The Platinum Metals in the Periodic System · 2014-06-24 · The Platinum Metals in the Periodic...

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The Platinum Metals in the Periodic System

“The six known platiniferous m etals , from a certain point o f view, m ay be rightly considered as form ing a separate and well-defined group . ”

K \ K I . k \ K I . < ) \ 1(11 M . M S. I 8 6 0

The gradual increase in the num ber of elements being discovered and isolated during the early part of the nineteenth century led to a num ber of attempts at their classification. As early as 1816 the great physicist André M arie Ampère (1775—1836), Professor of M athem atics and Mechanics at the École Polytechnique 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 elements 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 p latinum metals were thus recognised at this early date, but A m père’s method contained no numerical concept.

D o b e re in er ’s TriadsT hat 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 W ollaston’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

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“A History of Platinum and its Allied Metals”, by Donald McDonald and Leslie B. Hunt

© 1982 Johnson Matthey

of the middle one was the m ean of the other two. After discussing the halogens, the alkaline earths and the g roup of sulphur, selenium and tellurium among others, he turned to the similarities between iron, nickel and cobalt and then to the p latinum 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” ) w as one of the supposed elements discovered by O sann in 1827 in native p la tinum from the Urals and given that nam e from the two initial letters of Platinum a n d Urals. Only in 1844 was the true sixth member of the group, ru thenium , discovered by Klaus, as recorded earlier in C hap ter 12.

Very little was heard of D obere iner’s triads. Not until 1853 in fact did any serious notice appear to have been taken of them, but in that year Jo h n Hall Gladstone (1827-1902), a former student of Thom as G raham and Liebig, then a lecturer in chemistry at St. T h o m a s ’ Hospital in London and later Fullerian Professor of Chemistry at the Royal Institution, published a paper in The Philosophical M agazine, O n the Relations between the Atomic Weights of Analogous Elements. In the course of this he c o m m en ted :

“ 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) a n d another comprising osmium, platinum and iridium, again incorrectly a rranged by their then atomic weights, or rather the equivalents, that he employed.

T h e Schem es o f O d lin g and N e w la n d sA more comprehensive scheme for the classification of the elements was also published in 1857 by William Odling, at that time Professor of Chemistry at G u y ’s Hospital in London. In this he arranged forty-nine elements into thirteen groups of which the last contained the p latinum 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 reactions are altogether special” (5).

During their work on the p la tinum metals described in C hapter 15, Deville

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W illiam O dling1829-1921

T h e son of a L o n d o n do c to r . O d l ing e n te r e d G u v 's H o s p i ta l to s tudy m e d ic in e a n d c h e m is t ry , b e c o m in g a d e m o n s t r a to r in th e l a t te r su b je c t in 1850. A f te r a p e r io d of s tudy u n d e r G e r h a r d t in P a r i s he w as a p p o in t e d a le c tu r e r a n d th e n P ro fe s s o r of P ra c t ic a l C h e m is t ry in 1856. T h e first of his severa l p a p e r s on th e c lassif ication of th e e le m e n ts a p p e a r e d a yea r la ter . In 1859 he was e lec ted a Fellow of the R oya l Society a n d in th e fo l low ing y ea r he a t t e n d e d the K a r l s r u h e Congress . In 1868 he su c c e e d e d F a r a d a y as F u l l e r i a n P ro fe s so r of C h e m is t ry at th e Roya l In s t i tu t io n , m oving to O x fo rd as W a y n f le te P ro fes so r of C h e m is t ry in 1872. In tha t y e a r he m a r r i e d th e d a u g h t e r of A lf red Sm ee , the s u rg e o n to the B a n k of F n g l a n d . w h o s e w o rk o n th e e lec t ro p la t in g of th e p l a t in u m m e ta ls has b e e n d e s c r ib e d in C h a p te r 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”.

