CHAPTER XIV

THE ERBIUM AND YTTERBIUM GROUPS--YTTRIUM AND SCANDIUM

In his examination of the ‘Yttria’ of Gadolin and Ekeberg, during the years 1839 to 1843, Mosander, by methods based on differences in strength of the oxides as bases, separated the earth into three new oxides, yttria proper, the most strongly basic, terbia, intermediate in strength, and erbia,[348] the least basic. No further separation was effected until 1878, when Marignac, by fractional decomposition of the nitrates, separated from erbia a new oxide, for which he proposed the name Ytterbia; the new oxide was the least basic of the erbia earths. In the following year, Nilson[349] isolated from ytterbia a still less basic oxide, by the same method; he proposed the name Scandia, to recall the fact that it occurred in gadolinite and euxenite, which up to that time had been found only in Scandinavia. In 1879 also, Soret[350] announced the discovery of a new element X, evidence for the existence of which he had obtained during a spectroscopic examination of a mixture of erbia and terbia earths; the oxide of X was isolated in the same year by Cleve[351] from the old erbia, by fractional decomposition of the nitrates, and the name Holmium, from the town of Stockholm, was proposed for the new element. The same investigation led to the discovery of Thulium, which derives its name from Thule, an old name for Scandinavia.

[348] The reversed nomenclature of Delafontaine is here employed (see p. 184).

[349] _Compt. rend._ 1879, ~88~, 642, 645.

[350] _Ibid._ 1879, ~89~, 521.

[351] _Ibid._ 1879, ~89~, 478, 708.

Lecoq de Boisbaudran[352] in 1886 showed Cleve’s Holmia to be a mixture of at least two oxides; he retained the name Holmium for the element which gave the most characteristic absorption bands of the old holmium, and proposed the name Dysprosium (from δυσπροσιτος, difficult of access) for the second element. The name Erbia was retained for the oxide remaining after the removal of holmia, thulia, and dysprosia from the old erbia; the homogeneity of this erbia has been called in question, but is now fairly firmly established. The individuality of dysprosium[353] and holmium[354] may also be regarded as definitely established; that of thulium remains doubtful (see p. 204).

[352] _Ibid._ 1886, ~102~, 1003, 1005.

[353] Urbain, _Compt. rend._ 1906, ~142~, 785.

[354] Holmberg, _Zeitsch. anorg. Chem._ 1911, ~71~, 226.

The homogeneity of ytterbia was questioned by Auer von Welsbach[355] in 1906; by fractionation of the ammonium double oxalates, that author isolated the oxides of two new elements, for which he proposed the names Aldebaranium and Cassiopeium. By fractionation of the nitrates from nitric acid solution, Urbain[356] arrived at the same result, and proposed the names Ytterbium (Neoytterbium) and Lutecium, which have been adopted by the International Committee. The latter author, employing the same method in the fractionation of the gadolinite earths, has recently obtained very strong evidence of the existence in this group of another element, for which he proposes the name Celtium;[357] the discovery, however, awaits confirmation.

[355] _Monats._ 1906, ~27~, 935; 1908, ~29~, 121.

[356] _Compt. rend._ 1907, ~145~, 759.

[357] _Ibid._ 1911, ~152~, 141.

SEPARATION

In the separation of the yttrium elements, methods based on differences in electropositive character are of much greater importance than in the separation of the cerium and terbium groups, and the method of nitrate fusion has been very largely employed even in comparatively recent work. This method, which was introduced by Berlin in 1860, has been of great value in the separation of yttrium and the ytterbium elements from the erbium group; it was employed in the isolation of ytterbium by Marignac, and of scandium by Nilson.

If a concentrated solution of the nitrates be evaporated down, and the syrupy residue subjected to gradually increasing temperature, the nitrates of the ytterbium elements and scandium are converted first into the basic nitrates; at somewhat higher temperatures the erbium salts are decomposed, whilst yttrium nitrate and the nitrates of any cerium elements present are the last to break up. If the mixture of basic and neutral nitrates be dissolved in boiling water, the former, being less soluble, crystallise out on cooling, and may be separated by this means, the process being repeated with the filtrate containing the unchanged nitrates. In this way, the weakly basic scandia and ytterbia quickly collect in the first fractions, whilst the oxides of the erbia group are easily separated from the more strongly basic yttria. The presence of the intermediate terbium group renders the process much less easily workable.

