A metallic element, iridium’s atomic number is 77 and its chemical symbol is Ir. Iridium is placed in group 9 and is positioned between platinum and osmium. Similar to platinum, it is also a “transitional metal” and one of the “platinum metals” together with palladium, rhodium, osmium, and ruthenium and of course platinum.
This metallic element is hard as well as brittle. It has a silvery or whitish-yellow color and its density is little less compared to osmium. In fact, osmium is the densest metallic element having a density of 22.560 gm/cm3. In comparison, the density of pure iridium is 22.6 gm/cm3. In its pure form, iridium is known to be a very stable as well as dense “transitional metal”.
Most metals in the “platinum metal” group are resistant to corrosion, in its pure form, iridium is said to offer maximum resistance to corrosion as it is able to effectively fight by oxidation, salts, aqua regia (a blend of nitrochloric acid and hydric acid) as well as mineral acids. However, it is susceptible to attacks by molten salts like sodium cyanide and sodium chloride.
After osmium, iridium is considered to be the second densest metal. However, this view is still being debated by scientists. Similar to all other PGMs, iridium too melts at very high temperature and has excellent mechanical strength even at very high temperatures.
It has been established that among all metal elements, metallic iridium possesses the second highest elasticity modulus. In other words, metallic iridium is not only very stiff, but also opposed to deformation – attributes make this metal to be used for fabrication for utilizable parts. However, the same characteristics of iridium make the metal a valuable additive for strengthening alloys. For instance, an alloy of platinum with 50 percent iridium is almost ten times harder compared to iridium in its pure form.
An English chemist named Smithson Tennant is credited with discovering iridium. This metallic element derives its name from the Latin term iris or the Goddess of rainbow. Iridium has been named so owing to its various coloured salts. In fact, Smithson is also credited with discovering another “platinum metal” – osmium.
Unlike platinum which is present in alluvial soil in the rivers, iridium is present in gravel deposits. In fact, it is believed that it only occurs with alloys formed with other noble metals. In other words, it is not found in uncombined state. Iridium is often found in alloys formed with osmium – such as compounds known as iridiosmium and osmiridum, where the original names of the metallic elements are obvious. However, iridium is a rare metallic element as its presence on the Earth’s crust is just .001 ppm.
It is said that Smithson Tennant discovered iridium in 1804 while he was examining a platinum ore. Despite its discovery in 1804, crude iridium was extracted only 10 years later. It took scientists nearly 40 years after Tennant discovered iridium to produce the pure form of this metallic element.
It was John Isaac Hawkins, a practicing civil engineer and inventor, who first developed the commercial application of iridium in 1834. Hawkins, who is credited of being the co-inventor of pencil, was endeavouring to find a hard material to make pen tips that would not break or wear out even after repeated use. When he learnt about the characteristics of iridium, Hawkins obtained some metal containing iridium from William Wollaston, a colleague of Smithson Tennant, and started producing the first gold pens with iridium tips.
Johnson-Matthey, a British firm, took the lead in developing as well as marketing platinum-iridium alloys during the latter half of the nineteenth century. Among other things, initially this alloy was used in Witworth cannons, which were seen in action in the American Civil War.
In the era prior to the advent of iridium alloys, the vent pieces of most cannons, which embraced a cannon’s ignition, were infamous for being deformed owing to elevated combustion temperatures and repeated ignition. According to claims, the vent pieces made from iridium alloys were capable of retaining their form and shape for more than 3,000 charges.
Many years after this metallic element was discovered, Sir William Crookes is credited with designing the first ever iridium crucible in 1908. Crucibles are basically vessels that are used for undertaking chemical reactions at very high temperatures. These iridium crucibles were jointly produced by Crookes and Johnson Matthey. It was found that iridium crucibles offered great advantages over vessels made from pure platinum.
In the early 1930s, scientists developed the first ever iridium-ruthenium thermocouples. Thirty years later, the demand for this metallic element increased significantly in the 1960s for the wide use of dimensionally stable anodes (DSAs) made from it.
A significant development is the chloralkali process came with the development of these DSAs, which comprises titanium metal that was coated with PGM oxides. The chloralkali process is used to produce caustic soda and chlorine. As a result, even to this day, the dimensionally stable anodes remain one of the major iridium consumers.