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J o h n A lex a n d e r R e ina N ew lands1 8 3 7 - 1 8 9 8

H orn in L o n d o n of a Sco t t ish f a th e r a n d a n I ta l ia n m o th e r . N e w la n d s jo in e d G a r i b a l d i ' s rev o lu t io n a ry m o v e m e n t in I8 6 0 , r e tu rn in g in 1863 to s tudy u n d e r H o f m a n n a t t h e Royal College o f C h e m is t ry , la te r b e c o m in g a t e a c h e r of ch em is try a n d th e n in 1868 c h ie f c h e m is t to a s u g a r r e f inery . His n u m e r o u s p a p e r s on th e c lassif ica t ion of th e e le m e n ts w e re rece ived with sc e p t ic ism , bu t h is "L aw of Octaves '* w as a n i m p o r t a n t if d e f e c t iv e f o r e r u n n e r o f M ende leev *s P e r io d ic System

Klaus went on to show that the metals vertically above one another in his table resembled each other, the pairs ru then ium and osmium, rhodium and iridium, and palladium and p la t inum having identical reactions in the formation of their compounds.

K la u s ’s H o riz o n ta l Series

Principal Series Osmium Iridium PlatinumSecondary Series Ruthenium Rhodium Palladium

It will be seen that Klaus h ad 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).

O dling’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),

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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 multiple 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 p latinum metals and he achieved his arithmetical symmetry only by assigning one num ber to each of the pairs rhodium and ruthenium in the earlier series and to p latinum and iridium in the later, while he placed osmium alongside tellurium in another group. He also predicted that another element should exist between iridium and rhodium and another between palladium and platinum. Unfortunately for Newlands the Chemical Society declined to publish his paper, Odling, later the President, explaining that they “ made it a rule not to publish papers of a purely theoretical na tu re” .

Newlands continued to interest himself in arranging the elements so as to emphasise the family relationships, assigning consecutive atomic or “ o rd ina l” numbers to them and leaving blanks for elements as yet to be discovered, and in a small book he produced in 1884 he claimed with some justification to have been the first to publish a list of the elements in the order of their atomic weights and to have described the periodic law (11).

Meanwhile in 1864 Odling, probably unaware of New lands’ later publication, contributed a paper “ O n the Proportional Num bers of the Elem ents” to the Quarterly Journal o f Science in which he listed sixty-one elements in increasing order of atomic weight (12). In this he gave rhodium, ru thenium and palladium in that order and then platinum, iridium and osmium.

The K arlsruhe CongressThe accuracy of the atomic weights so far determined was in grave doubt and the subject of much controversy. Some values were only one half of their now established figures while some were twice as great. Friedrich Wohler had complained that “ the confusion can be tolerated no longer” . From this state of chaos order was restored by the well-known paper from Stanislao Cannizzaro, (1826-1910), Professor of Chemistry at Genoa, given at the Karlsruhe Congress in 1860 (13). This famous gathering of more than 120 chemists, the first international scientific conference, was proposed by August Kekule and some of his colleagues to secure more precise definitions of the concepts of atoms and molecules and to bring uniformity into the values of atomic weights. William Odling, one of the very few who had already read C annizzaro’s paper was among the signatories calling this meeting and he was present for the discussion.

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Earlier he had studied chemistry in Paris under Gerhardt, had translated Laurents’ M ethode de Chimie into English in 1855, and supported their unitary theory, the atomic weight to b e taken as the smallest quantity of an element present in the molecular weight of any of their compounds. But it was C annizzaro, whose paper, a reprin t of an earlier contribution given in 1858, was distributed by his colleague Angelo Pavesi, Professor of Chemistry at Pavia, after the close of the meeting that settled the whole problem of atomic weights based upon the earlier proposals of G erhard t and Laurent. Half a century of confusion was cleared up and it was now possible to ascribe the correct atomic weights to all the known elements.

