Osmium

                          Osmium is lustrous, silvery metal, one of the so-called platinum group of metals. It is the densest metal known, althoung only by the narrowest margins. Osmium is uneffected by water and acids, but dissolves with molten alkalis. Osmium powder reacts slowly with the oxygen of the air and gives off detacteble amounts of osmium tetroxide vapour.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.

Discovered:
                        Osmium was discovered in 1803 by Smithson Tennant in the residue left when crude platinum was dissolved by aqua regia (a mixture of hydrochloric acid and nitric acid). Smithson Tennant also discovered iridium at the same time. Osmium's name comes from the Greek word 'osme' meaning a smell associated with toxic osmium tetroxide (OsO4), released by the powdered metal in air.

                                           *Atomic number-76
                                           *Atomic mass- 190.2 g.mol -1
                                           *Electronegativity according to Pauling-2.2
                                           *Density-22.5 g.cm-3 at 20°C
                                           *Melting point-3045 °C
                                           *Boiling point-5027 °C

Reactions:
Reaction with air:mild, ⇒ OsO4                                                               Reaction with 6 M HCl:none
Reaction with 15 M HNO3:mild, ⇒ OsO2                                               Reaction with 6 M NaOH:none

Charasterstics:
                             Osmium is a rare, lustrous, very hard, brittle, bluish-white metal. It is the densest of all the elements. (Although osmium's density is very similar to iridiums, osmium's is slightly higher - both measured and calculated. Calculated: Osmium 22.587 ± 0.009 g/cm3 & Iridium: 22.562 ± 0.009 g/cm3 at 20 °C. See data source.) Osmium has the highest melting point and the lowest vapor pressure of the platinum group (ruthenium, rhodium, palladium, iridium, and platinum).

Physical Properties:
 *Melting point: 3306 [or 3033 °C (5491 °F)] K
 *Boiling point: 5285 [or 5012 °C (9054 °F)] K
 *Density of solid: 22610 kg m-3

Applications:
                Osmium is principally used alloyed with other metals in the platinum group to produce very hard alloys. An alloy of 90% platinum and 10% osmium is used in surgical implants such as pacemakers and replacement heart valves. Osmium tetroxide is used in microscopy as a stain for fatty tissue and in fingerprint detection. The metal is used in a few alloys and in industry as a catalyst. At one time it was to be encountered in the nibs of high-quality fountain pens, compass needles, long-life gramophone needles and clock bearings, thanks to its extreme hardness and corrosion resistance.


Health Effects of Osmium:

                 Osmium tetroxide, OsO4, is highly toxic. Concentrations in air as low as 10-7 g m-3 can cause lung congestion, skin damage, and severe eye damage. The oxide, in particular, should only ever be handled by a properly qualified chemist.

Osmium tetroxide can be absorbed into the body by inhalation of its vapour, by inhalation of its aerosol and by ingestion.

Inhalation risk: A harmful contamination of the air can be reached very quickly on evaporation of this substance at 20°C.

Inhalation: Burning sensation. Cough. Headache. Wheezing. Shortness of breath. Visual disturbances. Symptoms may be delayed. Skin: Redness. Skin burns. Pain. Skin discoloration. Blisters. Eyes: Redness. Pain. Blurred vision. Loss of vision. Severe deep burns. Ingestion: Abdominal cramps. Burning sensation. Shock or collapse.

Chemical dangers: The substance decomposes on heating producing fumes of osmium. The substance is a strong oxidant and reacts with combustible and reducing materials. Reacts with hydrochloric acid to form toxic chlorine gas. Forms unstable compounds with alkalis.

Effects of short-term exposure: Tear drawing. The substance is corrosive to the eyes, the skin, and the respiratory tract. Inhalation of this substance may cause lung oedema. Exposure to high concentrations may result in death. The effects may be delayed.

Effects of long-term or repeated exposure: Repeated or prolonged contact with skin may cause dermatitis. The substance may have effects on the kidneys.



                                                Published by Ravindra.K(Mechanical Engineering)