Chapter 73 Chapter 73
2. Mosander's activities: the first division
thorium and selenium, the latter in the mud of his own sulfuric acid factory. He was the first to produce elemental zirconium, silicon and thorium (Szabadv/try 1966, p.125).
Wilhelm Hisinger (1766-1852) was born in Skinnskatteberg, where his father was proprietor of a thriving iron-works. Hisinger studied mining and subsequently took over the family enterprise.
He occupied himself with many branches of science, with chemistry, mineralogy, zoology and cartography. In 1804, he was elected a member of the Swedish Academy of Science.
DISCOVERY AND SEPARATION OF THE RARE EARTHS 41 turned successively lighter as it became oxidized during storage in air. Mosander then studied the reactions of cerium with various substances: sulfur, selenium, phosphoric acid, organic acids, etc. This work was all carried out in 1826 (Berzelius 1828).
Certain phenomena, namely that the colour of cerium and its c o m p o u n d s varied in different experiments, and that slight differences were found in the densities of cerium oxides from different provenances, made Mosander assume that maybe the substance was not homogeneous after all, m a y b e something u n k n o w n was still hidden in it. He began to study the problem very thoroughly and for a very long time.
Mosander was a slow, conscientious and very reserved researcher. W6hler, the well-known chemist, the first to perform the synthesis of an organic compound, worked for a longer period in Berzelius's laboratory, where he was the colleague of Mosander and knew about his research work. W6hler, after his return to G e r m a n y , stood in correspondence with Berzelius until the death of the latter. Their cor- respondence appeared in b o o k f o r m too, it is a very good and entertaining source for getting acquainted with the scientific background of the period (Wallach 1901). In this correspondence W6hler inquired several times how Mosander stood with his investigations, all the more so, because he would have liked to get a paper from Mosander for the journal Annalen der Chemie (today generally termed Liebig's Annalen) whose editor he was.
'1 can't tell you anything new about Father Moses, he never says anything about what he has found, not so much because he is so reserved, but perhaps because he has not found anything. He is so busy with his mineral water factory that he has no time for anything else. Write to him directly, you may then perhaps get a paper from him for the Annalen'
wrote Berzelius once, apparently with the discontent of the boss. However, Mosander did progress and found very interesting things. He reported in 1839 that his assumption proved correct. Cerium oxide considered homogeneous earlier did contain, in an a m o u n t of two fifths, another element which he named lanthanum (from the Greek work lanthano meaning escape notice) (Mosander 1839, Berzelius 1840). The name was suggested by Berzelius to his co-worker who by then was his successor in the professorial chair. (The human attitude of Berzelius is well characterized by a letter to Magnus, another of his former pupils: In the spring of this year I'll give up my post at the Karolinska I n s t i t u t e . . . I feel it my duty to make over this post to Mosander, since otherwise his hair could turn grey as my first assistant (Hjelt 1900).) Berzelius was presumably led in this suggestion by the fact that it had remained hidden before him, the discoverer of cerite, that the substance was not homogeneous, but contained another element besides cerium.
Mosander discovered the new element in the following manner: he transformed cerium (III) oxide into the carbonate, which he dissolved in nitric acid and evaporated to dryness. He pulverized the residue to a fine powder and treated it with cold dilute nitric acid, in which the more basic lanthanum oxide was dissolved, while cerium oxide remained undissolved. He then separated the lanthanum from
Fig. 3. J6ns Jakob Berzelius.
the solution with sodium oxalate, by igniting the precipitate, he obtained pale brick- coloured lanthanum oxide. The oxide could not be reduced with metallic potassium, similarly to cerium oxide, but from the chloride the metallic lanthanum could be obtained. After purification with alkohol he obtained soft scales with a metallic glimmer, which dissolved in water accompanied by hydrogen evolution. In the aqueous solution slimy lanthanum hydroxide was formed, the solution changed the colour of litmus to blue. By treating the oxide with hydrogen sulfide he obtained lanthanum sulfide. The atomic mass of lanthanum being smaller than the atomic mass of the mixture earlier assumed to be pure cerium, the true atomic mass of cerium must be higher than the earlier value.
It was high time for Mosander to publish his results, because meanwhile, also in 1839, Axel Erdmann, a former pupil of Berzelius and Mosander, also detected an unknown element in a Norwegian mineral, which - a strange coincidence - he
DISCOVERY AND SEPARATION OF THE RARE EARTHS 43 proposed to name mosandrite. F r o m the mineral sample sent to Berzelius, Mosander stated that the element is identical with lanthanum. (W6hler was informed of lanthanum already earlier through Berzelius, he inquired with astonish- ment why Mosander did not publish his results. Berzelius replied that he also had encouraged him many times to publish, but Mosander only shrugged his shoulders irritably, maintaining that he could not publish anything before he had finally finished the investigations, and that takes much time. In fact, Berzelius, in his Jahresbericht, which in the Swedish original appeared in 1839, was ahead of Mosander's own paper on the discovery of lanthanum. Berzelius recognized the importance of the discovery and therefore - departing from his custom, to the luck of Mosander - he reported the work of Mosander executed in Berzelius's former laboratory without referring to an already printed source.)