Lothar M eyer and M e n d e le e vAmong those attending the K arlsruhe Congress was Julius Lothar Meyer, at that time Professor of Chem istry at Breslau, and he recorded later how on reading Cannizzaro’s paper during his return journey “ the scales fell from my eyes, doubt vanished, and was replaced by a feeling of peaceful certain ty” . W hen preparing a text-book M eyer was thus able to take account of the numerical relationships between the elements and in his Die M odernen

Ju liu s L o th a r Meyer1830-1895

A n a t iv e of th e sm a ll tow n of V are l n e a r O l d e n b u r g in no r th G e r m a n y , M e v e r was fa r f ro m ro b u s t as a child a n d was given a n o u t -d o o r e d u c a t io n u n d e r th e head g a r d e n e r to th e D u k e of O ld e n b u rg . H e b eg an his h ig h e r e d u c a tion a t Z ü r ich a n d th e n t r a n s f e r r e d to W ü r z b u r g . A f te r g r a d u a t io n he w ent to H e id e lb e r g to s tu d y u n d e r B u n s en a n d K i r c h h o f f an d in 1864 p u b l i s h e d his book on the m o d e rn th eo r ie s of c h e m is t ry , this c o n ta in in g a t a b le of most o f th e e le m e n ts a r r a n g e d in o r d e r of th e i r a tom ic weights . H e b e c a m e P ro fe s so r of C he m is try in the T e c h n i s c h e H o c h s c h u le at K a r l s r u h e in 1869 a n d in 1876 a c c e p te d a s im i la r c h a i r in th e I n iversi ty o f T iib in g en . In 1882 he a n d M e n d e le e v w ere join tly a w a r d e d th e Davy M edal by th e Royal Soc ie ty for the ir d e v e lo p m e n t of the p e r io d ic system

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Theorien der Chemie, written in 1862 but not published until 1864, he included a table of most of them (14). Here he placed correctly the three lighter members of the platinum group, ruthenium, rhodium and palladium, but wrongly gave the heavier three in the order platinum, iridium and osmium, an error tha t he was later to rectify.

In the meantime, however, the most clear and comprehensive trea tm ent of the elements and their classification was devised, as every chemist knows, by the outstanding genius Dmitri Ivanovich Mendeleev, the great Russian scientist whose name has ever since been firmly associated with the Periodic Table. Mendeleev had studied at St. Petersburg and he had been more than fortunate in his teachers. The senior of these, Professor Nikolai Nikolaevich Zinin (1812-1880) had travelled widely in western Europe, spending a year with Liebig at Giessen and returning to the University of Kazan where in 1841 he had been a close colleague of Klaus before being called to St. Petersburg in 1847. He accepted the new concepts of Gerhardt and Laurent, the first to do so in Russia, and he attended the Karlsruhe Congress in 1860. Mendeleev’s other mentor, who became closely attached to his brilliant student and gave him private lessons during a period of illness, was Professor Aleksyei Andreivich Voskressenskii (1809-1880) who had also spent some time with Liebig and who was affectionately known to his students as “ the grandfather of Russian chemistry” . He was also a disciple of Gerhardt and Laurent. Mendeleev, after graduating, visited Paris to study under Victor Regnault and then spent a period in Heidelberg where he opened a private laboratory. It was from there that he travelled to Karlsruhe and his appreciation of those discussions is shown in a letter he wrote to Voskressenskii that was published in the St. Petersburg Gazette.

This began:“The chemical congress just ended in Karlsruhe produced such a remarkable

effect on the history of our science that I consider it a duty, even in a few words, to describe all the sittings of the congress and the results that it reached”.

After giving a brief account of these discussions he concluded :“Cannizzaro spoke heatedly, showing that all should use the same new atomic

weights. There was no vote on the question, but the great majority took the side of Cannizzaro”.

