Title: Iridium
CAS Registry Number: 7439-88-5
Literature References: Ir; at. wt 192.217; at. no. 77; valences 1, 3, 4; also 2, 5, 6. Group VIII (9). Two naturally occurring isotopes: 191 (38.5%); 193 (61.5%); artificial, radioactive isotopes: 182-190; 191; 192; 194-198. Occurrence in earth's crust about 0.001 ppm. Discovered in 1804 by Tennant. Occurs in nature in the metallic state, usually as a natural alloy with osmium (osmiridium); found in small quantities alloyed with native platinum (platinum mineral) or with native gold. Recovery and purification from osmiridium: Deville, Debray, Ann. Chim. Phys. 61, 84 (1861); from the platinum mineral: Wichers, J. Res. Natl. Bur. Stand. 10, 819 (1933). Reviews of prepn, properties and chemistry of iridium and other platinum metals: Gilchrist, Chem. Rev. 32, 277-372 (1943); W. P. Griffith, The Chemistry of the Rarer Platinum Metals (John Wiley, New York, 1967) pp 1-41, 227-312; Livingstone in Comprehensive Inorganic Chemistry vol. 3, J. C. Bailar Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1163-1189, 1254-1274.
Properties: Silver-white, very hard metal; face-centered cubic lattice. mp 2450°; bp ~4500°. d420 22.65; highest sp gr of all elements. Specific heat 0.0307 cal/g/°C. Mohs' hardness 6.5. Pure iridium is not attacked by any acids including aqua regia; only slightly by fused (non-oxidizing) alkalies. It is superficially oxidized on heating in the air. Is attacked by fluorine and chlorine at a red heat; attacked by potassium sulfate or by a mixture of potassium hydroxide and nitrate on fusion; attacked by lead, zinc or tin. The powdered metal is oxidized by air or oxygen at a red heat to the dioxide, IrO2, but on further heating the dioxide dissociates into its constituents.
Melting point: mp 2450°
Boiling point: bp ~4500°
Density: d420 22.65
Use: In manufacturing crucibles; in hardening platinum; in making nibs for fountain-pen points.

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