RHODIUM

Listing description

Rhodium (pronounced /ˈroʊdiəm/ ROH-dee-əm) 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. Naturally-occurring rhodium is composed of only one isotope, ¹⁰³Rh. It is one of the rarest precious metals and, with a price of about US$80,000/kg in 2010, is the most expensive member of that class.^[3]

Detailed descrption

Rhodium was discovered in 1803 by William Hyde Wollaston. It is found in platinum- or nickel ores together with the other members of the platinum group metals. Rhodium is mostly used as a catalyst in the three-way catalytic converter. Rhodium is inert against corrosion and most aggressive chemicals, and because of that and 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 the optical impression. Rhodium detectors are used in nuclear reactors to measure the neutron flux level.

History

Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston,^[4][5] soon after his discovery of palladium.^[6][7] He used crude platinum ore presumably obtained from South America.^[8] 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 washing with ethanol, the rose red precipitate was reacted with zinc forming rhodium metal.^[9]

After the discovery the rare element had only minor applications, for example by the turn of the century rhodium containing thermocouples where used to measure temperatures up to 1800°C.^[10][11] The first major application was electroplating for decorative uses and as corrosion resistant coating.^[12] 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.^[13][14][15]

Characteristics

Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated.^[16] Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification.^[17] 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.

Chemical properties

Rhodium belongs to group 9 of the periodic table but has an atypical configuration in its outermost electron shells compared to the rest of the members. This can also be observed in the neighborhood of niobium (41), ruthenium (44), rhodium (45), and palladium (46).

Z	Element	No. of electrons/shell
27	cobalt	2, 8, 15, 2
45	rhodium	2, 8, 18, 16, 1
77	iridium	2, 8, 18, 32, 15, 2
109	meitnerium	2, 8, 18, 32, 32, 15, 2

Oxidation states of rhodium
+0	Rh4(CO)12
+1	RhCl(PH3)2
+2	Rh2(O2CCH3)4
+3	RhCl3, Rh2O3
+4	RhF4, RhO2
+5	RhF5, Sr3LiRhO6
+6	RhF6

Common oxidation states of rhodium is +3, but oxidation states from +0 to +6 are also observed.

PRICE

$970/TROY OZ

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