Occurrence and Uses
Rhodium is one of the rarest elements in the Earth’s crust (average concentration 0.001 ppm). It is found in small quantities associated with native platinum and some copper-nickel ores. It occurs in the minerals rhodite, sperrylite and iridosmine (or osmiridium).
Rhodium is used in corrosion-resistant electroplates for protecting silverware from tarnishing and in high-reflectivity mirrors for searchlights and projectors. It is also useful for plating optical instuments and for furnace winding. Rhodium serves as a catalyst for various hydrogenation and oxidation reactions. It is used for spinnerets in rayon production and as an ingredient in gold decorations on glass and porcelain.
Rhodium is alloyed with platinum and palladium to make very hard alloys for use in spinning nozzles.
There have been no significant experimental data indicating health problems with rhodium, its alloys or its compounds in humans. Although toxicity is not established, it is necessary to handle these metals carefully. Contact dermatitis in a worker who prepared pieces of metal for plating with rhodium has been reported. The authors argue that the small number of reported cases of sensitization to rhodium may reflect the rarity of use rather than the safety of this metal. The American Conference of Governmental Industrial Hygienists (ACGIH) has recommended a low threshold limit value for rhodium and its soluble salts, based on analogy with platinum. The ability of soluble salts of rhodium to give rise to allergic manifestations in humans has not been completely demonstrated.