Platinum


The name platinum is known to practically everyone, but few have actually seen it. It is well known to be rare and expensive. Chemistry texts give limited space to the platinum metals, though their important catalytic properties are always mentioned. The history of platinum is very imperfectly treated, even in encyclopedias, and many misconceptions are current. Platinum is not a silvery metal--it is grey--and is not found as bright, attractive nuggets like gold. For these reasons, I have thought that an article on platinum might be informative.

In 1748, Don Antonio de Ulloa published in Europe an account of his travels in New Spain in 1735. He brought to notice a heavy black sand that had been known for some two centuries in the placer gold operations in New Granada (Colombia and Panama), which he called platina del Pinto--"little silver of the river Pinto." It was certainly not very usable. If you hammered on it, you got a hammered black powder. If you heated it, you got a hot black powder, and no furnace then could fuse it. Apparently it could be dissolved in molten gold, but decolorized and ruined the gold and so was forbidden in trade by the Spanish colonial authorities. In fact, it was thrown away with the contaminated mercury used in the recovery of gold. The metal was not much used by pre-Coumbian natives, unless they liked heavy black sand. They could have hammered out the very rare nuggets, however. The report that it was used by the Egyptians is probably erroneous; they were not near any sources, and platinum was never mentioned in ancient times. Also, Ulloa did not discover platinum; he merely reported it.

Because of its chance association with gold in placers, it may have been mentioned in antiquity, but was not recognized as a metal or used for any purpose. As a heavy sand (density 14-19 g/cc in the native state) unaffected by chemical processes, it accumulated in placers like gold, and indeed in the same places. However, it is much rarer than gold, and does not have the same origin. Both metals are freed when the rocks containing them weather, but they come from different sources. Gold comes from veins of silica, and is deposited hydrothermally by active fluids. Platinum is present in the ultrabasic rocks associated with plate boundaries--serpentine, olivine and so forth. Like diamonds, platinum comes from the earth's mantle. When these rocks are weathered out, the durable, heavy platinum comes to rest at the same locations as the gold.

Platinum is the most abundant of a family of six heavy metals, all very similar to one another, and they are always found together. The platinum family consists of two triads analogous to the triad of iron, cobalt and nickel directly above them in the Periodic Table. Just under Fe, Co and Ni are the palladium triad ruthenium (Ru), rhodium (Rh) and palladium (Pd), while under them are the platinum triad osmium (Os), iridium (Ir) and platinum (Pt). These are nearly identical metals, differing only in numbers of interior d-electrons and in atomic weights. The densities of the palladium triad are 12.2, 12.41 and 12.16 (in the order Ru, Rh, Pd that will be used uniformly here), while those of the platinum triad are 22.48, 22.42 and 21.40 (in the order Os, Ir, Pt). The electronic structure of all is about the same, and the atoms are all of about the same size. The density difference is explained mostly by the difference in atomic weights, around 103 for the palladium triad and 193 for the platinum triad. Either osmium or iridium is the densest element known. Calculations say iridium, measurements osmium.

For reference, the atomic numbers and atomic weights are: Ru 44, 101.7; Rh 45, 102.91; Pd 46, 106.7; Os 76, 190.2; Ir 77, 193.1; Pt 78, 195.23. The electron configuration of Pt is 1s2 2s22p6 3s23p63d10 4s24p64d104f14 5s25p65d9 6s.

The Mohs hardnesses of Pd and Pt are 4.8 and 4.6 (or 4.3), respectively, which are quite high for metals, and the other members of the family are even harder. Hardest seems to be Os, at 7.0, while Ir and Ru are not far behind at 6.5. This means that platinum is as hard as iron, while osmium is as hard as quartz. A platinum-rhodium alloy is even harder, and is used for high-quality fountain pen nibs. Hardness does not imply that the metals are not malleable and ductile. In fact, platinum is very ductile and can be rolled into very thin sheets and drawn into fine wire. Its crystal structure is face-centred cubic, characteristic of soft metals.