However, the lanthanum oxides obtained in different experiments were not identical in colour, differences in shade were observable. N o r were the cerium oxide residues fully identical. F r o m these phenomena, based on his previous experiences, Mosander again began to suspect that the lanthanum separated was not a pure element, but may contain yet another new element. He continued his experiments and reported their success in 1842 (Mosander 1842). He detected a further new element, which he named didymium. This element figured under this name for 50 years in books on chemistry, until it was disclosed that didymium is not a homogeneous element, but the mixture of two elements. Today, when instrumental analysis presents innumerable high-performance techniques to the analyst, we can hardly imagine how difficult it was, how much skill and inventiveness was required to separate these elements by the traditional gravimetric processes. The analyst of our days would probably be unable to repeat it. By way of example I shall cite the process used in the discovery of didymium, as abstracted by Berzelius (Berzelius 1844a):
'Earlier studies made Mosander suspect that cerium oxide obtained from cerite contains a foreign substance. He attempted to separate it by shaking cerium oxide hydrate with water, introducing chlorine gas to transform cerium oxydul into cerium oxide and the unknown substance into chlorure. Insoluble yellow cerium oxide was precipitated in the operation. F r o m the filtrate he again precipitated the solute with potassium hydrate, shook the suspension and again introduced chlorine gas. Further cerium oxide was precipitated and the rest was dissolved. He repeated this operation several times, in this manner he succeeded to separate the total amount of cerium oxide and obtain a chlorure from which potassium hydrate precipitated a hydrate that did not turn yellow in air and when treated with chlorine, was completely soluble in water. Thus the separation was terminated, and the oxide which is not further oxidized by chlorine gas was termed lanthanum oxide, as generally known.
When treating a mixture of lanthanum oxide and cerium oxide with 50-200 parts of nitric acid diluted with water, lanthanum oxide was dissolved, but the residual cerium oxide was not yellow, but brownish-red, and the lanthanum oxide also had a more or less similar reddish tint. F r o m this phenomenon Mosander concluded that a third substance must be present, following in one experiment lanthanum oxide
completely, whereas in the other experiment it is distributed between cerium oxide and lanthanum oxide. Vast experimenting was needed to separate it with the certainty allowing Mosander to claim that the substance is the oxide of a previously unknown element. In no way did it appear possible to separate the substances in question completely, since all precipitating agents tested acted similarly on them.
Finally Mosander succeeded to separate their sulfates be repeated crystallization.
The sulfate of cerium oxidul is least soluble, that of lanthanum oxide somewhat more and that of the third metal oxide readiest. The salts of the third metal oxide have a fine amethyst purple colour with a pale violet tint. Their solutions are pink, with a blue tint. Mosander named the metal oxide separated in the above manner didymium oxide from the Greek word didymos, that is, twins, since it accompanies cerium and lanthanum as a twin in cerium minerals ...
The preparation of didymium oxide will give a good picture on the difficulties connected with the separation of these substances. The sulfates of the mixture of lanthanum oxide and didymium oxide are dissolved in small portions in 6 parts of cold water cooled from the outside, so that its temperature should remain below + 9 ' C . The solution is then heated to +40°C, where a slightly amethyst-coloured powder separates. It is the sulfate of lanthanum oxide contaminated with some didymium oxide. This precipitation results from the property of the lanthanum salt that at a certain temperature it changes its chemically bonded water content. The pure reddish solution is decanted, the precipitate is dried, again dissolved in 6 parts of water at +9~C, subsequently it is heated to 50°C and kept at this temperature until a precipitate is formed. This is again the lanthanum salt, now contaminated with less didymium. By repeating this operation 10-12 times, one obtains almost pure lanthanum salt and a solution containing both salts. This solution is red. It is mixed with an equal weight of water, acidified with sulfuric acid and left standing to evaporate at a lukewarm place. When the volume of the solution has decreased to one sixth, the usually yellow liquid is decanted from t h e salt mass at the bottom.
The latter consist of larger red crystals and smaller prismatic needles. A small amount of boiling w a t e r is poured on it and subsequently rapidly decanted. The remaining larger red crystals are then again dissolved in water, acidified with sulfuric acid and slightly evaporated. Again two types of crystals separate, long, narrow rhombic prisms and larger red ones. The former are meticulously singled out, the latter consist of the sulfate of didymium oxide.
