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For headlight reflectors.
(Gr. rhodon, rose) Wollaston discovered rhodium in 1803-4 in crude platinum ore he
presumably obtained from South America.
Rhodium occurs native 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 here 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 has a high reflectance and is hard and durable.
Rhodiums primary use is as an alloying agent to harden platinum and palladium. Such
alloys are used for furnace windings, thermocouple 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.
Sources: CRC Handbook of Chemistry
and Physics and the American Chemical Society.