WOLLASTON AND DISCOVERY OF RHODIUM
(Published in "Rhodium Express", 1993, No. 0 and No. 1. This less available journal, edited by Yuri Varshavsky, appeared in St. Peterburg during 1993-ca. 1998)
I. The Man Who Discovered Rhodium
Dr. Wollaston was endowed with bodily senses of extraordinary acuteness, and accuracy, and with great general vigour of understanding. Trained in the discipline of the exact sciences, he had acquired a powerful command over his attention, and had habituated himself to the most rigid correctness, both of thought and of language...In Chemistry, he was distinguished by the extreme nicety and delicacy of his observations; by quickness and precision with which he marked resemblances and discriminated differences; the sagacity with which he devised experiments, and anticipated their results; and the skill with which he executed the analysis of fragments of new substances, often so minute as to be scarcely perceptible by ordinary eyes. He was remarkable, too, for the caution with which lie advanced from facts to general conclusions; a caution which, if it sometimes prevented him from reachingat once to the most sublime truths, yet rendered every step of his ascent a secure station, from which it was easy to rise to higher and more enlarged inductions.William Henry, 1829
Rhodium was discovered in 1804 by W.H. Wollaston, and it seems natural to begin the story of the element with a short account of its discoverer.William Hyde Wollaston was born in East Derenham, Norfolk, on August 6,1766, as the third son (of fifteen children) of Francis Wollaston, a priest, who was fond of astronomy - a passion inherited by his son (inter alia W.H. Wollaston was the first to describe, in 1802, the dark lines in the solar spectrum, later investigated in detail by J. Fraunhofer). He began his working life however as a medical man; after five years at Caius College, Cambridge (1782-1787) he received his M.B. in 1788 and his M.D. in 1793. In that same year he was elected F.R.S. due to influential support from Cambridge. A physician's duties, however, weighed upon him strongly enough to cause 'the mental flagellation called anxiety', as he put it in a letter to a close friend: his highly sensitive nature probably made him unable to bear the atmosphere of uncertainty inherent in the treatment of the sick. Eventually he abandoned medical practice to begin, from 1800, the independent life of a man of science, earning a living through his own researches, mainly in chemistry and metallurgy.While at Cambridge, Wollaston met, and became friends with, Smithson Tennant (1761-1815), a talented chemist and a rich man, who must have stimulated Wollaston's chemical interests. From him Wollaston learned the latest techniques of operating with small quantities of substances, something to which he was especially suited by his extraordinarily keen eyesight and dexterity. Although at first, as he himself confessed, he could not dream 'of ever becoming Tennant's equal as a chemist', after giving up medicine Wollaston did just that.It was in Cambridge that the two scientists became interested in platinum, a strange new metal with unusual properties which was an enigma and a challenge to chemistry at the end of the eighteenth century. Besides, the metal could obviously be put to practical use; the first platinum crucible had been manufactured as early as 1784. Wollaston set himself a task of resolving the platinum problem, Le. of making it available commercially.From about 1800, the two friends began to collaborate closely on the problem, Wollaston being presumably the driving force, while Tennant coupled scientific fervour with financial support. Their far-reaching commercial aims are evident from the fact that they bought nearly 200 kg of native platinum. The primary operation was to treat the ore with aqua regia, and there was probably an informal agreement on the division of labour, so that Wollaston processed solutions and Tennant investigated the insoluble residues. Both achieved outstanding successes, discovering two new metals each: Tennant found Os and Ir, and Wollaston, Pd and Rh. Wollaston's task was especially difficult because of the extremely low concentrations of Pd and Rh in primary platinum solutions. It was certainly not just good luck, but hard work in combination with great intellectual powers and unusually keen sight. Wollaston's extraordinary abilities allowed him to manipulate the minutest quantities of substances; he usually conducted analytical reactions in a drop of solution on a small piece of glass; his bottles were fitted with special stoppers for obtaining single drops.After about a year of work a new metal was found. Unwilling to reveal the results of