(Gr. rhodon: rose) Wollaston discovered rhodium between 1803 and 1804 in crude platinum ore he presumably obtained from South America.
Rhodium occurs natively with other platinum metals in river sands of the Urals and in North and South America. It is also found with other platinum metals in the copper-nickel sulfide area of the Sudbury, Ontario region. Although the quantity occurring there is very small, the large tonnages of nickel processed make the recovery commercially feasible. The annual world production of rhodium is only 7 or 8 tons.
The metal is silvery white and at red heat slowly changes in air to the resquioxide. At higher temperatures it converts back to the element. Rhodium has a higher melting point and lower density than platinum. It is highly reflective, hard, and durable.
Rhodium's primary use is as an alloying agent to harden platinum and palladium. Such alloys are used for furnace windings, thermocoupling elements, bushings for glass fiber production, electrodes for aircraft spark plugs, and laboratory crucibles. It is useful as an electrical contact material as it has a low electrical resistance, a low and stable contact resistance, and is highly resistant to corrosion. Plated rhodium, produced by electroplating or evaporation, is exceptionally hard and is used for optical instruments. Rhodium is also used for jewelry, for decoration, and as a catalyst.
Exposure to rhodium (metal fume and dust, as Rh) should not exceed 1 mg/m^3 (8-hour time-weighted average, 40-hour week).
Rhodium costs about $1,000/troy oz.