Chapter 73 Chapter 73
3. Rearrangement and further divisions: 1843-1886
DISCOVERY AND SEPARATION OF THE RARE EARTHS 47 following t h a t several a m o n g the a b o v e six e l e m e n t s were l a t e r f o u n d t o c o n t a i n a d d i t i o n a l elements.
The first i m p o r t a n t aid in i d e n t i f i c a t i o n was s p e c t r a l a n a l y s i s i n t r o d u c e d in the fifties of the 19th c e n t u r y . T h e m e t h o d will be discussed later, let us, however, n o t e here t h a t the n a m e s g!ven by M o s a n d e r were f a t a l l y a n d p e r m a n e n t l y m i x e d u p in the c o u r s e of s p e c t r o - a n a l y t i c a l identification. D e l a f o n t a i n e w h o u n e q u i v o c a l l y p r o v e d and c o n f i r m e d the existence of the e l e m e n t s y t t r i u m , t e r b i u m a n d e r b i u m in 1864, n a m e d the p i n k c o m p o u n d e r b i u m a n d t h e white c o m p o u n d t e r b i u m , pre- s u m a b l y o u t of p u r e a b s e n c e of m i n d , a n y h o w , these n a m e s were t h e n k e p t on for g o o d ( D e l a f o n t a i n e 1864).
Carl Gustaf Mosander (1797-1858) - like so many chemists of the age - started his career as a pharmacist's apprentice in Stockholm. Later he entered the school for army surgeons and served for some years in the army as surgeon. Meanwhile he studied at the University of Medicine, and after graduation became an assistant at its department of chemistry headed by Berzelius. In 1832, after Berzelius's retirement, Mosander was appointed professor as successor of Berzelius and held that post until his death. In general, he was reluctant to write papers, most of his results survived in the Jahresberichte of Berzelius or as printed notes of his lectures (Kopperl 1974).
Heinrich Rose (1795-1864) studied at the University of Berlin, and subsequently spent two years in Stockholm in Berzelius's laboratory as his private assistant. In 1823, he was appointed an associate professor and later regular professor at the University of Berlin, where he worked till his death. Rose discovered niobium. His manual Handbuch der analytischen Chemie (1829) was pub- lished several times, it was the first systematic comprehensive book on analytical chemistry in its entirety on the level of the age (Szabadv~ry 1966, p. 165).
Marc Delafontaine (1837-1911) was born in Switzerland. He probably studied at the University of Geneva and worked there for some time. Later he emigrated to the United States and continued his activities in chemistry and geology there. I was unable to find any further biographical facts concerning Delafontaine.
Fig. 5. Jean Charles Marignac.
by Bunsen and Kirchhoff. The periodic system (Mendeleev 1869, Meyer 1870), on the other hand, showed the direction, one might say put a map into the hands of the researchers, of where to look for new elements and how many unknown elements may as yet be discovered. However, it still took a long time, until the periodic system was found to be capable of serving this purpose, until it became accepted that the periodic system was not just a game of scientists having no better ideas.
None the less, studying rare earth elements did not stop altogether. Papers appeared about investigations of the rare earth elements known at the time, though a few papers only, and they were exclusively concerned with the cerium, lanthanum and didymium groups. Up to 1864, for instance, I did not find a single paper on yttrium. However, the new leading man in the field of rare earth elements, who later increased their number from the known six by three further elements by further division, Jean Charles Marignac from Switzerland, had already made his ap- pearance. Marignac determined the atomic weights of several elements more exactly than earlier, including cerium, lanthanum and didymium. He improved the sepa- ration method developed by Mosander in order to obtain purer products. In 1848, he calculated the atomic weight of cerium from the reaction of Ce(III) sulfate with barium chloride, and found a value of 47.26 (Marignac 1848). In the same period
DISCOVERY AND SEPARATION OF THE RARE EARTHS 49 Rammelsberg found a value of 45.8, and Hermann of 46.0, while in 1853 Bunsen stated a value of 58.2 (Bunsen 1853). It may be seen that Marignac's value was closest to the true one. Let us keep in mind that all this happened before the con- gress in Karlsruhe and hence the concept of atomic weight was far from being defined unequivocally. It was frequently mixed up with equivalent weight, and its determination was based on an assumed atomic composition of compounds, easily leading to results that amounted to half or one third of the correct value, as in the above cases. At t h a t t i m e the researchers believed these elements to be 'bivalent'. I use the quotation marks, because the valence concept was unclear too, this statement can only be regarded as a conclusion from the calculation method used by the researchers. One year later (1849), Marignac published his results regarding the atomic weights of lanthanum and didymium: 47.04 and 49.6, respectively. The sequence of the values is correct again, the atomic mass of lanthanum is indeed somewhat less than that of cerium, while that of didymium is necessarily higher, since the atomic masses of its two constituting elements neodymium and prase- odymium are both higher than that of cerium (Marignac 1849). In 1853, Marignac studied the chemical reactions of didymium in detail: the colour, crystal shape, solubility and mode of preparation of the halogenides, sulfides, phosphates, sulfates, arsenic compounds and oxalates (Marignac 1853a), two years later he carried out similar studies with lanthanum (Marignac 1855).
