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Jai Ganesh

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Rhodium

Rhodium

Gist

Rhodium is a rare, silvery-white, and extremely hard transition metal with the chemical symbol Rh and atomic number 45. It's a member of the platinum group metals and is known for its high resistance to corrosion and high melting point. Rhodium is primarily used in catalytic converters for vehicles and in jewelry for plating white gold and sterling silver.

Rhodium is primarily used as a component in catalytic converters for vehicles, where it helps reduce harmful emissions from exhaust gases. It's also used as an alloying agent to improve the strength and corrosion resistance of platinum and palladium, particularly in high-temperature applications. Additionally, rhodium is used in jewelry plating, electrical contacts, and certain chemical processes.

Summary

Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isotope, which is 103Rh. Naturally occurring rhodium is usually found as a free metal or as an alloy with similar metals and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest and most valuable precious metals. Rhodium is a group 9 element (cobalt group).

Rhodium is found in platinum or nickel ores 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.

The element's major use (consuming about 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters in 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 is often plated with a thin rhodium layer to improve its appearance, while sterling silver is often rhodium-plated to resist tarnishing.

Rhodium detectors are used in nuclear reactors to measure the neutron flux level. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of acetic acid.

Details

Rhodium (Rh) is a chemical element, one of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table, predominantly used as an alloying agent to harden platinum. Rhodium is a precious, silver-white metal, with a high reflectivity for light. It is not corroded or tarnished by the atmosphere at room temperature and is frequently electroplated onto metal objects and polished to give permanent, attractive surfaces for jewelry and other decorative articles. The metal is also used to produce reflecting surfaces for optical instruments.

Rhodium added to platinum in small amounts yields alloys that are harder and lose weight at high temperatures even more slowly than pure platinum. Such alloys are used for laboratory furnace crucibles, spark-plug electrodes, and catalysts in very hot chemical environments (including automobile catalytic converters). In the industrial manufacture of nitric acid, gauze catalysts of rhodium–platinum alloys are used because they can withstand the flame temperature as ammonia is burned to nitric oxide. A wire of the alloy 10 percent rhodium–90 percent platinum joined to a wire of pure platinum forms an excellent thermocouple for measuring high temperatures in an oxidizing atmosphere. The international temperature scale is defined over the region from 660° to 1,063° C (1,220° to 1,945° F) by the electromotive force of this thermocouple.

Rhodium is a rare element comprising up to 4.6 percent of native platinum alloys. It also occurs in native alloys of iridium and osmium: up to at least 11.25 percent in iridosmine and up to at least 4.5 percent in siserskite. Rhodium occurs in nature in association with the other platinum metals, and its separation and refinement form part of the overall metallurgical processing of the group. Rhodium is generally obtained commercially as a by-product of the extraction of nickel and copper from their ores.

Natural rhodium consists entirely of stable isotope rhodium-103. The element was first isolated (1803) from crude platinum by the English chemist and physicist William Hyde Wollaston, who named it from the Greek rhodon (“rose”) for the red colour of a number of its compounds. Rhodium is highly resistant to attack by acids; the massive metal is not dissolved by hot concentrated nitric or hydrochloric acids or even by aqua regia. The metal dissolves in fused potassium hydrogen sulfate to yield a complex, water-soluble sulfate K3Rh(SO4)3·12H2O, in hot concentrated sulfuric acid, and in concentrated hydrochloric acid containing sodium perchlorate at 125°–150° C (257°–302° F).

Rhodium chemistry centres chiefly on the +1 and +3 oxidation states; a few compounds of the other positive oxidation states through +6 are recognized. Rhodium forms the dirhodium tetraacetate, Rh2(O2CCH3)4 and various derivatives containing two additional ligands—e.g., water, pyridine, or triphenylphosphine—in oxidation state +2. Complexes in oxidation state +1 chiefly contain carbon monoxide, olefins, and phosphines as ligands. All rhodium compounds are readily reduced or decomposed by heating to yield the powdered or sponge metal. Among these compounds rhodium trichloride, RhCl3 (in which rhodium is in the +3 state), is one of the most important. It provides a starting material for many of the other rhodium compounds in various oxidation states. In aqueous emulsions it can catalyze a number of useful organic reactions.

Element Properties

atomic number  :  45
atomic weight  :  102.905
melting point  :  1,966° C (3,571° F)
boiling point  :  3,727° C (6,741° F)
specific gravity  :  12.4 (20° C)
oxidation states  :  +1, +2, +3, +4, +5, +6.

Additional Information:

Appearance

A hard, shiny, silvery metal.

Uses

The major use of rhodium is in catalytic converters for cars (80%). It reduces nitrogen oxides in exhaust gases.

Rhodium is also used as catalysts in the chemical industry, for making nitric acid, acetic acid and hydrogenation reactions.

It is used to coat optic fibres and optical mirrors, and for crucibles, thermocouple elements and headlight reflectors. It is used as an electrical contact material as it has a low electrical resistance and is highly resistant to corrosion.

Biological role

Rhodium has no known biological role. It is a suspected carcinogen.

Natural abundance

Rhodium is the rarest of all non-radioactive metals. It occurs uncombined in nature, along with other platinum metals, in river sands in North and South America. It is also found in the copper-nickel sulfide ores of Ontario, Canada.

Rhodium is obtained commercially as a by-product of copper and nickel refining. World production is about 30 tonnes per year.

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