Osmium – Properties and Applications

Chemical Formula

Os

Topics Covered

Background
Basic Information
Occurrence
Isotopes
Production of Osmium
Health Aspects
Key Properties
Applications

Background

In 1803AD, Osmium was discovered by Smithson Tennant when dissolving an impure sample of platinum in aqua regia, a mixture of hydrochloric and nitric acids. He was able to determine that the black powder left behind after dissolving the platinum was actually a mixture of two new elements, indium and osmium.

The name Osmium is derived from the Greek word osme, which means odor. Osmium derived its name due to the strange smell of the compound Tennant was working with, which was osmium tetroxide.

Osmium’s claim to fame is that it is the densest of all the elements. It is also a member of the platinum group metals or PGMs (along with Ruthenium, Rhodium, Palladium, Iridium and Platinum) as well as being a transition metal.

Basic Information

Table 1. Basic Properties of Osmium

Name Osmium
Symbol Os
Atomic number 76
Atomic weight 190.23 (3) amu
Standard state solid at 298 K
CAS Registry ID 7440-04-2
Group in periodic table 8
Group name Precious metal or Platinum group metal
Period in periodic table 6
Block in periodic table d-block
Colour bluish grey
Classification Metallic
Melting Point 3306 K (3033°C or 5491°F)
Boiling Point 5285 K (5012°C or 9054°F)
Density 22.57 g/cm3
Ionization Energy 8.7 eV
Oxidation States +4, +3

Occurrence

Osmium is a rare element and found in the Earth's crust. Osmium-rich ores include Osmiridium and Iridosmine of which the former is the most common.Osmium is also found in all ores of platinum.

The largest known primary reserves of osmium include the Bushveld igneous complex in South Africa, however the huge copper-nickel deposits near the Norilsk in Russia and the Canada-based Sudbury basin are also important osmium sources.

Isotopes

Osmium consists of seven naturally occurring isotopes. Of these 6 are considered stable and the 7th, Osmium 186 has a half life of 2x1015 years, making it stable for all practical purposes. Several radioactive isotopes of Osmium also exist.

Production of Osmium

Osmium concentrates are produced as a by-product of nickel and copper mining or alternatively while isolating the platinum metal from its ores. Osmium is then separated from other platinum group metals by distillation or organic solvent extraction to yield volatile Osmium tetroxide OsO4 which is then collected and precipitated using KOH. The resultant salt is reduced and roasted to yield a fine power of Osmium.

Health Aspects

Some of the osmium compounds are very dangerous and cause irritation to throat, lungs, skin and eyes. Extreme care is needed while handling the compounds. One needs to be very careful while handling the most widely used osmium compound, osmium tetroxide (OsO4).

Key Properties

The key properties of osmium are listed below

  • Osmium is a bluish-white and shiny metal.
  • Osmium is very hard and is brittle even at very high temperatures
  • Osmium has the lowast vapor pressure and the highest melting point among the platinum group of metals
  • Osmium’s density is slightly more than iridium hence is credited as the heaviest element
  • Osmium can be dissolved by acids or aqua regia only if exposed to these liquids for a long period.
  • The metal reacts with oxygen to form osmium tetroxide (OsO4) after heating.
  • Osmium tetroxide is highly toxic and the only essential commercial compound of osmium.

Applications

Due to its rarity and hence expense, osmium has a limited number of applications. Applications of osmium include:

  • Osmium can be used as an alloying agent, where it adds hardness to the resultant metal. Examples include platinum or indium
  • Certain alloys of osmium and platinum are used in the manufacture of specialized laboratory equipment
  • Finely divided osmium metal can be used as a catalyst e.g. in the process of forming ammonia by combining hydrogen and nitrogen.
  • Hard alloys can be produced by using Osmium. Osmium alloys are used in ball point pen tips, fountain pen tips, record player needles, compass needles and electrical contacts.
  • Early light bulb filaments used osmium. This was later replaced by tungsten which was found to be easier to work with.
  • Due to its inertness, osmium-platinum is used to make surgical instruments and pacemakers
  • Osmium tetroxide is highly volatile and strongly oxidizing and is beneficial in the chemical industry as long as one takes sufficient precautions.
  • Osmium tetroxide is used for staining biological samples for microscopic examination and for fingerprint detection.

Source: AZoM.com

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