Rhodium Rh

Discovery

In 1803 the English chemist William Hyde Wollaston dissolved South American platinum ore in aqua regia, removed the platinum and palladium, and examined what remained. From the leftover solution he obtained a beautiful rose-red salt, and from that salt he isolated a new metal.

Only a year earlier Wollaston had found palladium in the same ore. He made a fortune from a process for turning platinum into a workable metal, and his research on that process led him to both elements in quick succession.

In the late twentieth century, tighter vehicle emission standards turned rhodium into an industrially vital metal, and its price became notoriously volatile.

Origin of the name

The name comes from the Greek rhodon, "rose", after the rose-red colour of its salts; the metal itself is silvery white. The symbol is Rh.

Main uses

• Catalytic converters: in three-way catalysts rhodium plays the key role of converting nitrogen oxides back into nitrogen. This accounts for most rhodium demand.
• Glass industry: platinum–rhodium alloys make bushings and vessels that withstand molten glass, including those used to spin glass fibre.
• Temperature measurement: thermocouples pairing platinum with platinum–rhodium alloy measure temperatures well above 1,000 °C accurately.
• Plating: a thin rhodium coating gives white gold jewellery a bright, white, scratch-resistant finish.
• Chemical catalysis: rhodium catalysts are used in large-scale processes such as acetic acid production and the hydroformylation of alkenes.

Isotopes

Rhodium has only one stable isotope, rhodium-103. Some radioactive isotopes, such as rhodium-101 and rhodium-102, are relatively long-lived but have few practical uses. In nuclear reactors, rhodium self-powered detectors absorb neutrons and produce a signal used to monitor neutron flux in the core.

In everyday life

Rhodium production is heavily concentrated in South Africa, followed by Russia. Annual output is only tens of tonnes, and nearly all of it is a by-product of platinum mining, so small shifts in demand can send prices soaring or crashing; at times rhodium has cost several times as much as gold.

In daily life it appears as the plating on white gold jewellery and inside catalytic converters. The high value of rhodium and other platinum group metals is why catalytic converter theft has become a problem in many countries. The metal has low toxicity, although some rhodium compounds can irritate the skin.