Rhodium is a silvery-white metal that belongs to the group of platinum metals. Rhodium is a rare metal with an atomic number of 45 and a molecular weight of 102.905. It has a density of 12.4 g/cm3 and a melting point of 1,771 °C. Rhodium is a very hard metal with a brilliant white metallic luster. Rhodium does not corrode in air and is not attacked by most acids, but is soluble in fused alkalis. Rhodium is found in nature as the free element and minerals, such as rhodonite and iridosmine.
Discovery of Rhodium
The discovery of rhodium is unique within the history of the platinum metals, with only iridium matching its accolades; both metals were discovered by William Hyde Wollaston at nearly the same time (rhodium in 1803, iridium in 1803), isolated as different elemental entities, and each has a unique history of discovery. Wollaston discovered the platinum metals after studying samples of what was thought to be platinum from South America.
After dissolving numerous other substances from the sample without obtaining the anticipated platinum, he concluded that it was not an element; however, he did manage to detect the presence of a new element within the sample. The rhodium from the sample was dissolved in aqua regia and then precipitated when ammonia was added.
The residue was melted with soda ash and cast into a bar which was sent to England.
The English chemist Smithson Tennant analyzed Wollaston’s sample and then claimed to discover a new element. He named the new element “rhodium” after the Greek word for rose, ῥόδον (rhódon). The symbol for rhodium is Rh.
Uses of Rhodium
The main uses for rhodium are as a catalyst in the production of ethylene oxide and propylene oxide. Rhodium is also used as a catalyst in the hydrogenation of organic compounds and as a precious metal in jewelry.
Rhodium is a very rare metal that is expensive and hard to find. The only known source of rhodium is the Ural Mountains in Russia and in South Africa, where it can be found in platinum ore. It has been estimated that all the rhodium ever mined to date would fit into an Olympic-size swimming pool. Rhodium is very expensive, about four times more than gold (gold price on June 2, 2017, is $1290/troy oz).
However, it dissolves readily in concentrated acids, such as HCl. It is insoluble in nitric acid and in aqua regia (the latter also dissolves platinum), but it does dissolve readily infused alkalis to give Rhodes.