Mendeleev had devoted long years to the accumulation of evidence for his developing ideas on the classification of the elements, carrying out hundreds of experiments, reading widely in the literature and corresponding with chemists throughout Europe to collect appropriate data. All this information, on their physical and chemical properties, on the nature of their combinations and on the isomorphism of their compounds, was then inscribed on to small white cards which he arranged until he was satisfied with their sequence. Early in 1869 he distributed privately a pamphlet entitled “ An Experimental System of the Elements based on their Atomic Weights and Chemical Analogies” , and then in the

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following March, at a meeting of the Russian Chemical Society which he had done so much to organise in the previous year, and with Zinin presiding, a paper from Mendeleev was read by his friend and colleague Professor Nikolai Mens- chutkin because the author had been taken ill. This was published in the first volume of the Society’s transactions (15), and was briefly referred to in the Germ an periodicals, giving their readers some indication of Mendeleev’s ideas. It was not, however, a complete periodic table as we know it bu t rather a preliminary study in which he merely arranged the elements in six columns and, as he later emphasised, he was unaw are of the publications of Meyer and Newlands and only of O dling’s first communication of 1857 and Lenssen’s even earlier work.

As with his predecessors, Mendeleev had difficulties with the p latinum group of metals on account of their close similarity and the very small differences in their atomic weights as then determined. In this first system he arranged them:

In a second paper read to a meeting of Russian scientists in Moscow in August 1869, “O n the Atomic Volum e of Simple Bodies” , (16), he produced a clearer table, a prototype of his final version, in which he showed the eighth group in the same order as before but he now assumed the presence of an empty period between the ru thenium a n d the osmium groups.

T hen in 1870 Lothar M eyer contributed a paper to Liebig’s Annalen, “T he N ature of the Chemical Elements as a Function of their Atomic W eights” , (17) in which he arranged the p la tinum metals in their correct order but with some uncertainty:

He commented:“To obtain this arrangement, some few of the elements whose atomic weights

have been found to be nearly equal, and which have probably not been very carefully determined, must be rearranged somewhat, Os before Ir and Pt, and these before Au. Whether this reversal of the series corresponds to the properly determined atomic weights must be shown by later researches” .

Partly arising from this p a p e r of M eyer’s, Mendeleev published a further account of his system a year later, and this was translated in full in the Annalen (18). Running to almost a hund red pages in the G erm an version, this gave a much clearer and more comprehensive description of his periodic system - the first time he actually used the ph rase - and dealt at length not only with the properties of the elements but also with their compounds. More courageous than

Rh 104.4 Ru 104.4 Pd 106.6

Pt 197.4 Ir 198 Os 199

Ru 103.5 Rh 104.1 Pd 106.2

Os 198.6? Ir 196.7 Pt 196.7

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I> jnna I. rp jn n » 11. r |iju n a HI- r p j n n » IV. r | i j o n t V. Tpinn* VI. rp jn n a VII. r p ja n * V III.n rfja I,

H = 1 1Tat>a*teeaie u e -

MCHru. L i= 7 B*=9,4 B—11 C =12 H = l . 0 = 1 « F = 1 9

5 * | P i n 1-i N i= 2 3 ■ « = * 4 A l= 27 ,3 P = 31 8 = 3 1 C 1=36,4

- i « K = 39 C a= 40 - = 4 4 Ti=5»>? V=S1 ¡Cr=32 M n=M F e — 5«, Co=69, N i=5® , C u= 63

i f f - 3 “ (Ouz=63) Zn=6ft - =_68 - = 7 2 Ac = 75 S e = 7 8 Br=8<)

i l l Rb=85 S r z 87 (?Y t=88?) Zrrz90 N b -9 4 Mu—»6 - = l t t l R n= 104,R u= 104,P d = H » 4 ,A f= l0 8

5 j l - M i l l - 6-1

(**-=106)

C*=133

LU—»11

Ba—137

la = 1 1 3

- = 137

Sn= 118

C —-I38?

