Osmium Chemistry

Detection

The element osmium is detected via its compound osmium tetroxide. The detection reaction is specific, however, it is not recommended due to its toxicity.

The solution containing the osmium is dropped on a filter paper which has been previously moistened with a benzidine or potassium hexacyanoferrate solution. In case of the benzidine solution, the filter paper turns purple. In the case of the potassium hexacyanoferrate solution, the paper changes its colour to light green.

However, more and more instrumental techniques are being used today to detect osmium. Examples include atomic spectrometry, neutron activation analysis and voltammetry. With these methods, a more accurate determination is possible.

Production

The production of osmium is very complex and usually takes place during the extraction of other metals, such as platinum or gold.

The starting material for osmium production is usually anode sludge from gold or nickel production. The anode sludge is dissolved in aqua regia, causing platinum and gold to dissolve. The other platinum metals and silver remain.

The silver forms insoluble silver chloride which can be separated by nitric acid and lead carbonate. Subsequently, sodium hydrogen carbonate is melted and leached out.

Rhodium is dissolved and removed as rhodium sulphate. The residue is melted down together with sodium peroxide, causing ruthenium and osmium to dissolve. Iridium remains in the insoluble residue.

Chlorine is added to the solution. This produces the volatile substances osmium tetroxide and ruthenium tetroxide.

After the addition of alcoholic caustic soda lye, only osmium tetroxide is dissolved and can therefore be separated from ruthenium tetroxide.

To obtain elemental osmium, it is precipitated as a complex with ammonium chloride.

Finally, hydrogen is reduced to metallic osmium.

OsO2(NH3)4Cl2 + 3 H2 → Os + 4 NH4+ + 2 Cl- + 2 OH-

Characteristics

In the periodic table, osmium is located in the sixth group of the transition elements. Within this group, it can be found in the sixth period.

Osmium has the atomic number 76 and an atomic mass of 190.23 u. Its electron configuration is [Xe] 4f14 5d6 6s2.

Osmium is non-toxic in its crystalline form as long as a temperature of 400 °C is not exceeded.

Osmium has a bluish-silvery lustre. It is the hardest platinum-group metal and having a density 22.61 g / cm³, it is the element with the highest density in the periodic table. Osmium is very difficult to process.

Osmium has a hexagonal close packed crystal structure.

Osmium is not particularly reactive.

Osmium reacts directly only with chlorine, fluorine and oxygen.

If osmium is compact, it is resistant against non-oxidising acids in water and air.

Finely dispersed osmium slowly oxidises to osmium VIII oxide:
Os + 2O2 → OsO4

After rhenium and tungsten, osmium, at 3033 °C, has the highest melting point of all metals.

Its boiling point is 5000 °C.

At low temperatures, osmium is a superconductor.

In addition, osmium has the largest bulk modulus of all elements. With a value of 462 GPa, it even surpasses diamond.

Chemical compounds

Osmium tetroxide is a well-known osmium compound. It is formed by the reaction of oxidants such as nitric acid with elemental osmium. Osmium tetroxide is a volatile solid with a strong oxidising effect.

Crystalline osmium is not a compound, but a specific crystallisation form of the pure element which, due to its chemical properties, is non-toxic up to 400 °C.

Unlike most oxidants, oxidation with osmium tetroxide can occur under stereochemical control. Although it is an expensive and toxic compound, osmium tetroxide has some applications.

For example, osmium tetroxide is used in fingerprint forensics. It also serves to enhance the contrast of cell membranes in electron microscopy.

Other types of compounds include the coordination complexes of osmium. The so-called osmates, anionic oxygen coordination complexes, derive from osmium tetroxide.

Coordination complexes also exist with other ligands such as ammonia, cyanide, carbon monoxide, and nitric oxide. If the ligand is organic, an osmocene may be formed.

Isotopes

Osmium consists of a mixture of seven stable isotopes: Osmium-192 (41 percent), Osmium-190 (26.4 percent), Osmium-189 (16.1 percent), Osmium-188 (13.3 percent), Osmium-187 (1.6 percent), Osmium-186 (1.58 percent) and Osmium-184 (0.02 percent).

The only natural radioactive isotope is Osmium-186 with a half-life of about two quadrillion years.

In addition, there exist 27 short-lived isotopes, of which Osmium-194 has the longest half-life (six years).

The ratio of the isotope Osmium-187 to the isotope Osmium-186 is used in rhenium-osmium chronometers. These are used to determine the age of iron meteorites.