research not yet completed, Wollaston published only an anonymous note on 'Palladium, or New Silver', an episode mentioned in many books on the history of chemistry. More than two years passed before he was ready to present his papers on the two metals discovered to the Royal Society. The second of them, rhodium, was first observed as deep red crystals in a mixture of salts obtained by evaporating a complex solution containing all three metals, Pt, Pd, and Rh.It is worth noting that in a sense the discoveries of the two new metals, as well as of osmium and iridium, were by-products of Wollaston's goal of obtaining commercial quantities of malleable platinum. On the other hand, it was precisely because Wollaston had come to know the properties of four new metals present in crude platinum that he was able to arrive at the pure metal suitable for final processing - by hot-pressing pure platinum powder. Although his invention of this last operation has been disputed, he was certainly the one who made it viable. He produced his first platinum articles in about 1805. His technology was highly advanced for the age, and won him both considerable wealth and world-wide fame.Wollaston lived and worked alone, always careful to preserve his privacy; nobody was admitted to his laboratory, set up at the back of his house. He renounced the secrecy only when he learned that he was seriously, probably terminally, ill. He gave the Bakerian lecture, 'On a Method of Rendering Platina Malleable', a mere five weeks before his death on December 22, 1828, in London.Even in such a short note some of this man's other achievements cannot be ignored. As early as 1804 he demonstrated that 'electricity and Galvanism . . . are both essentially the same' and that in Galvanism 'the oxidation of the metal is the primary cause of electric phenomena observed'. He was also a pioneer in photochemistry and studied various chemical effects of light, including the decomposition of silver chloride in invisible (UV) rays.His medical experience led him to some 'biochemical' investigations. Being of opinion that 'in any case a chemical knowledge of the effects of diseases will assist us in the cure of them', he analyzed human 'urinary calculi' and discovered, among other things, the first of the amino-acids -cysteine, called by him 'cystic oxide'.A convinced supporter of Dalton's atomic theory, he found a special case of 'the law of simple multiples' in 'super-acid and sub-acid salts'. Unfortunately his 'equivalents' (a word he had coined and introduced into chemistry) proved an impediment to the acceptance of Dalton's theory, at least in Britain.A coordination chemist would be impressed by Wollaston's 'stereo-chemical' ideas (1808). According to his views, the proportions in which atoms unite cannot be fully explained without acquiring 'a geometrical conception of their relative arrangement in all the three dimensions of solid extension'. Proceeding from the simplest hypothesis that 'their virtual extent' is spherical, he considered stable and unstable configurations; one of those where a 'stable equilibrium' occurs is a regular tetrahedron surrounding a single spherule. He confessed that 'this geometrical arrangement of the primary elements of matter is altogether conjectural', but tried to utilize these ideas to explain the forms of crystals (1810). He found support for his hypothesis about polyhedrons made up of spherical atoms in the fact that metals crystallize 'in octahedral form as they would do if their particles were spherical'. Nevertheless, for composite substances, 'any attempts to trace a general correspondence between the crystal-lographical and supposed chemical elements of bodies must, in the present state of these sciences, be premature'.Those interested in the history of scientific instrumentation should give Wollaston credit for his numerous inventions in that field; the list includes both fundamental apparatus, such as the reflective refractometer and goniometer, the main principles of which have been retained to this day, and a lot of ingenious smaller items, from a 'pocket blow pipe' to platinum filaments as thin as 0.001 mm, 'excellent for applying to the eye-pieces of astronomical instruments'.Leaving aside Wollaston's work in astronomy, optics, crystallography, and botany, we might conclude with the words of his contemporaries: 'To Dr Wollaston every part of science seemed equally familiar'... 'He died as he had lived, self-possessed, stern, and silent.'The above text is based on essays devoted to W.H. Wollaston in the Dictionary of Scientific Biography, vol. 14, the Dictionary of National Biography, vol. 62, and on sources quoted there, as well as on Wollaston's own works.
