Rhodium

Rhodium was discovered in 1803 by British chemist William Hyde Wollaston. Wollaston had already discovered the element palladium, but for some chemists, discovering one element isn’t enough!

Wollaston had been experimenting on a sample of platinum ore that had been smuggled out of Latin America and acquired by his business partner, chemist Smithson Tennant. Tennant and Wollaston were working to develop a way to purify large amounts of platinum.

By dissolving the platinum ore in the acid mixture aqua regia—whose name means “royal water”—Wollaston was able to carry out a process that eventually yielded rhodium metal. Wollaston named the element after the Greek word “Rhodon,” meaning “rose,” in reference to the reddish tint of the sodium rhodium chloride he produced.

As it happened, Tennant discovered the elements osmium and iridium in the same ore sample that Wollaston had found rhodium. Forget two birds with one stone—it's more like three elements, one ore!

Rhodium metal is silvery white and hard, and it looks a lot like platinum. However, rhodium has a higher melting point and lower density than platinum.

For a long time after rhodium was discovered, no one was quite sure what to do with it. As a result, it wasn’t used much. That changed in the 1970s, when Volvo developed the three-way catalytic converter, which helps remove harmful chemicals from automobile exhaust. Today, around 85% of rhodium is used in catalytic converters.

The next biggest use of rhodium is in the glass industry, which uses rhodium as a coating that can protect glass and make it more reflective.

In the chemical industry, rhodium is used as a catalyst in the production of nitric acid and acetic acid. Rhodium also plays a role in the production of menthol, which you’ll find in everything from chewing gum and cigarettes to toothpaste. You can thank rhodium for your fresh breath!

Unlike other elements, rhodium doesn’t turn into an oxide when heated: rhodium will absorb oxygen from the atmosphere, but then release it again once it cools off.

In nature, rhodium comes in just one isotope—rhodium-103. Scientists have made around twenty unstable, radioactive, rhodium isotopes, with half-lives ranging from over four years to much less than a second.

Rhodium is one of the rarest elements, and it’s not easy to separate from the elements that often are found alongside it, such as gold, palladium, platinum, and silver. Most rhodium—more than 80% produced annually—comes from platinum ores mined in South Africa. Russia and Zimbabwe also produce rhodium.

Worldwide, only around 20 tons of rhodium are produced each year. Rhodium is also obtained by recycling, oftentimes from catalytic converters in vehicles.