Osmium is a transition metal that has a lustrous silvery blue color in its natural solid state. It forms a tetroxide when exposed to air, and is the densest element known. Osmium has an atomic weight of 190.23, a melting point of 5491 °F, and a boiling point of 9054 °F. Osmium is extremely volatile and it is also very toxic; for these reasons osmium is rarely used alone. Osmium alloys are often used in the tips of fountain pens, electrical contacts, and phonograph needles.
Osmium occurs in nature in the ore iridosmium; this compound is composed of iridium and osmium. Osmium also occurs frequently with platinum. Isolation procedures of osmium are complex.
The volatility, toxicity, and complex isolation procedures of osmium make small scale laboratory isolations nearly impossible. The process of osmium isolation begins with treating the ore or byproduct that was formed from processing other metals in order to remove silver, palladium, platinum, and gold from the mixture. For this process, the residue is heated with sodium bisulphate until it melts. Water extractions yield a solution that contain rhodium sulfate; insoluble osmium residues are collected and then melted with sodium peroxide. Extraction of this solution in water yields osmium salts, iridium oxides, and ruthenium. Salt and chlorine gas treatments results in the formation of osmium oxides. These oxides are then dissolved through treatment with alcoholic sodium hydroxide to form a sodium hydroxide osmium complex. The complex is then treated with ammonium chloride to precipitate out the osmium chloride oxygen compound. Further evaporation and drying under hydrogen gas yields the pure osmium product.
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