They are again dissolved in water, and precipitated with potassium hydrate. The precipitate is didymium oxide hydrate. When filtered on paper it is bluish violet.
During filtration and wash it rapidly takes up carbonic acid and thereby changes its colour to a slightly reddish violet. After ignition didymium oxide is o b t a i n e d . . . ' (Berzelius 1844a,b).
T o be sure, this work did need lots of patience!
W6hler, Berzelius's permanent correspondence partner, was not satisfied with the name didymium. He wrote that it sounds like childish babble, it is not worthy to specify an element. He urged Berzelius to persuade Mosander to choose a better name. We are informed from Berzelius's reply that in addition to 'twinship' Mosander had other points speaking for the name didymium.
DISCOVERY AND SEPARATION OF THE RARE EARTHS 45
Fig. 4. Carl Gustaf Mosander (Courtesy Library Karofinska Institutet Stockholm).
'No, dear friend, I don't like the name either, but can't ask him to change it, since he has already publicly announced it. You are surely unable to understand our friend Father Moses. He doesn't accept suggestions from anyone. He would be offended by a proposal to change a name given by him. He was led by the point of view that the name should begin with a D, since this letter is as yet free as a chemical symbol. You're right the repetition of the consonants and vowels is displeasant to the ear, but one can get accustomed to it ...'.
Well, M o s a n d e r invented the name didymium in vain in favour of the symbol D, it did not remain permanently in the list of the elements, as will be seen in what follows. Also, the name m o s a n d r i u m was later introduced by others several times for elements, but no element by this name now exists.
Let us return, however, to Mosander, since he had not nea[ly finished his successful activities in the complex world of the rare earth elements. He hunted up two further elements. Encouraged by the success with cerite, he now started investigating gadolinite. M a y b e yttrium is no homogeneous element either? And he proved to be correct.
His suspicion, of course, had antecedents. Heinrich Rose discovered earlier that yttrium chloride itself is not volatile, although it was thought to be so earlier. What appears volatile is the impurity beryllium chloride. This finding spoke in favour of yttrium not being a pure element as believed earlier. Rose was the first to prepare metallic yttrium by reducing yttrium chloride and yttrium fluoride with metallic sodium. It turned out later, however, that this yttrium metal was still largely contaminated.
Also, Berzelius observed that when working with yttrium oxide and adding ammonia to a solution of yttrium nitrate, precipitation takes place in the following sequence: first, beryllium hydroxide is separated, and, subsequently, a yellowish substance mixed with the whitish yttrium hydroxide (Berzelius 1844b).
Mosander reported in 1843 on his results connected with gadolinite. He found not only one, but two new elements in it. The yellowish substance precipitated first with ammonia he termed erbium. The residue again precipitates in two fractions. He retained the name yttrium for the first colourless substance, and gave the name terbium to the second, pale amethyst-coloured substance. This mode of separation, however, was very cumbersome and uncertain. He found a better method for separating the substances by fractioned precipitation of the oxalates: Erbium oxalate is the first to precipitate, subsequently the terbium salt contaminated with important amounts of yttrium, and finally the yttrium salt. By dissolving and repeatedly precipitating the substances obtained, the products will become purified successively (Mosander 1843).
In his not too ingenuous naming of the new elements (he only diversified the name of the place of discovery, Ytterby, from which the name yttrium also stemmed), Mosander was presumably again led by his intention to find free letters for the symbols of the new elements. Both E and T were free. However, his symbol concepts did not survive. I cannot see the reason why, since the letter T in itself is not used as a chemical symbol up to the present day, although we know ten elements whose names begin with a T. The reason was perhaps that the elements tellurium, tantalum, titanium and thorium all known earlier than terbium were given symbols in Berzelius's nomenclature consisting of two letters, and so terbium got the symbol Tb. Presumably per analogiam erbium was given the symbol Er, though no element whose name begins with E was known at the time, and only later was europium (Eu) discovered with the continuing division of rare earth elements.
Thus, Mosander's activities led to the originally two-element division into a six- element division. The cerium compounds are yellow at the higher oxidation level and colourless at the lower oxidation level, lanthanum compounds are white, didymium compounds are red, yttrium and erbium compounds are white, terbium compounds are pink. Chemists existed, of course, who disputed the existence of these elements. Unequivocal identification of elements was, however, possible in later times only. In the period in question, the main characteristics on the basis of which a substance could be qualified as a new element were separability, colour, crystal shape and reactivity. Even atomic mass determinations were largely un- certain, particularly in the group of the rare earth elements, it will be seen in the
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.