Rhodium is a chemical element in the periodic table that has the symbol Rh and atomic number 45. A rare silvery-white hard transition metal, rhodium is a member of the platinum group, is found in platinum ores and is used in alloys with platinum and as a catalyst.
Rhodium is a hard silvery white and durable metal that has a high reflectance. If slowly cooled from a red hot state it changes in air to the sesquioxide, which at higher temperatures converts back to the metal. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids and only dissolves in aqua regia.
The primary use of this element is as an alloying agent for hardening 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;
It is used as an electrical contact material due to its low electrical resistance, low and stable contact resistance, and its high corrosion resistance.
Plated rhodium, made by electroplating or evaporation, is extremely hard and is used for optical instruments.
This metal finds use in jewelry and for decorations.
It is also a highly useful catalyst in a number of industrial processes (notably it is used in the catalytic system of automobile catalytic converters and for catalytic carbonylation of methanol to produce acetic acid by the Monsanto process).
Rhodium (Greek rhodon meaning "rose") was discovered in 1803 by William Hyde Wollaston soon after his discovery of palladium. Wollaston made this discovery in England using crude platinum ore that he presumably obtained from South America.
His procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum metal by adding ammonium chloride, NH4Cl, as ammonium chloroplatinate. The element palladium was removed as palladium cyanide after treating the solution with mercuric cyanide. The material that remained was a red rhodium(III) chloride: rhodium metal was isolated via reduction with hydrogen gas.
The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. It is found in platinum ores and obtained free as a white inert metal which it is very difficult to fuse. Principal sources of this element are located in South Africa, in river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective. Main exporter of rhodium is South Africa (>80%) followed by Russia. However, the annual world production of this element is only about 20 tons and there are very few rhodium minerals.
It is also possible to extract Rhodium from burned-out nuclear fuel, which contains a few percent of rhodium. Rhodium produced in such a way contains radioactive isotopes with half-lives of up to 45 days and it must be checked very carefully for radioactivity before being sold.
Naturally occurring rhodium is composed of only one isotope (Rh-103). The most stable radioisotopes are Rh-101 with a half-life of 3.3 years, Rh-102 with a half-life of 207 days, and Rh-99 with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (Rh-93) to 116.925 u (Rh-117). Most of these have half-lifes that are less than an hour except Rh-100 (half-life: 20.8 hours) and Rh-105 (half-life: 35.36 hours). There are also numerous meta states with the most stable being Rhm-102 (0.141 MeV) with a half-life of about 2.9 years and Rhm-101 (0.157 MeV) with a half-life of 4.34 days.
The primary decay mode before the only stable isotope, Rh-103, is electron capture and the primary mode after is beta emission. The primary decay product before Rh-103 is ruthenium and the primary product after is palladium.
Compounds that contain rhodium are not often encountered by most people and should be considered to be highly toxic and carcinogenic. Rhodium compounds can stain human skin very strongly. This element plays no biological role in humans.