The Platinum Metals in Early Nineteenth Century Chemistry
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“A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt 14 The Platinum Metals in Early Nineteenth Century Chemistry “Only one who himself has worked with the chemistry of the platinum metals can fully understand and evaluate the difficulties Berzelius had to overcome. He not only deter mined the atomic weights of platinum, palladium, rhodium, iridium and osmium but also investigated a large number of the most important compounds of these metals. ” HENRIK GUSTAVSODERBAUM The first quarter of the nineteenth century saw the transformation of chemistry into an exact science. Following the great contributions of Lavoisier, his appreciation of the role of oxygen, his use of the balance and his introduction of the new nomenclature, a second major advance was made with the establish ment of the quantitative basis of chemical combination. It is possible here only to summarise very briefly the development of this numerical approach and to set the scene for the great change brought about in inorganic chemistry and its impact on the study of the platinum metals. This began with the rather obscure and somewhat neglected writings of Jeremias Benjamin Richter (1762—1807), the originator of the term “stoichiometry” and of the concept of equivalent weights of bases and acids. Richter studied under the great German philosopher Immanuel Kant at Königsberg and his choice of subject for his doctoral thesis, “The Use of Mathematics in Chemistry”, was most probably inspired by Kant. His book, published in 1792, was prefaced with a quotation in Greek from the Apocrypha: “Thou hast ordered all things in measure and number and weight.” (1) In 1799 Joseph Louis Proust, whose work on platinum in the Royal Laboratory in Madrid has been described in Chapter 6, put forward his Law of Constant Composition, a concept that he later summarised in the phrase: 253 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt John Dalton 1766-1844 The concept of atomic weight introduced by Dalton together with his laws of constant and multiple proportions in chemical combination gave a great impetus to inorganic and analytical chemistry and prompted Berzelius to embark on his systematic investigations “A compound is a substance to which Nature assigns fixed proportions; it is in a word a being which she never creates even in the hands of man, except with the aid of a balance. ” (2) This brought Proust into a long controversy with Claude Louis Berthollet (1748-1822), who maintained that fixed proportions were exceptional rather than a general rule, but the atomic theory propounded by John Dalton, incorporating his doctrine that the elements were composed of atoms of constant weight and that compounds were formed by their union in simple numerical proportions, put an end to this argument. O n October 21st, 1803 Dalton read a paper to the Manchester Literary and Philosophical Society - published only later in 1805 - in which he concluded: “An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new: I have lately been prosecuting this enquiry with remarkable success.” (3) Dalton began his determination of atomic weights with the gases, but by 1804 he had turned to the metals and the calculation of their atomic weights from the published analyses of other workers, selecting the values which disagreed least with the often inconsistent analyses of their oxides and adding new results as further literature appeared. His views on platinum, about whose atomic weight he remained uncertain, may be seen in the reproduction here of a passage from “A New System of Chemical Philosophy” published between 1808 and 1810. Dalton also introduced his “arbitrary marks” or symbols to represent the elements. 254 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt Dalton began publishing his ‘New System of Chemical Philosophy' in 1808. the second part following two years later. This extract The weight of the ultimate particle of pla from Part II shows his thoughts on platinum ting cannot be ascertained from the data we at that time. He also devised a number of have at present: from its combination with symbols for the elements, three of them oxygen, it should seem to be about 100; but, illustrated here judging from its great specific gravity, one would be inclined to think it must be more. Indeed the proportion of oxygen in the oxides of platina cannot be considered as ascertained. Platina is chiefly used for chemical pur poses ; in consequence of its infusibility, and the difficulty of oxidizing it, crucibles and other utensils are made of it, in preference to every other metal. Platina wires are extremely useful in electric and galvanic researches, for like reasons. At first Dalton’s views made very little impression, but in 1807 Thomas Thomson, Professor of Chemistry in the University of Edinburgh and the founder of the Annals of Philosophy, published the third edition of his book, A System of Chemistry, and included in this an account of the atomic theory based upon a conversation he had had with Dalton in Manchester in 1804. A year after this Thomson read a paper to the Royal Society in which he confirmed the law of multiple proportions by the analysis of some neutral compounds (4). Immediately following this a paper from W. H. Wollaston not only gave further support to Dalton but disclosed that he had himself been turning his mind in the same direction: “I thought it not unlikely that this law might obtain generally . and it was my design to have pursued the subject with the hope of discovering the cause, to which so singular a relation might be ascribed. But since the publication of Mr. Dalton’s theory of chemical combustion the enquiry which I had designed appears to be superfluous as all the facts that I had observed are but particular instances of the more general observations of Mr. Dalton that in all cases the simple elements of bodies are disposed to unite atom to atom singly or if either is in excess, it exceeds by a ratio to be expressed by some simple multiple of the number of its atoms. ” (5) The Massive Researches of Berzelius It was this paper of Wollaston’s, reproduced in Nicholson’s Journal in November 1808, that first gave Berzelius any indication of Dalton’s theory as the Napoleonic blockage had prevented his receiving earlier publications from England. He was in the course of preparing a text book of chemistry in Swedish and had been impressed by his reading of Richter’s work on stoichiometry when Wollaston’s paper reached him and he realised at once the enormous 255 © 1982 Johnson Matthey “A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt significance of Dalton’s generalisation. Immediately Berzerlius set himself the task of determining “the definite and simple proportions in which the consti tuent parts of inorganic substances are united with each other”, proposing to establish the atomic weights of all the elements then known and to this end to improve the infant techniques of quantitative analysis. By 1814 he was able to publish in Thomson’s Annals of Philosophy (translated much later into German) a tentative table of atomic weights, based on oxygen taken as 100, including those for rhodium, platinum and palladium, with all the evidence on which he had based his calculations, while at the same time, becom ing wearied with the need to refer repeatedly to the elements by their full names, he proposed the use of our now familiar symbols based upon the initial letter of their Latin names (or upon this followed by a second letter from the body of the name) to replace the rather cumbersome circular symbols devised by Dalton. Thus we acquired the symbol Pt for platinum, while his first suggestion for palladium PI, was later revised to Pa to avoid confusion with platinum and finally to Pd. Similarly he proposed I for iridium later revised to Ir, and R, changed to Rh, for rhodium. (6) In 1818 a fuller version of the atomic weights of almost all the elements ap peared but still excluding iridium and osmium (7). These were again all based upon the atomic weight of oxygen taken as 100 and were reasonably close to modern values except that several were in error, being twice their real value because of his assumption that most oxides had a simple formula, consisting of one atom of metal and two of oxygen, an error that he later corrected. This amazing achievement was surpassed some ten years later in so far as one of his favourite pursuits, the chemistry of the platinum metals, was concer ned. Reference was made on page 240 to Count Kankrin sending to Berzelius two small samples of native platinum from the Urals together with a personal note asking him to analyse them and to be good enough to report his results. He had already noticed that the early results obtained in the Russian mining laboratory and in Paris by Laugier showed marked differences and he realised that there was a great deal still to be established about the means of separating the five metals of the platinum group so far known. Berzelius therefore embarked upon a thorough examination of these specimens. He began with rhodium and quickly found that his earlier work (6) carried out on a sample presented to him by Wollaston, contained errors in his calculation of its atomic weight. This he proceeded to correct, and he then went on to determine the atomic weights of palladium, iridium, platinum and osmium.