Delafontaine took up to work with gadolinite again in 1864 to study Mosander's yttrium elements. He studied erbium and its compounds in greatest detail, using various methods including the blowpipe. He unequivocally confirmed the existence of erbium, but was not unequivocal regarding the existence of terbium (Delafontaine 1864). In the same year another chemist, O. Popp, questioned the existence of both erbium and terbium. He had already made use of absorption spectrometry, and based on its results he asserted that erbium is didymium contaminated with cerium.
In the same paper he reported the properties and preparation of yttrium and its compounds, he prepared metallic yttrium by reduction with sodium (Popp 1864). At the same time, Bahr and Bunsen also studied gadolinite. They asserted that erbium was a true element, but denied the existence of terbium. They dissolved the mineral in hydrochloric acid, filtered off silica and precipitated with oxalic acid. After dissolution of the precipitate in nitric acid, ceria earths were precipitated as sulfates, they precipitated the filtrate again with oxalic acid and repeated these operations until the didymium spectrum disappeared. The nitric acid solution then contained only yttria earths. It was evaporated to dryness, they took up the residue in water and let the solution crystallize. The pink erbium salt crystallized first, yttrium remained in the solution. This fractional crystallization they repeated several times.
In what was earlier considered terbium they could only discover spectral lines of erbium, yttrium and cerium (Bahr and Bunsen 1866). Cleve also denied the existence of terbium (Cleve later became an important figure in the study of the rare earth elements). Delafontaine, however, continued to hold on to the opinion that terbium exists, though not identical with Mosander's terbium. He proposed a new name for it: mosandrium, while he considered Mosander's terbium identical with what Bahr and Bunsen separated as erbium (Delafontaine 1865). It may be seen that things
around yttrium and its companion elements became very tangled. Erbium was accepted, since Young demonstrated its existence in the solar spectrum too (Young 1872).
For some time everything was quiet around terbium, only Delafontaine returned to the issue, persisting and confirming his statement that the element mosandrium exists with new observations (Delafontaine 1874). Bunsen, in his famous spectros- copic investigations, found erbium, yttrium, cerium, lanthanum and didymium chloride spark spectra very characteristic, and also the flame spectrum of erbium and the absorption spectra of erbium and didymium salt solutions, but still did not find terbium (Bunsen 1875). Delafontaine, however, continued his search. In the mineral samarskite, discovered by G. Rose in 1839 and containing yttrium group elements, Delafontaine again detected the oxide discovered by Mosander, but denied by Bunsen and Cleve which he named mosandrium, but this time - following Marignac's suggestion - he proposed the name terbium for it, and suggested that what was originally terbium and separated by Bunsen shall remain erbium (Delafontaine 1877). In the next year he reported in detail about his terbium, which finally was what we call terbium up to the present time. In analyzing samarskite, his starting point was that the formates of the cerium group are slightly soluble, while the formates of the yttrium group can be separated by fractional crystallization.
Terbium compounds are distinctly different from erbium compounds (Delafontaine 1878a). In his next paper he remarked that probably - in addition to terbium - another very similar unknown element was also present, he even gave it a name, he termed it philippium (symbol Pp) after a certain Philippe Plantamour. Following Mendeleev he predicted its atomic weight between 90 and 95 and outlined the characteristics of its compounds, with less success, however, than his great con- temporary. Mendeleev also - as is well known - predicted new elements and their properties, but at least he did not name them. Anyhow, with time Mendeleev's elements were actually discovered, whereas Delafontaine's philippium turned out to be a mistake (Delafontaine 1878b).
Whenever something is started, it will get going easily. This was the case with mistaken element discoveries too. Delafontaine thought he discovered another new element in samarskite, and gave it a name: decipium, symbol Dp, expected atomic weight 122. He even predicted its absorption spectral line: 2 ~ 416. This element persisted in books on chemistry for a rather long time, one even meets it here and there in the first years of our century (Delafontaine 1878c).
