The Platinum Metals in the Periodic System

“A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt 18 The Platinum Metals in the Periodic System “The six known platiniferous metals, from a certain point of view, may be rightly con sidered as forming a separate and well-defined group.” K \ K I. k \KI.<)\ 1(11 M. M S. I860 The gradual increase in the number of elements being discovered and isolated during the early part of the nineteenth century led to a number of attempts at their classification. As early as 1816 the great physicist André Marie Ampère (1775—1836), Professor of Mathematics and Mechanics at the École Polytechni que but at this stage of his career very interested in chemistry and in the whole concept of classification, put forward a scheme of ordering the elements that would bring out “the most numerous and essential analogies and be to chemistry what the natural methods are to botany and zoology” (1). All the ele ments then known were classified into five groups, one of these being called the “Chrysides”, derived from the Greek word for gold, and including palladium, platinum, gold, iridium and rhodium. Osmium, however, he grouped with titanium. Some of the similarities between the platinum metals were thus recog nised at this early date, but Ampère’s method contained no numerical concept. Dobereiner’s Triads That such a quantitative component was necessary was first recognised by J. W. Dôbereiner who noticed in 1817 that the molecular weights for calcium oxide, strontium oxide and barium oxide formed a regular series or triad with that of strontium being the arithmetic mean of the other two. Twelve years later he published his paper on the Classification of the Elements in Poggendorff’s Annalen der Physik und Chemie, curiously immediately following an abridged translation of Wollaston’s paper on the production of malleable platinum given to the Royal Society in 1828 (2). Expressing first his great interest in the atomic weights of Berzelius, Dôbereiner again showed that when the elements were arranged in groups of three resembling each other chemically the atomic weight 333 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt of the middle one was the mean of the other two. After discussing the halogens, the alkaline earths and the group of sulphur, selenium and tellurium among others, he turned to the similarities between iron, nickel and cobalt and then to the platinum metals: “The interesting series of analogous metals that occur in native Platina, namely Platinum, Palladium, Rhodium, Iridium, Osmium and Pluran, fall according to their specific and atomic weights into two groups. To the first belong Platinum, Iridium and Osmium, to the other Palladium, Rhodium and Pluran, which last corresponds with osmium, as rhodium does with iridium and palladium with platinum”. His Pluran, to which he referred in a footnote (“The existence of Pluran is however somewhat doubtful”) was one of the supposed elements discovered by Osann in 1827 in native platinum from the Urals and given that name from the two initial letters of Platinum and Urals. Only in 1844 was the true sixth member of the group, ruthenium, discovered by Klaus, as recorded earlier in Chapter 12. Very little was heard of Dobereiner’s triads. Not until 1853 in fact did any serious notice appear to have been taken of them, but in that year John Hall Gladstone (1827-1902), a former student of Thomas Graham and Liebig, then a lecturer in chemistry at St. Thomas’ Hospital in London and later Fullerian Professor of Chemistry at the Royal Institution, published a paper in The Philoso phical Magazine, On the Relations between the Atomic Weights of Analogous Elements. In the course of this he commented: “Who has failed to remark that the platinum group has double the atomic weights of the palladium group” (3). Four years later Ernst Lenssen, one of the young assistants in Professor Fresenius’ analytical laboratory in Wiesbaden, also speculated on the triads, grouping the elements by their chemical and physical characteristics and even by the colour of their oxides (4) included one consisting of palladium, ruthenium and rhodium (in that order) and another comprising osmium, platinum and iridium, again incorrectly arranged by their then atomic weights, or rather the equivalents, that he employed. The Schemes of Odling and Newlands A more comprehensive scheme for the classification of the elements was also published in 1857 by William Odling, at that time Professor of Chemistry at Guy’s Hospital in London. In this he arranged forty-nine elements into thirteen groups of which the last contained the platinum metals and gold. He wrote: “The propriety of associating gold with the platinum group is very questionable. Palladium appears to present a relation of parity with rhodium and ruthenium, platinum with iridium and possibly with osmium, though indeed many osmic reac tions are altogether special” (5). During their work on the platinum metals described in Chapter 15, Deville 334 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt William Odling 1829-1921 The son of a London doctor. Odling entered Guv's Hospital to study medicine and chemistry, becoming a demonstrator in the latter subject in 1850. After a period of study under Gerhardt in Paris he was appointed a lecturer and then Professor of Practical Chemistry in 1856. The first of his several papers on the classifica tion of the elements appeared a year later. In 1859 he was elected a Fellow of the Royal Society and in the follow ing year he attended the Karlsruhe Congress. In 1868 he succeeded Faraday as Fullerian Professor of Chemistry at the Royal Institution, moving to Oxford as Waynflete Professor of Chemistry in 1872. In that year he married the daughter of Alfred Smee, the surgeon to the Bank of Fngland. whose work on the electroplating of the platinum metals has been described in Chapter 1 1 and Debray also emphasised the resemblances between these elements. In 1859 they wrote: “The family of the platinum metals has a particular character, completely apart from the more or less natural families formed by the other metals. It is true that they are not entirely analogous on every point, but they have their own character, a common appearance that separates them, while from the point of view of a rational classification one should separate them from the diverse families of elements” (6). Odling returned to this subject later, revising and extending his classification in 1861 and again in 1864, but in the interval Karl Klaus presented a paper on the platinum metals to the Academy of Science in St. Petersburg in which he also recognised them as a distinct group of elements (7): “These metals may be arranged in two superimposed series, the superior horizontal which I designate the principal series because the metals which constitute it predominate in the various platinum ores. This series is characterised equally by an elevated atomic weight and by almost the same specific gravity . The second horizontal series contains the remainder of the platiniferous metals, which also possess almost identical atomic and specific weights, but have in this respect but half the quantities of the principal series”. 335 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt John Alexander Reina Newlands 1 8 3 7 -1 8 9 8 Horn in London of a Scottish father and an Italian mother. Newlands joined Garibaldi's revolutionary move ment in I860, returning in 1863 to study under Hofmann at the Royal College of Chemistry, later becoming a teacher of chemistry and then in 1868 chief chemist to a sugar refinery. His numerous papers on the classification of the elements were received with scepticism, but his "Law of Octaves'* was an important if defective forerun ner of Mendeleev *s Periodic System Klaus went on to show that the metals vertically above one another in his table resembled each other, the pairs ruthenium and osmium, rhodium and iridium, and palladium and platinum having identical reactions in the formation of their compounds. Klaus’s Horizontal Series Principal Series Osmium Iridium Platinum Secondary Series Ruthenium Rhodium Palladium It will be seen that Klaus had his metals in the correct order as established much later on. In his famous Lecture on Platinum, given to the Royal Institution in February 1861, Faraday clearly accepted these conclusions and quoted Klaus almost verbatim (8). Odling’s revised and enlarged scheme of 1861 included fifty-seven elements arranged in seventeen groups, the last two being very similar to those of Klaus (9), 336 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt and then in 1863 the first of a long series of papers by J. A. R. Newlands appeared in Chemical News, followed by several more in the next three years (10). In his final table of the elements, arranged numerically in the order of their atomic weights, he pointed out that “the numbers of analogous elements generally differ either by seven or by some mul tiple of seven; in other words members of the same group stand to each other in the same relation as the extremities of one or more octaves in music . This relationship I propose to provisionally term the Law of Octaves”. Newlands was uncertain how to deal with the platinum metals and he achieved his arithmetical symmetry only by assigning one number to each of the pairs rhodium and ruthenium in the earlier series and to platinum and iridium in the later, while he placed osmium alongside tellurium in another group.

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