When crude platinum came to Europe, no fire succeeded in melting it. The melting point of platinum is 1755°C, which is high, but no competition for tungsten, which melts at 3370°C, tantalum at 2850°C or even molybdenum, melting at 2620°C. Platinum cannot quite make a good incandescent filament, but it could be heated in the open if it were so used. Unable to melt it, the chemists of the time did succeed in dissolving it in hot aqua regia, or at least most of it. Rhodium and iridium are not dissolved by aqua regia. Eventually the crude platinum was separated into a more or less pure part that was mostly platinum, and into a dark mess called Polyxen that was assumed to contain a mixture of many yet-unknown metals. These metals were gradually isolated on the basis of the very slight differences in their chemical behaviour. Osmium and iridium were isolated by Tennant in 1803. Osmium forms the volatile oxide OsO4 (mp 40°C, bp 130°C) that volatilizes in steam and has a very unpleasant chlorine-like odour, which gave it its name. The Greek osmh means an odour or smell, good or bad. The powdered metal gives off the tetroxide by catalyzing its own oxidation by the oxygen of the air. This "osmic acid" is very damaging to the lungs and extremely poisonous. This substance can be used as a histologic stain; it is reduced by the tissue and opaque osmium is deposited. IrO4 exhibits iridescent colours that gave the metal its name. Palladium was also identified in 1803; it will dissolve in nitric acid alone. It was named after the recently-discovered asteroid Pallas. Rhodium was separated from iridium in 1804 by Wollaston; its name comes from Greek 'rodon, a "rose," presumably from the colour of one of its compounds. The metal itself is grey. Ruthenium was finally found in 1844 in Russian crude platinum (whence the name, from the Latin for "Russia"), and the complete family was then known. The metals were by no means pure when initially recognized; in 1857 the density of osmium was given as about 10; it is actually 22.48.

The problem still remained of how to get platinum metal that behaved like a metal, not like a heavy black dust. The chemist Achard found that platinum could be fused with the aid of arsenic, which on further heating evaporated. He made the first platinum crucible in 1783. Unfortunately, on heating the material became porous. In 1828, Wollaston found that this powder could be welded into a coherent whole by heat and pressure. In that year, he made a platinum crucible by forming it from a paste of platinum black and water, then applying pressure and considerable heat. Such sintering methods are still used for fabricating some of the platinum metals, especially those of the higher melting points. Possibly, this was the origin of powder metallurgy.

In 1822, on the eastern slope of the Urals, in the Upper Tura River near the town of Nizhni Tagil, a large platinum placer was discovered. After this, Russia became the primary source of platinum. The granules were larger here than in Colombia, and even some fairly large nuggets were found. However, platinum nuggets never approach the size of large gold nuggets, and are extremely rare. From 1824 until 1845 platinum coins were minted in denominations of 1, 2 and 4 ducats (3, 5 and 12 rubles), the platinum considered as three times the value of gold by weight. Since 1 ducat = US$2.27 at the time, the coins would have been rather small. My source does not say how the platinum was consolidated in these coins, but it was probably by the same welding or sintering process discovered by Wollaston. These placers became exhausted in the early 20th century, and lode mining of platinum was begun, since the source of the placers had been found.

Presently, the largest source of platinum metals is (or recently was) as a by-product of the nickel mines of Sudbury, Ontario. This is a lode source, developed after 1935, in the Sudbury ring complex, an intrusive structure that is well-known for its sulphide ores of Ni and Cu. On the average, 1 oz of platinum is produced for every ton of nickel. Native platinum is found here, as well as the very rare mineral Sperrylite, PtAs2, the only platinum mineral. The platinum is accompanied by nickel and copper, as might be expected. Platinum also is often accompanied by iron, silver and gold, and is always found with the other members of its family. South Africa and Russia also produce platinum from lode mining.

Platinum can be melted in the oxyhydrogen flame, probably in a graphite crucible. All the chemical purification processes leave platinum in colloidal form as "platinum sponge" or "platinum black." There may be no fundamental difference between materials described by these two terms. Platinum sponge is said to be made by heating ammonium chloroplatinate, and platinum black by reducing chloroplatinic acid solution with zinc. For the use of platinum as a chemical catalyst, this is indeed just the form desired. For other uses, the platinum is fused, and the ingot can be rolled into foil or drawn into wire.

All the platinum metals form square complexes like PtCl4-- or octahedral complexes like PtCl6--, as well as similar complexes involving NH3 and CN. This is what allows platinum to be dissolved in aqua regia, forming chloroplatinic acid, H2PtCl6. Like gold, platinum has very little chemistry. The chloroplatinates and the oxides are about all that is of importance. Many compounds of the platinum metals are highly coloured.

The platinum crucible was a great boon to chemistry, since it allowed strong heat to be applied without danger of reaction of the charge with the crucible. The alternatives are nickel and porcelain crucibles, which are much cheaper but cannot be used at higher temperatures, and react more readily with a charge. Graphite crucibles can be taken to high temperatures without melting, but are reactive and are even consumed themselves through heating in air. There is a surprisingly long list of substances that should not be heated in platinum. Hg, Sn, Pb and Bi alloy readily with platinum. C, Si, P and S embrittle the crucible. Fluorine and chlorine gas attack platinum readily. Strong alkalies, such as NaOH and KOH should not be heated, but carbonates are all right. Ferric chloride is also forbidden, since the chlorine will react with platinum. Most of the attacks are not really extensive, but platinum is such an expensive material that it must be carefully protected.

An alloy of 76.7 Pt, 23.3 Co is a good permanent magnet material, but the price of platinum puts it out of reach for most uses. Its (BH)max is said to be twice that of Alnico V. Many new permanent magnet materials using rare earths are, however, even better and much cheaper.