Sb= 132 T e*_ l28? J — 127

-

h i : : : - — -

1

T « = 1 8 2 W - 184r

0 » = lt* 9 ? ,Ir= 1 9 8 ? , 1*1=197, A ii = 197

i l l - 9-" (A n = J9 7 j H|T=*Jf. T l= 204 P b = * l7 Bi=2U8 - -

111 - HW> - - T h r r132 — L’r= 2 4 0

B ucuiai c o j h h m

OBIICb. K * 0 R’O* a jm KO R*0* RH)‘ a j a HO* «»O' R’O- m i RWI K'O’ 11*0- i i j « RO*

Bucioee ao*o- poxnoe coejaaeine. (RH*?) RH‘

i

RH* RH* RH

T h e rev ised a n d c o m p r e h e n s iv e P e r io d ic T a b l e p u b l i sh e d by M e n d e leev in th e J o u r n a l of the R u s s ia n C h e m ic a l Society in 1871. T h e p l a t i n u m m e ta l s w e re now p laced in the ir co r rec t o r d e r in G r o u p V I I I , a l th o u g h this a lso in c lu d e d c o p p e r , s i lver a n d gold. A space was left for as yet u n d i sc o v e re d m e m b e r s o f the p l a t i n u m g ro u p b e tw e e n th e l igh te r an d the h e a v ie r t r ia d s a n d th e a to m ic w eigh ts w e re g iven only in ro u n d f igures b e c a u s e of M e n d e leev 's u n c e r t a in ty a b o u t th e i r a c c u ra c y

Lothar Meyer, he ascribed different atomic weights to a num ber of elements, based solely upon their chemical analogies, calling for new determinations to be made. In the case of the three heavier platinum metals he wrote:

“Three elements stand in the system in succession between W = 184 and Hg = 200. Their atomic weights are actually smaller than W, but the succession does not correspond to expectations, for in considering that Os, Ru, Fe are similar, but that Ru and Fe have smaller atomic weights than Pd and Ni, it is to be expected that the atomic weight of Os is smaller than that of Pt, and that Ir, standing between Pt and Os, has a middle value of atomic weight. Moreover, the inaccuracy of the atomic weight determinations of the Pt metals is readily understood, not simply because their separation from one another is difficult but also because their compounds that have been used for atomic weight determinations are not of great stability”.

Mendeleev’s table of 1871 is reproduced here from the original Russian version. It will be seen that in addition to iron, cobalt and nickel and to the platinum metals he had included copper, silver and gold in Group VIII but had left alternative positions for them in Group I, and that, unaware of most of the rare earth elements, he had again left gaps for an extra series between the two

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K a r l F r ie d r ic h O tto S e u b e r t 1851-1921

T h e s o n of a P ro fe s s o r a t th e K a r l s r u h e T e c h n i s c h e H o c h s c h u le , S e u b e r t first s tu d ie d p h a r m a c y th e r e a n d th e n tu r n e d to c h e m is t r y , s e rv in g fo r se vera l y ea r s as a s s is ta n t to L o th a r M e y e r a n d a c c o m p a n y in g h im to B res lau a n d th e n to T ü b in g e n o v e r a p e r io d of tw en ty y e a r s a n d su c c e e d in g h im as P ro fe s s o r . H is d o c to r a l thes is was on th e a to m ic w e ig h t o f i r id iu m a n d he w en t 011 to r e - d e t e r m i n e th e a to m ic w eigh ts of all the o th e r p l a t i n u m m e ta l s , c o n f i rm in g th e o r d e r in w h ich M e n d e le e v h a d p l a c e d th e m

platinum metal triads. This h a d the inevitable result of prom pting the reexamination of native platinum for the apparently missing elements and led to a num ber of “ discoveries” that will be referred to a little later. Also, as is well known, he successfully predicted from a knowledge of their adjacent elements all the essential properties of as yet undiscovered elements, later to be identified as germanium, scandium, gallium, rhen ium and technetium.

