PTL:Iridium
From ChemEd Collaborative
Iridium is a transition metal in Group VIIIB (Group 9) and the 6th period. Other members of the group are cobalt (Co) and rhodium (Rh). It is one of the six platinum group elements: ruthenium (Ru), osmium (Os), rhodium (Rh), iridium (Ir), palladium (Pd), and platinum (Pt).
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Discovered
The element was discovered in 1803 by S. Tennant in the black residue left after dissolving crude platinum in aqua regia.
Name
He named the element after the Latin word for rainbow because the salts of iridium display many different colors.
Characteristics
Iridium is exceedingly rare in the earth's crust (0.001 ppm), but ten times more abundant than its neighbor rhodium. As a consequence, Ir is an expensive element; it cost about $4200 per 100 grams in 1991--about the same as gold. Like rhodium, iridium is a by-product of the nickel processing industry.
Iridium is thought to be the densest known substance. Its density is reported to be 22.65 g/cc, while that of osmium is only slightly less, 22.61 g/cc.
Iridium metal is very hard and brittle and so is hard to machine. However, it is the most corrosion-resistant metal known, and so was used, as a 90% Pt - 10% Ir alloy, in making the standards for mass and length. In the solid state, the metal has a face-centerd cubic structure.
Reactions
Iridium is not attacked by any acids, including aqua regia, but it does react with molten salts such as NaCl and NaCN.
Uses
Its principal use is as a hardening agent in making alloys with other platinum group metals such as Pt. It is also used in [airplane spark plugs], electrical contacts, needles for [hypodermic syringes], [labware], and, as an alloy with osmium, it is used for [fountain pen tips].
Emission Spectra
This is the emission spectra for Iridium.
