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Rhodium

physical science Maturity 7-9 Next article →

Rhodium is a rare and highly valuable chemical element. It is identified by the symbol Rh and has the atomic number 45. As a transition metal, it is characterized by its dark, silvery-white color and extreme hardness. Rhodium is classified as a noble metal, meaning it is exceptionally resistant to corrosion and chemical attack. It is also a member of the platinum group, a family of similar metals. Because it is so difficult to find, rhodium is one of the most expensive precious metals on Earth.

Rh price
Rh price

In nature, rhodium is extremely scarce. It exists in the Earth's crust at concentrations of less than one part per billion. It is usually found as a free metal or as an alloy with other metals. Sometimes, it appears in specific minerals like bowieite or rhodplumsite. Most rhodium is extracted from platinum or nickel ores.

Rhodium daily Price 1992-2022
Rhodium daily Price 1992-2022

Because it is so rare, the global annual production fluctuates around only 30 tonnes. This scarcity causes the market price to be highly volatile. For example, the price reached a peak of $14,197 in July 2022.

Understanding how rhodium was first identified requires looking at the work of William Hyde Wollaston. In 1803, Wollaston discovered the element while studying crude platinum ore, likely from South America. He named the metal after the rose-red color of one of its chlorine compounds.

Wollaston William Hyde Jackson color
Wollaston William Hyde Jackson color

His extraction process was complex and involved several chemical steps. He first dissolved the ore in aqua regia, a strong acid mixture. He then used ammonium chloride to precipitate platinum. By adding zinc, he created a mixture containing copper, lead, palladium, and rhodium. After using nitric acid to remove copper and lead, he used hot ethanol and additional zinc to finally release the rhodium as a free metal.

Today, the most significant use of rhodium is as a catalyst in automobiles. A catalyst is a substance that speeds up a chemical reaction without being consumed. About 80% of all rhodium produced globally is used in three-way catalytic converters.

HRh(CO)P3again
HRh(CO)P3again

These converters are essential for reducing harmful exhaust emissions. Specifically, rhodium helps reduce nitrogen oxides, also known as NOx, into less harmful gases. While older converters used platinum or palladium, the introduction of the three-way converter by Volvo in 1976 greatly increased the demand for rhodium. In 2012, for instance, 24,300 kg of the 30,000 kg produced was used for this single purpose.

Beyond cars, rhodium plays a vital role in industrial chemistry and medicine. It is used in asymmetric hydrogenation, a process that helps form precursors for drugs. One famous example is the Nobel Prize-winning route used to create the drug L-DOPA. Rhodium catalysts are also used in the hydroformylation process, which converts alkenes into aldehydes. This process is used to manufacture everything from fragrances to detergents.

Wilkinson's-catalyst-2D
Wilkinson's-catalyst-2D

These rhodium-based catalysts are 1,000 to 10,000 times more active than cheaper cobalt-based alternatives. This high activity allows chemical plants to run reactions at much lower temperatures and pressures.

In the world of jewelry, rhodium is used for its beauty and durability. It is often used in a process called electroplating. Jewelers apply a thin layer of rhodium, known as "rhodium flashing," over white gold to create a bright, reflective surface. It is also plated onto sterling silver to prevent tarnishing. Because rhodium is inert, it does not react with most aggressive chemicals. This makes it an excellent protective coating for items that must withstand high temperatures or corrosive environments.

Finally, rhodium has specialized uses in high-tech science and energy. In nuclear reactors, rhodium detectors are used to measure the level of neutron flux. It also has a role in the study of isotopes. Naturally occurring rhodium consists of only one stable isotope, 103Rh. This isotope has a nuclear spin that makes it useful for nuclear magnetic resonance spectroscopic studies. Even though rhodium is a fission product in used nuclear fuel, extracting it from that source is currently too complex and expensive for large-scale use.

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File:Wollaston William Hyde Jackson color.jpg
Wollaston William Hyde Jackson color.jpg
File:Wilkinson's-catalyst-2D.png
Wilkinson's-catalyst-2D.png
File:Rhodocene dimerisation.svg
Rhodocene dimerisation.svg
File:Rh price.png
Rh price.png
Rhodium daily Price 1992-2022.webp
File:HRh(CO)P3again.png
HRh(CO)P3again.png
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