Platinum and palladium in colloidal form are excellent catalysts for hydrogen reactions. All the platinum group metals show catalytic action, but it is best known in these two. The colloidal form, with is very large surface area per unit weight, is essential to the catalytic action. Palladium absorbs 900 times its own volume of hydrogen at room temperature, releasing it when strongly heated. These metals were used in the manufacture of catalytic sulphuric acid (oxidizing SO2 to SO3), and as well in nitric acid manufacture (oxidizing NO to NO2; an iron catalyst is now used). They are also used in petroleum refining ("catalytic cracking"). A tiny amount of arsenic reacts readily to "poison" the catalyst by blocking the active sites, so arsenic must be rigorously avoided when platinum is used. Other, much cheaper, catalysts such as nickel or iron have been substituted for platinum in recent years, so that platinum is retained only when no substitute can be found. Colloidal platinum also will ignite hydrogen flames in the presence of oxygen or carbon monoxide. Platinum black was used as an igniter for town gas (which includes both H2 and CO), but it probably will not work with natural gas. It will explode a mixture of H2 and O2. Fine platinum wire will catalyze the reaction of methanol to form formaldehyde, and become red-hot from the heat released. Catalytic converters on motor cars, which remove hydrocarbon residues, surely use some cheaper and less delicate catalyst at the present time. Tetraethyl lead poisoned the catalyst. The elimination of leaded motor fuel was as much to protect catalytic converters as to remove lead.

Platinum is perhaps the best material for electrical contacts, since it is not only hard, but will not tarnish. It can be used to make resistance wire for resistance thermometers and electric furnaces, as well as for thermocouples. The resistivity of platinum is about 10 μΩ-cm. Alloys of 90 Pt 10 Rh and 87 Pt 13 Rh are used in thermocouples against Pt. The thermoelectric power is low, but the thermocouples are very stable and reliable, and can be used at high temperatures. The coefficient of thermal expansion of Pt is 8.99 x 106 °C-1, about the same as that of soda-lime glass, so platinum wires can be used in seals in glass apparatus.

What were called "platinum prints" were introduced by William Willis in 1873. A web source says they were iron salts developed in oxalate, giving a pleasing color to the image. Platinum is not mentioned anywhere, except to say that palladium was used when the price of platinum rose; this would be of no importance if the prints contained no platinum.

For a while in the 19th century, counterfeiters alloyed platinum with copper to give an alloy of the same density as gold, then gold-plated the result. These were apparently very effective counterfeits, but became uneconomic with the rising price of platinum. Recently, the USSR minted 150-ruble platinum coins, and the Isle of Man mints a series of platinum coins, for the interest of speculators.

It is a sad commentary on human nature that platinum is used in jewellery, simply because it is rare. Metallic platinum has all the charm of zinc. It is not, as stated in the Handbook of Chemistry and Physics, "a beautiful silvery-white metal," as anyone who has seen a platinum crucible knows. The image in the title shows a tasteful diamond-platinum ring, and will give an idea of the appearance of platinum. It is a dull-grey metal that does not even look as good as stainless steel. Its beauty is solely in the cupidity of the beholder. If it is stamped "Platinum" it means that it is 98.5% pure or better. A hard alloy of 90 Pt 10 Ir is often used in jewellery (Ir is the most expensive platinum-group metal). Highly prized are articles of platinum and diamond--an unattractive metal combined with a common gem whose price is artificially inflated. In fact, an imperfect, badly-coloured natural gem is considered more valuable than a perfect artificial gem. A gold and garnet piece may be of very great beauty, but is less prized because it is reasonably priced. The price of platinum in 1990 was $500 per troy ounce. In 1920, it was considered 8 times more valuable than gold by weight. The least expensive of the platinum group is palladium. A palladium-gold alloy is called "white gold." The platinum metals are six very homely sisters, but useful ones when put to nonvulgar jobs.

References

J. L. Bray, Non-Ferrous Production Metallurgy, 2nd ed. (New York: John Wiley & Sons, 1947). pp. 411-418.

F. X. M. Zippe, Geschichte der Metalle (Wien: W. Braumüller, 1857). pp. 296-305.

L. Pauling, General Chemistry (New York: Dover, 1988). pp. 693-695.

W. N. Jones, Inorganic Chemistry (Philadelphia: Blakiston, 1949). pp. 709-713.

R. C. Weast, ed. Handbook of Chemistry and Physics, 56th ed. (Cleveland: CRC Press, 1975), p. B-27.

An internet search will turn up over 2,000,000 sites on platinum, most not dealing with the metal, but with the use of the name in commerce. Some are "periodic table" sites, giving the usual information. The image of the ring in the title was from Jewellers.Net, whom I thank for it. This website offers platinum jewellery for sale.


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Composed by J. B. Calvert
Created 24 February 2004
Last revised 6 March 2004