The process may be modified by raising the temperature to such an extent that the soluble basic nitrates are converted into insoluble superbasic nitrates, the temperatures at which this change occurs increasing from element to element as the positive character becomes more marked; the mixture of basic and superbasic salts is then extracted with dilute nitric acid which leaves that latter undissolved and removes the more positive elements in solution.

Fractional precipitation of the hydroxides by means of ammonia, alkalies, or alkaline earths has also been frequently employed. A modification of this process is the precipitation with aniline, carried out by Kruss;[358] in this method, the solution of the chloride in warm dilute alcohol is treated with an alcoholic solution of the organic base. Another modification is the ‘Oxide process’ employed by Auer von Welsbach[359] for the separation of the cerium elements, and by Drossbach[360] in the yttrium group. The concentrated solution of the mixed salts is thoroughly digested with the oxides obtained by precipitating a fraction of the earths; the more strongly basic oxides tend to displace the less basic, so that these accumulate in the insoluble part. The solution is filtered from the undissolved oxides, another fraction precipitated, and the oxides obtained from the precipitate digested with the concentrated solution as before.

[358] _Zeitsch. anorg. Chem._ 1893, ~3~, 108, 353.

[359] _Monats._ 1883, ~4~, 630.

[360] _Ber._ 1902, ~35~, 2826.

The more modern methods of separation combine the above processes with the methods of fractional crystallisation, for which the bromates and alkylsulphates of these elements are well adapted. The procedure[361] which experience shows will lead to a fairly rapid separation is roughly represented in Fig 9. The double sulphates (B), left in solution after removal of the cerium and part of the terbium group, are transformed into the bromates, which are separated by fractional crystallisation into five main fractions. The least soluble portion, fraction 1, contains the terbium elements with some dysprosium; in the fractionation of the terbium group by means of the nitrates and double nitrates, the dysprosium, with some terbium, collects in the final fractions (fraction 6). Fraction 2 contains terbium, dysprosium, holmium, and yttrium as the bromates; these are converted into the anhydrous chlorides, from which, by treatment with sodium ethylsulphate in alcoholic solution, the ethylsulphates are obtained. By fractional crystallisation, dysprosium may be obtained in a fairly pure condition (fraction 7), the least soluble part (fraction 6) containing the terbium with some dysprosium. Holmium and yttrium collect in the most soluble part (fraction 8), from which pure holmium can be obtained by the method of nitrate fusion. Fraction 3 contains yttrium and erbium, with small quantities of dysprosium and holmium; the latter are readily separated by the nitrate fusion, which will also allow of a fairly complete separation of yttrium (fraction 9). Fraction 4 contains yttrium and erbium; scandium if present will also collect here. Erbium can be obtained pure by the nitrate fusion; the second fraction from this process contains both yttrium and erbium, and may be further worked up with the fraction of similar composition (fraction 10) from fraction 3.

[361] James, _J. Amer. Chem. Soc._ 1912, ~34~, 757.

The mother-liquors from the bromate separation (fraction 5) contain thulium and the ytterbium elements; the crystallisation is continued, and allows of complete separation of thulium and ytterbium, and probably of lutecium, though the most soluble fractions do not seem to have been fully separated.

THE ERBIUM GROUP

The oxides of this group, as contrasted with the ytterbia oxides, give rise to coloured salts, which in solution show definite absorption spectra in the optical region; the spectrum of erbium salts is particularly definite and characteristic. Erbium has among the yttrium elements the place of neodymium among the cerium elements; after yttria, erbia is the commonest oxide of the yttria group, though on account of the difficulties of separation the chemistry of erbium is by no means so complete and definite as that of neodymium. The oxides in order of decreasing basicity, as shown by the order in which they are thrown down by ammonia, are: dysprosia, holmia, erbia, thulia; the electropositive character becomes weaker, therefore--as generally in the rare earth series--as the atomic weight of the elements increases.

~Dysprosium~, Dy = 162·5

Compounds of this element were probably prepared in the pure state for the first time by Urbain[362] in 1906, by the fractional crystallisation of the ethylsulphate. He showed that after fourteen recrystallisations, the absorption spectrum of the salts and the mean atomic weight of the element remain unaltered, and that after removal of terbium by the very efficient ethylsulphate method, all remaining traces of yttrium could be rapidly removed by crystallisation of the nitrate. The salts have generally a more or less pronounced yellow colour.

