Two men, two centuries, four metals

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Bill Griffith introduces us to the colourful characters behind the discovery of four rare metals 200 years ago

The years 2002-2005 mark the bicentennials of the discovery of four rare members of the platinum group metals: rhodium (1802-3), osmium and iridium (1804) and palladium (1802, finally announced in 1805).

Two remarkable British chemists, William Hyde Wollaston (1766-1828) and Smithson Tennant (1761-1815) discovered and isolated the metals in London, UK. They were firm friends and made an interesting pair. Wollaston was an energetic and brilliant scientist; although Tennant was rather lazy he was just as bright as Wollaston in his own way. Theirs was to be a highly productive partnership.

These elements are industrially and chemically very important, both as metals (particularly rhodium, palladium and iridium) but also as their compounds for catalysis and many other uses. Indeed, Nobel prizes are associated with them - Noyori and Knowles for work on rhodium catalysis and Sharpless for osmium catalysis in 2001 and by another London chemist, Wilkinson, in 1973.

Wollaston and Tennant first met at Cambridge University in 1783 where they had been studying medicine. By 1795 both had, for different reasons, become disillusioned with the subject.

Wollaston, the more outgoing of the two and not a rich man, wanted to make his fortune, and thought that studying the metallurgy of platinum might be a way to do this. Tennant had a large inheritance from his parents and so was able to help his friend financially.

They bought, on Christmas Eve in 1800, 5 959 Troy ounces (a mass unit used to measure precious metals) of alluvial platinum ore, ’platina’ from the Spanish colony of Nueva Granada - now Colombia. This platina had been illegally smuggled out of that country via Jamaica, for the then considerable cost of ?795, money that was principally put up by Tennant. Wollaston devised a method, which he kept secret until the year of his death, for rendering platinum malleable, which made him a substantial sum of money, leaving ?30 000 in his will.

Platinum had been known since at least the 15th century, when it had been discovered by the indigenous population of what is now Colombia and Ecuador. The Spanish conquistadors found platina in the river beds of Colombia, and for a long time it was regarded as worthless because it was thought to debase silver. However, in the mid 18th century its beauty and remarkable resistance to corrosion was recognised and it became much prized.

Recognising this, the Spanish banned its export from the only mines then known, those in South America, and of course this made it even more desirable, so that an extensive and profitable smuggling trade in the metal and its ores developed. There were, however, problems in fabricating objects from platina due to impurities, particularly iridium, in the metal which made it brittle.

In the course of his platinum work Wollaston noted - as had some French chemists - that platina was not entirely soluble in aqua regia, a black residue always remained. He persuaded Tennant to draw what might be regarded as the chemical short straw and work on this intractable material. Wollaston worked on the aqua regia soluble portion, which he knew contained platinum, but also, he suspected, at least one other new element.

Before describing their work something should be said about these two remarkable scientists.

Wollaston was born in East Dereham, Norfolk, on 6 August 1766 and studied medicine at Gonville and Caius College, Cambridge, where he first met Tennant. After a brief medical career, in 1789 in Ivy Bridge Street, near the Strand in London, he began to produce some 50 papers on an astonishing variety of topics, including aspects of chemistry, optics, physiology, pathology, crystallography, electricity, astronomy, mechanics, botany and mineralogy.

In mineralogy his name lives on in the minerals wollastonite and pseudowollastonite as it does in his endowment in 1828 of the Wollaston medal, a sort of Nobel prize for geology, cast in solid palladium and still awarded annually.

Prominent among his many innovations was a camera lucida, the development of the equivalent weights concept and the invention of a reflecting goniometer to accurately measure the positions of crystal faces in minerals.

He was one of the most influential scientists of his time, and became a fellow of the Royal Society (FRS) in 1794. In 1802 he was awarded the Copley medal, then, and still now, the most prestigious award of the Royal Society. He at last became rich, mainly from his process for rendering platinum malleable. It was said of him that ’he was...a just and most honourable man, candid, open and free from envy with perfect straightforwardness; [he possessed] a relish for acquiring money with a generosity in parting with it when it could be generously bestowed’.

Tennant was in his quieter way as outstanding a person as Wollaston. He was born on 30 November, 1761, in Finkle Street in Selby, near the Abbey where he was baptised. His father died when Tennant was only 10, and when he was 20 he also lost his mother to a tragic horse riding accident which was to be uncannily repeated for Tennant himself some 30 years later.

In 1781 he went to Edinburgh University where he attended lectures by Joseph Black, then in 1782 went to Christ’s College, Cambridge, to study chemistry and botany. By this time he had received a considerable inheritance from his parents and moved to Emmanuel College, Cambridge. Here he studied medicine until 1796, by which time he and Wollaston had become friends.

He moved to London to a house in Garden Court in the Temple, off the Strand, where he isolated iridium and osmium. Although nowhere near as prolific as Wollaston he showed in some expensive experiments involving the combustion of diamond, that the latter and graphite were both allotropes of carbon. He also developed the double distillation technique, established the constitution of carbon dioxide and the natures of calcium phosphate, emery and corundum, and a number of alkaline earth carbonates.

