Osmium

Osmium: the essentials

 Brief description: ruthenium, rhodium, palladium, osmium, iridium, and platinum together make up a group of elements referred to as the platinum group metals (PGM).

Osmium metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and lowest vapour pressure of the platinum group. The metal is very difficult to fabricate, but the powder can be sintered in a hydrogen atmosphere at a temperature of 2000°C. The solid metal is not affected by air at room temperature, but the powdered or spongy metal slowly gives off osmium tetroxide, which is a powerful oxidising agent and has a strong smell. The tetroxide is highly toxic, and boils at 130°C (760 mm). Concentrations in air as low as 10^-7 g m^-3 can cause lung congestion, skin damage, or eye damage.

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Isolation

Isolation: it would not normally be necessary to make a sample of osmium in the laboratory as the metal is available, at a price, commercially. The industrial extraction of osmium is complex as the metal occurs in ores mixed with other metals such as ruthenium, rhodium, palladium, silver, platinum, and gold. Sometimes extraction of the precious metals such as iridium, rhodium, platinum and palladium is the main focus of a partiular industrial operation while in other cases it is a byproduct. The extraction is complex because of the other metals present and only worthwhile since osmium is useful as a specialist metal and is the basis of some catalysts in industry.

Preliminary treatment of the ore or base metal byproduct is required to remove silver, gold, palladium, and platinum. The residue is melted with sodium bisulphate (NaHSO₄) and the resulting mixture extracted with water to give a solution containing rhodium sulphate, Rh₂(SO₄)₃. The insoluble residue contains the osmium. The residue is melted with Na₂O₂ and extracted into water to extract the ruthenium and osmium salts (including [RuO₄]^2- and [OsO₄(OH)₂]^2-). The residue contains iridium oxide, IrO₂. Reaction of the salt with chlorine gas gives the volatile oxides RuO₄ and OsO₄. The osmium oxide is dissolved by treatment with alcoholic sodium hydroxide to form Na₂[OsO₂(OH)₄], and the osmium precipiated out as pure OsCl₂O₂(NH₃)₄ by treatment with NH₄Cl. Evaporation to dryness and burning under hydrogen gas gives pure osmium.

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