Discovery and naming
In the initial days, all elements belonging to the group of platinum metals created a difficult problem for the chemists. Often these metals are found mixed with other materials on the Earth’s crust. Therefore, it is possible that when a scientist was examining a platinum sample, it actually was a mixture of rhodium, iridium, osmium as well as some other metals. A case in point is French chemist Pierre-Francois Chabaneau’s work in this field. The Spanish government asked Chabaneau to study platinum and gave him all its platinum supply. Interestingly, during his experiments with the samples, Chabaneau himself was puzzled with the results. At times, Chabaneau found that the platinum he was working with could be easily hammered down to flat sheets. On other occasions, when he hammered the platinum sample, it proved to be too brittle and got shattered. At that time, Chabaneau failed to realize that the “platinum” samples he was working on actually comprised different amounts of a variety of other noble metals also.
Several chemists were engaged in separating platinum metals in the early part of the 1800s. One of these chemists was Smithson Tennant (1761-1815), an Englishman credited with discovering osmium and iridium. Like several other people, Tennant developed a keen interest in chemistry from a tender age. It is said that when Tennant was just nine years old, he prepared gunpowder that is used in fireworks.
Tenant liked to experiment and in 1803, he used aqua regia in an attempt to liquefy platinum in it. It is worth mentioning here that aqua regia is a combination of two potent acids – hydrochloric acid and nitric acid. He was surprised to find that while most of platinum dissolved in the liquid, a little residue was left in the form of a black powder. In fact, no chemist was interested in analyzing the black powder left as residue after most part of platinum dissolved in aqua regia. However, Tennant was interested to examine the substance and he did. During his study, Tennant found that the properties of the black powder were different from those of platinum. He then became conscious that he had discovered another element that was hitherto unknown to scientists. Tennant named the new element iridium, a name that has its origin in the Greek mythology. Iris, in Greek mythology, is the Goddess of Rainbow. Tennant named the element iridium because the compounds of this metallic element have a variety of colors.
Although iridium has a number of applications, primarily it is used for manufacturing alloys – a combined metal formed by melting and blending two or more metallic elements. Hence, it is quite natural that the properties of an alloy are different from the metals it is made up of. Often iridium is mixed with platinum with a view to produce a metallic compound whose strength is more compared to platinum. Generally, alloys of platinum or iridium are very expensive and solely utilized for special purposes. For example, helicopters use sparkplugs that are manufactured from an iridium-platinum alloy. In addition, this type of alloys is used for making special varieties of electrical wires, electrical contacts and even electrodes.
As mentioned above, iridium has multiple uses. An iridium alloy formed by combining osmium is used in gold-tip pens. This metallic element is also used for hardening platinum. Last, but not the least important, it is also used for making very specialized crucibles. Like platinum, even iridium can be used in cancer irradiation and also for making surgical pins and hypodermic syringes.
One very important use of this metallic element in the weights and measures standard, especially for the kilogram. In fact, the kilogram is made from an alloy comprising platinum (90 percent) and iridium (10 percent). Since it has been found that rocks belonging to the Cretaceous as well as the Tertiary periods contain considerable amounts of iridium, some people assume that this “platinum metal” was actually a constituent of an asteroid that hit the Earth long ago, thereby leading to the extermination of the dinosaurs.
Iridium is known to be a very stable metallic element, which hardly ever has any chemical reaction. Moreover, among all metals known to mankind, it is the most corrosion resistant metallic element.
A high dose of iridium in radiation therapy helps to treat prostate and other cancer forms. It is also used for manufacturing crucibles – vessels which are used for melting other metals at very high temperatures. Iridium is also used for making other equipment that is capable of resisting extreme heat. The alloy formed by combining iridium with platinum is used to make tough and corrosion resistant electrical contacts like technically advanced and high performing spark plugs, which are used in aviation and automobile industries.
Iridium is extremely brittle, but very difficult to machine. Commercially, it is available in various forms such as powder, wire, sheet and even sponge. It also works as a catalyst in the production of acetic acid. The alloy formed by combing it with osmium is used in pivot bearings like those present in surgical tools, compasses and other specialized apparatus. This alloy is also used to make fountain pen nibs, especially pens with gold nibs.