It was said of Tennant that ’his appearance, notwithstanding some singularity of manners, and great negligence of dress, was on the whole striking and agreeable. [His] rooms exhibited a strange, disorderly appearance of books, papers and implements of chemistry, piled in heaps and thrown in confusion together...when at a loss for a piece of linen to filter some of his preparations he never scrupled taking a part of a cambric handkerchief, or cutting off a piece of shirt.’

Tennant became an FRS in 1785 at the remarkably early age of 24, earlier even than Wollaston, and received the Copley medal in 1804 for his discovery of osmium and iridium. In his congratulatory speech for the award of this medal Sir Joseph Banks, president of the Royal Society, (an event at which Tennant - perhaps typically - was not present) exhorted Tennant to make greater efforts in his chemistry.

He died in 1815 in a riding accident. He had given up equestrianism after his mother’s fatal accident in 1781, but some 15 years later he had been advised to take it up again to improve his health, and thereafter rode daily (famously on one occasion leaving a diamond burning in his work on its allotropy with graphite to take his daily ride).

On 22 February 1815 he rode over a wooden bridge near Boulogne; a bolt securing it was weak, it snapped, and the bridge collapsed. He was thrown from his horse, which fell on top of him in the deep ditch beneath, and he died an hour later from a fractured skull.

Although the concept existed earlier, it was Lavoisier’s precise definition in 1789 of the element as ’the last point that analysis can reach’ - ie, a material that can not be broken down chemically into simpler constituents - that led to an explosion in the discovery of many new elements in the 19th and 20th centuries.

Wollaston recognised in 1802 that at least one unknown element was present in platina’s soluble aqua regia extract, and in 1803 he isolated rose-red crystals of a chloro complex (probably Na₃[RhCl₆].nH₂O). He published this in the Philosophical Transactions of the Royal Society in 1804, naming the new metal rhodium after the Greek rhodos (???o?), a rose, from the chloro compounds’ colour. To get pure rhodium, he reduced its trichloride with copper.

Also in 1802 he isolated a material called ceresium, after the recently discovered asteroid Ceres; later renaming it palladium, after another asteroid, Pallas. He then adopted an extraordinary procedure unparalleled in the long and varied history of elemental discoveries.

Instead of publishing his work in one of the learned journals he advertised samples of the element as ’Palladium, or New Silver.a new Noble Metal’ for sale at five shillings, half a guinea or a guinea. This handbill was displayed in the window of 26 Gerrard Street, Soho, the shop owned by Mrs Forster, whose husband Jacob obtained platinum supplies from South America for supply to the UK via Jamaica.

The entire stock of palladium was bought up by Richard Chenevix, an Irish chemist living in London, who after two weeks work on it concluded that it was not an element but was a platinum-mercury alloy. A year later Chenevix published his finding in Philosophical Transactions, and seemingly on the basis of this was awarded the Copley medal in the same year.

An anonymous letter was published in December 1803, almost certainly by Wollaston, offering a reward of ?20 to anyone who could ’make’ palladium from these metals in public. The challenge was not taken up and in 1805 Wollaston at last published full details of his isolation of palladium. Despite this contretemps he and Chenevix remained good friends and often dined together; Chenevix left chemistry after this episode and became a successful playwright.

Tennant, it will be remembered, had been left with the intractable black material when platina was dissolved in aqua regia. He had not been as idle in this matter as he was often said to be. He found that alkaline fusion at red heat of the black powder gave a yellow solution with a pungent smell: this smell greatly increased on acidification when a vapour came off which ’stains the skin of a dark colour which can not be effaced...[it has] a pungent and penetrating smell...from the extrication of a very volatile metal oxide...this smell is one of its most distinguishing characters, I should on that account incline to call the metal osmium.’

The name is derived from the Greek word osme (o???), smell. Reducing an aqueous solution of the oxide with copper, silver or zinc gave him black osmium metal.

Still working with a solution of the black residue from platina in aqua regia, Tennant, by using slightly different reactions, isolated red crystals of a new material (probably Na₂[IrCl₆].nH₂O).

The ammonium salt of these crystals when heated gave the new metal iridium, named after the Latin word iris, a rainbow. Tennant wrote that iridium ’appeared of a white colour, and was not capable of being melted, by any degree of heat I could apply...I should incline to call this metal iridium, from the striking variety of colours which it gives, while dissolving in marine acid’.

Some 15 years after Lavoisier’s definition of the element four new metals were discovered in London by two friends in memorable partnership, both personal and financial. These important metals went on to find uses in metallurgy, industrial and academic chemical catalysis, and there are also medical applications for osmium (in tissue microscopy) and palladium (for making dental plates).

The world at large, it seems, should be grateful that these friends chose not to pursue their trained professions but followed their chemical instincts.

Bill Griffith is professor of inorganic chemistry, department of chemistry, Imperial College London, UK