Periodic table: Rh, Appearance: silvery white metallic
General properties: Name, symbol, number: rhodium, Rh, 45
Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed with similar metals, and never as a chemical compound. It is one of the rarest precious metals, and the most costly.
Rhodium is a so-called noble metal, resistant to corrosion, found in platinum- or nickel ores together with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds, produced after it reacted with the powerful acid mixture aqua regia.
The element's major use (about 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters of automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. White gold jewelry is often plated with a thin rhodium layer to improve its optical impression while sterling silver jewelry is often rhodium plated for tarnish resistance.
Rhodium detectors are used in nuclear reactors to measure the neutron flux level.
William Hyde Wollaston
Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium. He used crude platinum ore presumably obtained from South America. His procedure involved dissolving the ore in aqua regia and neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum by adding ammonium chloride, NH4Cl, as ammonium chloroplatinate. Most other metals like copper, lead, palladium and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but palladium and rhodium, which were dissolved in aqua regia, and the rhodium was precipitated by the addition of sodium chloride as Na3[RhCl6]·nH2O. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which displaced the rhodium in the ionic compound and thereby released the rhodium as free metal.
After the discovery the rare element had only minor applications, for example by the turn of the century rhodium-containing thermocouples were used to measure temperatures up to 1800°C. The first major application was electroplating for decorative uses and as corrosion resistant coating. The introduction of the three way catalytic converter by Volvo in 1976 increased the demand for rhodium. The previous catalytic converters used platinum or palladium while the three way catalytic converter used rhodium to reduce the amount of NOx in the exhaust.
A 78g rhodium sample
Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.
Rhodium-plated white gold wedding ring
Rhodium finds use in jewelry and for decorations. It is electroplated on white gold and platinum to give it a reflective white surface. This is known as rhodium flashing in the jewelry business. It may also be used in coating sterling silver to protect against tarnish, which is silver sulfide (Ag2S) produced from the atmospheric hydrogen sulfide (H2S). Solid (pure) rhodium jewelry is very rare, because the metal has both high melting point and poor malleability (making such jewelry very hard to fabricate) rather than due to its high price. Additionally, its high cost assures that most of its jewelry usage is in the form of tiny amounts of powder (commonly called rhodium sponge) dissolved into electroplating solutions.
Rhodium has also been used for honors, or to symbolize wealth, when more commonly used metals such as silver, gold or platinum are deemed insufficient. In 1979, the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.
Rhodium foil and wire
Rhodium is used as an alloying agent for hardening and improving the corrosion resistance of platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles. Other uses include:
An electrical contact material due to its low electrical resistance, low and stable contact resistance, and high corrosion resistance.
Plated rhodium, made by electroplating or evaporation, is extremely hard and is used for optical instruments.
It is also used as a filter in mammography systems because of the characteristic X-rays it produces.
Rhodium is also used to coat some parts of high-quality fountain pens due to its high reflectance, as well as its chemical and mechanical resistance. These pens include Graf von Faber-Castell and Caran D'ache.
Rhodium neutron detectors are used in combustion engineering nuclear reactors to measure neutron flux levels – a method that requires a digital filter to determine the current neutron flux level, as there are three signals generated: immediate, a few seconds later, and a minute later, each with its own signal level, and all three are combined in the rhodium detector signals. The three Palo Verde nuclear reactors each have 305 rhodium neutron detectors, 61 detectors on each of 5 vertical levels, providing an accurate 3-D "picture" of reactivity and allowing fine tuning to most economically burn the nuclear fuel.