[362] _Compt. rend._ 1906, ~142~, 785.

The _oxide_, Dy₂O₃, is a white powder which does not alter in composition when strongly heated in reducing or oxidising atmospheres. It is remarkable in that it is the most strongly paramagnetic oxide known, having a coefficient of susceptibility much greater than that of ferric oxide.[363] The _chloride_ crystallises with 6, the _sulphate_ with 8, and the _nitrate_ with 5 molecules of water of crystallisation. The _bromate_, Dy(BrO₃)₃,9H₂O,[364] obtained by double decomposition, melts at 78°. The _platinocyanide_, Dy₂[Pt(CN)₄]₃,21H₂O, forms bright red cubic crystals, with greenish fluorescence.

[363] _Compt. rend._ 1908, ~146~, 922.

[364] Jantsch and Ohl, _Ber._ 1911, ~44~, 1274.

Several other salts are described by Urbain, and by Jantsch and Ohl (_loc. cit._).

~Atomic Weight.~--Urbain and Demenitroux[365] determined this constant from the ratio Dy₂(SO₄)₃,8H₂O : Dy₂O₃. The mean value of six determinations carried out with material obtained by fractional crystallisation of the nitrate was 162·52; with material purified by the ethylsulphate crystallisation, the mean of six determinations gave the value 162·54. The International Atomic Weight is 162·5.

[365] _Compt. rend._ 1906, ~143~, 598.

~Detection.~--Lecoq de Boisbaudran[366] and Urbain[367] give the position of the following absorption maxima in the visible and ultraviolet regions respectively:

┌──────┴──────┐ 753 368·5 338 475 379·5 332·5 451·5 365 427·5 351

[366] _Ibid._ 1886, ~102~, 1003.

[367] _Ibid._ 1906, ~142~, 785.

The arc spectrum of Urbain’s material was examined by Eberhard,[368] who gives as most suitable for detection of the element in a mineral or oxide mixture the following lines:

3385·16 3531·86 3536·17 3645·54 3898·69 3944·83 4000·59 4078·11 4187·00 4211·82

[368] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60.

See also Exner and Haschek, and Eder and Valenta.[369]

[369] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 9.

The ultraviolet arc spectrum and the cathode phosphorescence have also been examined by Urbain.[370]

[370] _Loc. cit._

~Holmium~, Ho = 163·5

The individuality of this element can hardly be regarded as perfectly established, though Holmberg[371] has prepared salts which in solution show only faint indications of erbium and dysprosium, when tested spectroscopically. That author fractionated the yttrium elements obtained from euxenite by a long process of separation, which involved crystallisation of the _m_-nitrobenzenesulphonates, of the simple nitrates (two series), of the double ammonium oxalates, and finally fractional precipitation of the hydroxides by ammonia.

[371] _Zeitsch. anorg. Chem._ 1911, ~71~, 226; see also Langlet, _Abstr. Chem. Soc._ 1907, ~92~, ii. 955.

He determined the _Atomic Weight_ as 163·5, which is the value accepted by the International Committee, and mapped the absorption spectrum. The _oxide_, Ho₂O₃, is a pale yellow powder; the _salts_ are yellow, with a faint orange tinge.

~Erbium~, Er = 167·7

Although erbia was separated by Mosander seventy years ago, it is doubtful if the perfectly pure oxide has ever been prepared. Whilst the individuality of the element is well established, its homogeneity has frequently been called in question. The name ‘Neo-Erbia’ was given by Cleve[372] to the residue left after the separation from the old erbia of ytterbia, scandia, thulia, and holmia (with which dysprosia (_q.v._) was also separated), but the spectrum examination of Kruss and Nilson[373] led them to regard Cleve’s oxide as still complex. Their results, however, were explained by the work of Hofmann and his pupils,[374] who consider erbia to be a homogeneous product; the homogeneity of the element, therefore, may be considered as established, though it would be strengthened by a more complete knowledge of the neighbouring elements, holmium and thulium.

[372] _Loc. cit._

[373] _Ber._ 1887, ~20~, 2134.

[374] _Ber._ 1908, ~41~, 308; also Hofmann, _ibid._ 1910, ~43~, 2631.

