Gunnar Nordberg
Occurrence and Uses
Osmium (Os) is found almost exclusively in osmiridium, a natural alloy consisting of osmium and iridium, and in all platinum ores. The major ore deposits are located in the Urals, Canada and Colombia, with less important ores in Australia and in Alaska, California and Oregon in the United States.
Osmium alloys readily with the other platinum metals and with iron, cobalt and nickel. It also forms brittle intermetallic compounds with tin and zinc. One of the distinctive features of osmium is the ease with which it forms osmium tetroxide (OsO4). Osmium powder always has the characteristic odour of its tetroxide because even at normal temperatures it oxidizes in air to OsO4, even if only to a slight degree. The tetroxide is extremely volatile and has an unpleasant odour, from which the name of the element was derived (osme=odour). It is a powerful oxidizer and is easily converted to osmium dioxide (OsO2) or even to metallic osmium. With alkalis it forms unstable compounds such as OsO4·2KOH. When heated, osmium readily forms osmium disulphide (OsS2). The fluorides OsF4, OsF6 and OsF8 are also formed. Various chlorides are formed when osmium is treated with chlorine at high temperatures. With carbon monoxide, it forms carbonyls. It also forms a number of compounds with the complex anion containing osmium, as for example ammonium osmium hexachloride ((NH4)2OsCl6).
Osmium is used as a catalyst in the synthesis of ammonia and in the hydrogenation of organic compounds. As an alloy with indium it is used for the manufacture of compass needles and fine machine bearings. It is found in the parts of watch and lock mechanisms and in fountain pen points. Osmium tetroxide, sometimes incorrectly termed osmic acid, is used as an oxidizing agent, particularly for converting olefins to glycols. The chloro-osmiates are used in place of gold salts in photography.
Hazards
The metal is innocuous, but persons engaged in its production are exposed to the effects of vapours from acids and chlorine. Osmium tetroxide vapours are poisonous and extremely irritating to the eyes even at low concentrations, causing weeping and conjunctivitis, and to the upper respiratory system, causing bronchitis, bronchial spasms and difficulty in breathing, which may last for several hours. Longer exposure can result in damage to the cornea, blindness, disturbances of the digestive system and inflammatory disorders of the lungs and kidneys. Upon contact, it discolours the skin green or black and causes dermatitis and ulceration.
Safety and Health Measures
During the production of osmium, local exhaust ventilation should be provided and the apparatus should be sealed if gaseous chlorine is used. An enclosed ventilated area or hood is necessary in order to control the release of osmium tetroxide vapours into the work environment and prevent eye and respiratory irritation. Exposed workers should wear protective clothing, hand protection, gas-tight chemical safety eye protection and appropriate respiratory protective equipment. Containers must be stored in naturally ventilated premises. The vapour has a pronounced and nauseating odour which should serve as a warning of toxic concentration in the air, and personnel should leave the polluted area immediately. Determination in air and blood is possible by colourimetry of the complex with thiourea.