Rhodium
Rhodium (Rh) is a rare, silvery-white, hard, corrosion-resistant, and chemically inert transition metal. It is a member of the platinum group of metals and is found in platinum ores and nickel ores. Its atomic number is 45 and its atomic mass is 102.90550(2) u.
Discovery and History: Rhodium was discovered in 1803 by William Hyde Wollaston, shortly after he discovered palladium. He extracted rhodium from platinum ore obtained from South America. Wollaston named rhodium for the rose color (Greek rhodon meaning "rose") of one of its chlorine compounds.
Occurrence and Production: Rhodium is one of the rarest elements on Earth, occurring in the Earth's crust at an average of around 0.001 parts per million. It is found uncombined in nature and as a component of platinum and nickel ores. Major sources include South Africa, Russia, and North and South America. Rhodium is commercially produced as a byproduct of platinum and nickel refining. The process involves a complex series of chemical reactions to separate rhodium from other platinum group metals.
Properties: Rhodium possesses a high melting point (1964 °C or 3567 °F), a high boiling point (3695 °C or 6683 °F), and good electrical conductivity. It is remarkably resistant to corrosion, even at high temperatures. Rhodium does not readily form oxides, even when heated. It is insoluble in most acids.
Uses: Rhodium is primarily used as a catalyst, particularly in catalytic converters in automobiles to reduce nitrogen oxides in exhaust gases. It is also used as a hardening agent for platinum and palladium. Other uses include:
- Jewelry: Rhodium plating is used to give white gold and silver jewelry a bright, reflective, and durable finish.
- Electrical Contacts: Due to its corrosion resistance and good electrical conductivity, rhodium is used in electrical contacts and spark plugs.
- Laboratory Equipment: Rhodium is used in high-temperature thermocouples, crucibles, and furnace windings.
Compounds: Rhodium forms a variety of compounds, primarily with oxidation states of +2, +3, and +4. Common compounds include rhodium(III) chloride (RhCl3), which is used as a starting material for other rhodium compounds, and rhodium(II) acetate dimer (Rh2(OAc)4), a popular catalyst.
Toxicity: Rhodium metal is generally considered to have low toxicity. However, some rhodium compounds can be toxic and should be handled with care. Inhalation of rhodium dust can cause respiratory irritation.
Isotopes: Rhodium has only one naturally occurring isotope, 103Rh. Several radioactive isotopes are known.