This material is a silver-white metallic element that is highly reflective and resistant to corrosion. It is considered the rarest and most valuable precious metal in the world — above Gold or Silver. The name comes from the Greek word “rhodon,” meaning rose, named for the rose-red colour of its salts.
It is one of the six Platinum group metals: Platinum, Palladium, Rhodium, Osmium, Iridium and Ruthenium. It is also classified as a noble metal, meaning that it does not react to oxygen easily, acts as a fantastic catalyst, and is resistant to corrosion and oxidation. Some common characteristics of the Platinum group metals include high melting points, general non-toxicity, as well as resistance to wear, oxidation and corrosion.
Rhodium was discovered in 1803 by English chemist William Hyde Wollaston shortly after he had discovered the element Palladium. Wollaston extracted Rhodium from a piece of Platinum ore that he had obtained from South America.
Commercial Rhodium is generally obtained as a byproduct of Copper and Nickel refining. In nature, it can occur uncombined or with other Platinum minerals. It can be found in river sands in North and South America and in Copper-Nickel sulfide ores in Ontario, Canada.
It is the rarest of the Platinum group, only occurring up to one part per 200 million in the Earth’s crust. It has a lower density and a higher melting point than Platinum. It is unaffected by air and water up to 600 degrees Celsius. It is also resistant to most acids. All Rhodium compounds are easily reduced or decomposed through heating to create powdered (or sponge) metal.
The main use for this material is in catalytic converters designed to clean vehicle emissions. It accomplishes this by reducing Nitrogen oxide in exhaust gas. Without Rhodium catalysts, the air in our cities would be much worse due to vehicle exhausts.
In the chemical industry, it is used as a catalyst in the making of nitric acid, acetic acid and hydrogenation reactions.
Other uses for this material are coating optic fibers, crucibles, thermocouple elements and headlight reflectors. Since it has a low electrical resistance and is highly resistant to corrosion, it is used as an electrical contact material as well.
It is often alloyed with Platinum and Iridium to make an oxidation-resistant metal that can withstand high temperatures. These alloys are used in furnace windings, pen nibs, phonograph needles, high-temperature thermocouple and resistance wires, electrodes for aircraft spark plugs, bearings and electrical contacts.
Although it seems to have found a solid industry niche in cleaning car exhausts, many new research projects are showing that the precious metal may hold much more potential. This is particularly true when used as a catalyst in a new chemical process that aims to use the Sun’s thermal energy to convert carbon dioxide and water into high-energy fuels.
Art and Design
If you have a piece of very shiny jewellery that dates back to the 1930s, then chances are high that you have a piece of Rhodium-plated jewellery. Since Rhodium is quite brilliant and resistant to tarnishing, it has often been used as a finish for jewellery and mirrors. It is applied as a protective coating on jewellery pieces to protect them from scratches and tarnish.
Rhodium has no known biological use and no known use for life processes. While some compounds of Rhodium are carcinogenic, there are almost no reported cases of humans being affected by this element in any way. This may be because Rhodium compounds are encountered so rarely. Tests on plants have shown that it is the least toxic member of the Platinum group of metals.