The element forms a rose-coloured oxide, and rose-coloured salts, which give to the compounds of the mixed erbia earths their characteristic colour. The oxide gives a very definite and characteristic reflection spectrum, but the salts do not possess this property;[375] the reflection spectrum remains unchanged in the presence of foreign oxides, provided no combination occurs. From the atomic weight determinations, it seems clear that the salts described by Cleve and his pupils[376] were not pure erbium compounds; a few salts only appear to have been recently obtained in the pure state for the atomic weight determination (_q.v._).

[375] See Kruss and Bugge, _Ber._ 1908, _41_, 3783.

[376] See _Compt. rend._ 1880, ~91~, 381.

The _sulphate_ separates from aqueous solutions at ordinary temperatures as the octohydrate, Er₂(SO₄)₃,8H₂O, which forms rose-coloured monoclinic crystals isomorphous with the corresponding sulphates of the whole group. The anhydrous sulphate is formed by long heating at 400°, more quickly at 475°, and can be heated to 630° without decomposition. At 845° a basic salt, Er₂O₃,SO₃, is formed, which begins to decompose at 950°; at 1055° the transformation to the oxide is complete. The ammonium and potassium double sulphates are easily soluble in cold water.

The _oxalate_ is thrown down in rosettes of bright rosy plates, which according to Hofmann[377] have the formula Er₂(C₂O₄)₃,10H₂O, even when dried in the air. Cleve believed the salt to be thrown down as the enneahydrate. When kept _in vacuo_ over phosphoric anhydride, the decahydrate passes into the trihydrate, which when heated decomposes, passing into the oxide at a temperature of 575°. The _nitrate_, Er(NO₃)₃,5H₂O, separates from aqueous solution as the pentahydrate, in large stable red crystals. The _platinocyanide_, Er₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour with green fluorescence. The _formate_, Er(HCOO)₃--Cleve, _loc. cit._--is a red powder, obtained by dissolving the oxide in formic acid; it crystallises from water as the dihydrate.

[377] _Loc. cit._

~Atomic Weight.~--The determinations of the earlier workers, being carried out with impure material, gave results which differ very widely, and are quite unreliable. Cleve’s value of 1880, for material free from ytterbia, but not apparently free from earths of lower equivalent, was 166·25; Brauner,[378] using the same material in 1905, obtained the much higher value 167·14. The determinations of Hofmann and Burger[379] in 1908 gave the mean value 167·38; with purer material, Hofmann in 1910[380] obtained the mean value 167·68, on which is based the value accepted by the International Committee, 167·7.

[378] Abegg, III. i. 318.

[379] _Loc. cit._

[380] _Loc. cit._

~Detection.~--Salts of erbium give in solution absorption spectra which are well defined and highly characteristic, though not so intense as those of praseodymium and neodymium. Hofmann and Bugge[381] give the following absorption maxima for a 10 per cent. solution of their pure nitrate in a layer of 15 mm. thickness:

667 weak 654 strong 541 very weak 523 very strong 519 shadowy 492 487 strong 450 442 weak

[381] _Ber._ 1908, ~41~, 3783.

The arc spectrum has been mapped by Eder and Valenta[382] and Exner and Haschek. The following lines are used by Eberhard[383] for purposes of detection:

3230·73 3264·91 3312·56 3372·92 3499·28 3692·85 3896·40 3906·47 3938·79

[382] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 18.

[383] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60.

~Thulium~, Tm = 168·5

The thulia isolated in 1879 was described by Cleve[384] as pale rose in colour; in the following year, having obtained it in larger quantity, he found that it was white, and dissolved in acids to form colourless solutions which showed absorption bands in the red and blue. The spectra of the thulium compounds prepared by Cleve were examined by Thalèn,[385] who concluded that a new element was certainly present, though it had not been freed from ytterbium and erbium. Incidental observations on the new oxide were made by various investigators, but no extensive researches were carried out upon it until 1911, when James[386] published an account of the separation and purification by the bromate method, stating that after some 15,000 operations, his products remained unaltered; he gives, however, no spectroscopic determinations, though part of his material, spectroscopically examined by Sir William Crookes, was described as ‘Very good thulium, with a trace of ytterbium.’ In the same year Auer von Welsbach[387] published an account of a spectroscopic investigation, as a result of which he concludes that thulium is a mixture of at least three elements, of which the second, Tm II, agrees fairly well in properties, so far as the two accounts allow of comparison, with the thulium of James.

[384] _Loc. cit._

[385] _Compt. rend._ 1880, ~91~, 376.