In the same year Marignac again studied gadolinite and confirmed the presence of terbium in it. He assigned an atomic weight of either 99 or 148.5 to the element, depending on whether its oxide has the composition T b O or T b 2 0 3 (Marignac 1878a). It is an interesting episode that in the same year a chemist called Smith also discovered terbium in samarskite, but termed it mosandrum (Smith 1878). A priority dispute began between Delafontaine and Smith, ending with Delafontaine's victory. In 1880, Soret reported the absorption spectrum of terbium together with other elements, including the non-existing philippium and decipium (Soret 1880). In 1882 Roscoe and Schuster reported the exact spectrum of terbium: 194 lines, and thereby the existence of terbium was definitively confirmed (Roscoe 1882).
DISCOVERY AND SEPARATION OF THE RARE EARTHS 51
Fig. 6. Marc Delafontaine.
Following the adventurous story of terbium it is actually impossible to decide by now who was the true discoverer, Mosander, Delafontaine or Smith? The element names, as indicated above were applied inconsistently, and we cannot know whether they referred to the same substance. Did Mosander find the same substance and called it erbium that finally became terbium with Delafontaine, or was Bunsen correct and consequently Mosander's fraction was a mixture only? N o d a t a were reported that would allow us to state now, at this late date, what substances were identical, no characteristic spectral lines, no exact atomic weight values are at our disposal as yet.
During that same period, progress was achieved also in the field of cerite.
Delafontaine the indefatigable, a figure unfairly neglected in the history of chemistry, investigated the cerium group in the same mineral samarskite. He noted that in his opinion, d i d y m i u m was not homogeneous, since didymium separated from samar- skite had an absorption spectrum not fully identical with that separated from cerite:
certain lines in the blue region were missing and lines in the violet also differed (Delafontaine 1878c). His assumption was confirmed in the following year by Lecoq de Boisbaudran, whose n a m e became famous earlier, in 1875, by his spectrographic discovery of gallium, the first a m o n g the eka-prefix elements predicted by
Mendeleev, evidencing that Mendeleev's periodic table was not mere speculation, but a system reflecting reality based on some deeper law.
It is of interest that Lecoq de Boisbaudran found a new element in the didymium precipitate obtained from samarskite while simultaneously disproving Delafontaine's statement that the spectra of didymium obtained from cerite and samarskite differ. He reported simultaneously that he found in samarskite - in the stage before didymium separation - new lines by spark spectrography indicating the presence of a further hitherto unknown element. He gave approximate values for the corresponding wave lengths and stated that the double salt with potassium sulfate of the new element precipitates together with the corresponding didymium salt, while its sulfate is less soluble than didymium sulfate. Its oxalate separates before didymium oxalate and its hydroxide precipitated with ammonia behaves similarly (Lecoq de Boisbaudran 1879a). In a next paper he gave the exact wave length values for the characteristic lines of the new element (2 = 480, 463.5) and described a complicated fractional separation method, by means of which he succeeded in separating the new element in a relatively pure state from didymium and also from decipium. He gave it the name samarium, referring to the mineral in which he detected in (Lecoq de Boisbaudran 1879b).
The results around 1878-1880 made the family relations of rare .earth elements rather puzzling. T o facilitate finding one's bearings, it appears expedient now to trace a genealogical table to aid in following this tangled story. The various divisions, multiplications and propagations discussed and the ramifications to be discussed in the following taking place around 1880, and moreover, leaping ahead in time, the last events in the history of the discovery of the rare earth elements which took place in the years following the turn of the century shall all be included.
As can be seen in the genealogical table, many novelties came forth from gadolinite too. Marignac confirmed Delafontaine's statements that terbium is an existing element. He determined its atomic weight amounting to either 99 or 148.5, depending on its oxide corresponding to the formule T b O or T b 2 0 3 . At that time many discussions went on as to the valence of rare earth elements, we shall come back on this issue. Terbium oxide is orange and turns white upon ignition in H2, it is readily dissolved in dilute acids, its solutions are colourless and presumably have no absorption spectrum (Marignac 1878a). As to the erbium precipitate obtained in the separation, Marignac stated that it is not homogeneous, but the mixture of two elements. The oxide of one of them is pink, with a characteristic absorption spectrum. Marignac retained the name erbium for this element. The other oxide is colourless and so are the salts. According to Soret there are no characteristic lines in its visible and UV absorption spectra. Its neutral chloride does not give a precipitate with hyposulfurous acid, while erbium chloride does, based on this property it can be separated from erbium. Its atomic weight is high, 115, or 172.6 in case it is tetravalent. Marignac gave the name ytterbium to the new element, since it stands between yttrium and erbium in its properties (Marignac 1878b).