In this table Mendeleev separa ted the elements into their m ain and subgroups, while he also confined his atomic weights to round numbers as he could not be sure of their accuracy. H is predictions about the correct order for the platinum metals were fully confirmed a few years later by Karl Seubert (1851-1921), a student and later a colleague of Lothar M eyer’s at Tiibingen. Seubert also distrusted some of the old values and set about their redetermination, arriving at the following arrangement and so confirming Mendeleev’s views (19):

Ru 101.4 Rh 102.7 Pd 106.35Os 190.3 Ir 192.5 Pt 194.3

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Some Spurious P latinum M etalsSeveral supposedly new elements of the platinum group were claimed before the publication of Mendeleev’s Periodic Table, but the gaps he left, as already mentioned, led to further claims for the isolation of further members of the group. In 1877 Sergius Kern of the Obouchoff Steel Works in St. Petersburg wrote to Chemical N ew s that he had “ perceived the presence of a new metal of the platinum group which has been called by me davyum in honour of the great English chemist Sir H um phry Davy” (20). This claim was investigated in 1898 by Professor J. W. Mallet, an Irish chemist who had settled in America to become Professor of Chemistry at the University of Virginia, and who thought this might indeed be a member of the missing triad of p latinum metals. While he was able to confirm K ern ’s experimental observations, he quickly showed that the new metal was merely

“a mixture of iridium and rhodium with a little iron, and hence that we have not yet reason to believe in the existence of a third group of platinum metals” (21).

A further “ discovery” was claimed by a French chemist, Antony Guyard in 1879:

“Some years ago, about 1809, I discovered in some commercially fabricated platinum from Russian mineral a new member of the platinum group to which I give the name of Ouralium to commemorate its origin” (22).The atomic weight was given as 187.25, its specific gravity as 20.25 and its

ductility was said to be greater than that of platinum, but the experimental work was of a very low order and ouralium was again almost certainly a mixture of platinum with some iridium and rhodium. These two fallacious discoveries, together with several others, were reviewed in more detail by Dr. W. P. Griffith in 1968 (23).

T he M odern P er iod ic T ab leMendeleev continued for the remainder of his life to take an active interest in his Periodic Law and used it as a base in his famous text-book, The Principles of Chemistry, first published in Russia in 1869 and in m any later editions, with an English translation in 1891 and Germ an and French versions a few years later. His chapter on the platinum metals opens with a statement of “ the naturalness of the transition” from zirconium, niobium and m olybdenum to silver, cadmium and iridium through ruthenium, rhodium and palladium, and similarly from tantalum and tungsten through osmium, iridium and platinum to gold and mercury. This is followed by an account of the chemistry of the platinum metals that would have been creditable to an author many years later, as for that m atter would the whole of the book.

But it was not until the discovery of the electron by Sir J . J . Thom son in 1879 and then Moseley’s work on the X-ray spectra of the elements that led to the concept of atomic num ber ju st before his death in 1915 in the European W ar that a sound theoretical basis could be established for the periodic system.

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The Platinum Metals and their Neighbours in the Periodic Table

Group VIA Group VIIA Group VIII Group IB

First long period C r 24 M n 25 Fe 26 C o 27 N i 28 Cll 29

Second long period M o 42 T c 43 R u 44 R h 45 P d 46 Ag47T hird long period w74 R e 75 0S76 Ir 77 Pt78 Au 79

The part of the modern tab le in which the platinum metals occur is reproduced above with their a tom ic numbers, this including of course rhenium discovered in 1925 and predicted by Mendeleev as dri-manganese, and technetium, his eka-manganese, discovered only in 1937 in the bom bardm ent of molybdenum by deuterons in a cyclotron. Jus t as Mendeleev emphasised, the greatest similarities are found in the vertical groups; there is a strong resemblance between ruthenium and osmium, between rhodium and iridium, and between palladium and platinum . At the same time there are obvious analogies in the horizontal series, between for example palladium and silver and between platinum and gold, while ru thenium and osmium more closely resemble technetium and rhenium , or in certain respects m olybdenum and tungsten, than they do iron. R hod ium and iridium are more closely allied to cobalt than to any other metal, while p latinum and palladium have close analogies with nickel.