II. Discovery of Rhodium
The rhodium discoverer, W. H. Wollaston has been characterized in the preceding note; here is a short description of the discovery itself which dates formally by the day, 24 June 1804, when Wollaston read his paper 'On a new Metal, found in Crude Platina' before the Royal Society.It should be remembered that those were times when science remained, and especially in Britain, essentially an affair of gentlemen interested in it; most of scientific work was done in home laboratories. As to chemistry, the times were fruitful; Lavoisier's chemical revolution offered a new level of general knowledge for special discoveries. Both these background features manifest themselves in the story of the discovery.As has been mentioned in the Note I, rhodium was found as a byproduct in the course of working out means to obtain pure platinum on a commercial scale. Wollaston's technology included dissolution of the crude metal in a mixture of nitric and muriatic (HC1) acids and processing the acid solution by alternate precipitation and dissolution operations, as shows the following scheme:metallic material + aqua regia → solution ;solution + NH4Cl → precipitate of ammonium chloroplatinate + mother liquor;mother liquor + Fe (or Zn) → metallic residue;metallic residue + aqua regia → solutionetcIt was already known that sal ammoniac gave platinum containing precipitate with such a solution, leaving possible admixtures, among them gold, in the mother liquor. To recover the rest of platinum from the liquor, Wollaston treated it by iron and obtained 'the first metallicresidue' that could be treated by the same method of dissolving in aqua regia and precipitating ammonium chloroplatinate by sal ammoniac. The solution left after precipitation was used for obtaining 'the second metallic residue' which was first presumed to be suitable for the same processing by aqua regia.'As I was not at first prepared to expect any new bodies', Wollaston wrote, 'I proceeded to treat a second precipitate, as the former, by solution and precipitation. But soon I observed appearances which I could not explain by supposition of the presence of any known bodies, and was led to form conjectures of future discoveries, which subsequent inquiry has fully confirmed'.It became obvious that 'the second metallic residue' contained unknown substances, so Wollaston returned to the solution wherefrom the residue had been obtained. Perhaps at this stage of his search he found, basing on Tennant's results on Os and Ir, the optimal process for dissolving crude platinum 'leaving as much as possible of the shining powder' that is minerals containing Os and Ir; thus he could prepare the solution nearly free from indium. A portion of such solution he ' suffered... to evaporate without heat and obtained a mixture of various crystals very different from each other in form and colour. From these, I selected for examination some that were of a deep red colour, partly in thin plates adhering to the sides of the vessel, and partly in the form of square prisms having a rectangular termination.A portion of these crystals being heated in a small tube, yielded sal ammoniac by sublimation, and left a black residuum, which, by a greater heat, acquired a brilliant metallic whiteness, but could not be fused under the blowpipe'.That was rhodium, but the task remained of obtaining it in a quantity large enough to characterize it as a new metal different from the others.This stage of Wollaston's research remains undisclosed; he but mentioned 'unsuccessful experiments made upon the properties of this metal, in hopes of discovering means by which its separation from platina might be effected'. We can think, nevertheless, his first attempt was to go by a conventional way of treating 'the second metallic residue' by aqua regia and processing the acid solution in alternate precipitating, and dissolving operations according the above scheme. This way lead to nowhere as 'the third metallic residue' was stable against almost any reagent; we can understand now it was a resistant alloy of rhodium with platinum and palladium.The advance was probably attained - there is a hint in one of his papers - when he found that caustic soda could be a good neutralizer for acidic Pt-containing solutions; thus, he learned to prepare sodium chloroplatinate and corresponding palladium and rhodium salts.The effective way to pure rhodium consisted in washing the first metallic residue by very diluted nitric acid to remove Cu and Pb and then dissolving it in moderately diluted aqua regia; about ten percent of the metal was left undissolved, probably that containing Ir. A slight excess of sodium chloride was added to the solution, and the mixture was slowly evaporated to dryness. 'The residuum, which I had found, from prior experiments, would consist of the soda-muriates of platina, of palladium and of rhodium, was washed repeatedly with small quantities of alcohol, till it came off nearly colourless. There remained a triple salt of rhodium, which by these means is freed from all metallic impurities'.For this study Wollaston used 1000 grains of crude platinum and obtained as the pure salt about 4 grains, that is less than 0.5 g of rhodium; we can but admire his extraordinary skill, since this quantity was for him enough to determine the main properties of the element. Having recrystallized the salt and found it 'formed rhomboidal crystals of which the acute angle was about 75°' he dissolved the salt in water, and one half of the solution was used for testing its reactions with acids, salts, alkali, and metals, whereas the other half had been transformed into metallic rhodium to test properties of the simple body. Some examples of these tests would be instructive.From the solution, 'pure alkalis precipitated a yellow oxide, soluble in excess of alkali; and also soluble in every acid I have tried'; a drop of the oxide solution in nitric acid 'being placed upon pure silver, occasioned no stain. On the surface of mercury a metallic film was precipitated, but did not appear to amalgamate'.Rhodium metal was obtained by substitution reaction with zinc and tested, too, as to its ability to alloy with Au, Ag, Bi, Cu, and Pb. Simultaneously its density was estimated, as follows:'The specific gravity of rhodium, as far as could be ascertained by trial on so small quantities, seemed to exceed 11. That of an alloy consisting of 1 part of rhodium and about 2 parts of lead, was 11.3; which is so nearly that of lead itself, that each part of this compound may be considered as having about the same specific gravity'. Thus the long story of rhodium began.