In 1877, Mallet discovered an yttrium niobate mineral called sipylite in Virginia (USA). Analyzing it he found that it contained niobium and cerium and yttrium group elements (Mallei 1877). In 1878, Delafontaine identified Marignac's ytterbium in this mineral too (Delafontaine 1878d).
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However, the division of erbium did not end here. After the Swiss results from 1878 it was again the Swedish school of rare earth elements that took a great step forward in 1879: erbium again multiplied into four elements. Nilson repeated the separation of erbium described by Marignac and confirmed the existence of ytterbium and the statements of the Geneva master regarding it, with the only difference that according to Nilson, the atomic weight is 116 instead of 115 as reported by Marignac. (Owing to a slip of the pen, Nilson's paper cites 132 and 131, respectively, obviously not the atomic weight data of the element, but the molecular weight data of the oxide). Nilson also thought tetravalence conceivable, in that case the atomic weight would amount to 174 (Nilson 1879).
By an exceedingly intricate fractioning method consisting of 13 steps and starting from the nitrates, Nilson finally obtained a 'basic' nitrate from gadolinite, dissolved in nitric acid it yielded a weak absorption line in the red and in the green. A precipitate was formed with oxalate. Nilson considered it a new element and termed it scandium after his wider homeland Scandinavia, following thereby the patriotic line of giving names to elements started with Lecoq de Boisbaudran's gallium and repeated many times in the following decades with new elements. Nilson studied the spectrum of scandium with Thal6n's assistance very thoroughly and found several identifiable lines. The chemical properties of the new element: the oxide is white, sparingly soluble in nitric acid, more readily in hydrochloric acid; its nitrate decomposes at a relatively low temperature as compared to other rare earth elements. In the case of ScO the atomic weight is 80, however, by reason of the ready decomposition of the nitrate Nilson assumed that the element is tetravalent. He pointed out that by its properties and assumed valence scandium fits very well into the empty square in Mendeleev's periodic table between tin and thorium (?) (Nilson 1879).
In a somewhat later French version of that same paper Nilson discussed the properties of scandium in greater detail, he does not, however, mention tetravalence, but assumes that - similarly to the other rare earth elements - scandium is trivalent.
On this basis its atomic weight amounts to 44, and this value, he writes, 'cor- responds to the eka-boron predicted by Mendeleev'. Based on its atomic weight scandium is located between beryllium and yttrium. Nilson, though referring to Mendeleev's prediction, avoids the expression 'periodic system' (Nilson 1880), maybe because in another paper published in the same year with Peterson and dealing with the atomic weight determination of beryllium, he declared that 'in its present state the periodic system cannot be considered the adequate expression of the properties of elements'. The reason for his negative attitude was that in his atomic weight determinations of beryllium he found 13.65 and with this value he could not place beryllium in the periodic system. Therefore he preferred to believe in the incorrectness of the system rather than accept that his own determinations gave false results (Nilson and Peterson 1880).
The quality of the analytical performance cannot be demonstrated better than by the quantitative control investigation performed in the same year by Cleve: starting from 4kg gadolinite mineral he obtained 0.Sg scandium oxide (Cleve 1879a).
In 1880, Nilson, by processing a larger amount of auxenite (after digestion with
DISCOVERY AND SEPARATION OF THE RARE EARTHS 55
Fig. 7. Per Teodor Cleve (Courtesy Library University of Uppsala).
potassium sulfate in a platinum vessel, precipitation with ammonia, dissolution of the precipitate in nitric acid, precipitation with oxalic acid, dissolution in nitric acid, evaporation to dryness, precipitation with hydroxide, evaporation of the filtrate and subsequent fractionation consisting of 30 operations to separate the individual rare earths) obtained pure ytterbium oxide and scandium oxide. From there he de- termined the atomic weights (Yb 173, Sc 44) and prepared numerous compounds (Nilson 1880). As the reader may have already noticed, it is astonishing how immediately the colleagues reacted to true or assumed discoveries by repeating, confirming or refuting.
Cleve studied the erbium remaining after the separation of ytterbium in 1879 and decided that the fraction is still not homogeneous. His starting point was the spectrum taken by Thal6n. He separated the substance into three fractions, one close to yttrium, the second to ytterbium and the third to erbium. Among the assumed spectral lines of erbium, one was present only in the fraction close to ytterbium, but not in ytterbium itself, a second similarly only in the fraction close to yttrium, but not in yttrium itself, both lines were present very weakly in the spectrum of the erbium fraction. Based on this finding, Cleve assumed the existence of two further elements and immediately gave them names: thulium (originating from the legen- dary old name of Scandinavia), and holmium (from the medieval Latin name of