In the two platinum metal tr iads the hardness and mechanical strength decrease from left to right and a re greater in the second triad than in the first. Ruthenium and osmium, both close-packed hexagonal in crystal structure are brittle although they can be fabricated with difficulty at high temperatures, while palladium and platinum faced-centred cubic metals, are soft and readily workable in the cold. A review of these similarities in properties and of the relevant chemical properties of th e group was contributed some years ago by the writers’ colleague A. R. Powell (24).

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References for C h a p te r 18

A. M. Ampère, Ann. C him , 1816, 1, 295-308; 2, 5-32J. W. Dobereiner, Ann. Phys. ( Poggendorff), 1829, 15, 301-307J. H. Gladstone, Phil. M ag., 1853, 5, 313—320E. Lenssen, Ann. Chem. ( Liebig), 1857, 103, 121-131; 104. 177-184W. Odling, Phil. M ag., 1857, 13, 422-^39; 480-497H. Sainte-Claire Deville and H. Debray, Ann. Chim., 1859, 56, 385-389C. Claus, J . prakt. Chem., 1860, 79, 28-59; 80, 282-317; Chem. News, 1861, 3, 194-195; 257-258M. Faraday, A Lecture on Platinum, bound with The Chemical History of a Candle, London, 1861, 173-204; Chem. News, 1861, 3, 136-141 W. Odling, A Manual of Chemistry, Part 1, London, 1861, 3J. A. R. Newlands, Chem. News, 1863, 7, 70-72; 1864, 10, 59-60; 94-95; 1865, 12. 83J. A. R. Newlands, On the Discovery of the Periodic Law, London, 1884 W. Odling, Q_.J. Sci., 1864, 1, 642-648S. Cannizzaro, Il Nuovo Cimento, 1858, 7, 321—366; English translation in Alembic Reprint 18; for an account of the Karlsruhe Congress see C. de Milt, J . Chem. Ed., 1951, 28, 421—425J. L. Meyer, Die modernen Theorien der Chemie, Breslau, 1864D. I. Mendeleev, ^[hur. Russ. Khim. Obshch., 1869, 1, 60-77D. I. Mendeleev, Proc. 2nd Meeting Scientists, 23 Aug, 1869, 62-71 J. L. Meyer, Ann. Chem. ( Liebig), 1870, Supp. VII, 354—364D. I Mendeleev, %hur. Russ. Khim. Obshch., 1871, 3, 25—56; Ann. Chem. ( Liebig), 1871, Supp. VIII, 133-229K. Seubert, Ann. Chem. ( Liebtg), 1891, 26 1, 272-279 S. Kern, Chem. News, 1877, 36, 4; 114-115; 164 J . W. Mailet, Am. Chem .J., 1898, 20, 776 A. Guyard, Moniteur Scientifique, 1879, 9, 795-797 W. P. Griffith, Chemistry in Britain, 1968, 4, 430^134 A. R. Powell, Platinum M etals Rev., 1960, 4, 144-149

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Henri Louis Le Chatelier1 8 5 0 - 1 9 3 6

Born in Par is . I,e C ha te l ie r s p e n t some t im e u n d e r S a in te -C la ire Devil le a t the Ecole Norm ale b u t his ed u ca t io n was in te r ru p te d by the F ra n c o /P ru s s ia n W ar. I . a te r he s tud ied at th e Ecole des Mines an d b ecam e a m in ing en g in ee r bu t in 1877 he re tu rn e d th e re to teach chem ist ry , being ap p o in ted P ro fe s s o r ten years la ter . H e was th e first to employ a p la t in um agains t rh o d iu m -p la t i n u m alloy th e rm o c o u p le , so in i t ia t ing a re l iab le m eans o f d e te rm in in g high t e m p e ra tu re s



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