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Rhodium, together with ruthenium, palladium, osmium, iridium, and platinum form a group of elements referred to as the platinum group metals (PGM). Rhodium metal is lustrous and silvery white. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. Upon heating it turns to the oxide when red and at higher temperatures turns back to the element. Rhodium it is unaffected by air and water up to 600 C. It is insoluble in most acids, including aqua regia, but is dissolved in hot concentrated sulfuric acid and it is attacked by molten alkalis. Applications Most metal (85%) goes into catalytic converters for cars. The major use of the metal is in alloys with platinum and iridium, giving improved high-temperature strength and oxidation resistance. These alloys are used in furnace windings, pen nibs, phonograph needles, high-temperature thermocouple and resistance wires, electrodes for aircraft spark plugs, bearings and electrical contacts. |