[386] J. _Amer. Chem. Soc._ 1911, ~33~, 1333.

[387] _Zeitsch. anorg. Chem._ 1911, ~71~, 439.

Thulia is described by James as a dense white powder, with a greenish tinge, which ‘emits a carmine coloured glow, when carefully made to incandesce.’ The salts have a greenish tint, very susceptible to traces of erbium; addition of erbium compounds turn the solution first yellowish-green, then colourless, and finally pink. von Welsbach describes Thulium II as forming an almost white sesquioxide, which, when heated in the flame, gives a purplish light quickly succeeded by a splendid characteristic glow; the salts are pale yellowish-green by daylight, emerald-green by artificial light, the colour being almost complementary to that of erbium salts. In solution, salts of Tm II give the bands at 685 and 464 ascribed by James and other workers to thulium.

Until further researches on these interesting results are published, the elementary nature of thulium cannot be considered definitely settled; it appears probable, however, that homogeneous salts of a definite element were obtained by James. The following salts are described by James (_loc. cit._).

The _chloride_, TmCl₃,7H₂O, separates at ordinary temperatures from the concentrated solution of the oxide in hydrochloric acid as greenish crystals, very soluble in alcohol and water. The _bromate_, Tm(BrO₃)₃,9H₂O, forms pale bluish-green hexagonal prisms, isomorphous with the analogous salts of the group. The _sulphate_ and _nitrate_ separate as the octohydrates. The precipitated _oxalate_ has the formula Tm₂(C₂O₄)₃,6H₂O, and is soluble in excess of alkali oxalate. The _acetylacetone derivative_ was prepared by dissolving the precipitated and well-washed hydroxide in alcoholic acetylacetone; it recrystallises from absolute (?) alcohol as the dihydrate, Tm₂(C₅H₇O₂)₆,2H₂O. The _phenoxyacetate_, Tm₂(C₆H₅·O·CH₂·COO)₆,6H₂O, was obtained in a similar manner by addition of the hydroxide to a solution of phenoxyacetic acid in dilute alcohol.

~Atomic Weight.~--Cleve gave the value 170·7 for this constant, but his material was very impure. In a footnote to a paper published in 1907, Urbain[388] pointed out that the value could not be above 168·5. Analyses of the salts prepared by James agree fairly well with the theoretical values calculated on this basis, but a systematic determination with pure material has not yet been made. The International Committee (1912) have adopted the value 168·5.

[388] _Compt. rend._ 1907, ~145~, 760.

~Detection.~--The element can be detected in solution by its absorption spectrum, the most intense bands being in the neighbourhood of λ = 685, and λ = 464. For provisional arc spectra see Exner and Haschek, and for spark spectra Auer von Welsbach (loc. cit.) and Eder and Valenta.[389]

[389] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 103.

~Ytterbium~ (Neoytterbium, Aldebaranium), Yb = 172·0. ~Lutecium~ (Cassiopeium), Lu = 174·0.

The first indication of the complexity of Marignac’s Ytterbium was furnished on spectroscopic grounds by Auer von Welsbach in 1905;[390] he showed that a separation could be effected by the fractional crystallisation of the ammonium double oxalates from concentrated ammonium oxalate. Three years later[390] he published a full account of his method, gave atomic weight determinations, and mapped the spectra of the two new elements. In 1907, Urbain[391] independently effected a separation by the fractional crystallisation of the nitrates from nitric acid, and proposed the names Lutecium (from the old name for Paris) and Neoytterbium for the elements.

[390] See _Monats._ 1908, ~29~, 204.

[391] _Compt. rend._ 1907, ~145~, 759.

The two new elements resemble one another so closely in chemical properties that the account given by Astrid Cleve in 1902[392] of the compounds of the old ytterbium applies in practically every detail to the new elements. The oxides are white, and yield colourless salts, showing in solution no absorption bands in the visible region.

[392] _Zeitsch. anorg. Chem._ 1902, ~32~, 129.

The _oxides_, R₂O₃, though perfectly white, are coloured yellow or brown by the faintest traces of thulium. They are attacked by acids only slowly in the cold, but dissolve readily on warming; lutecia is slightly the less strongly basic. The _chlorides_ crystallise with six molecules of water, and are extremely soluble and deliquescent; when heated in a stream of hydrogen chloride, they form oxychlorides of the type ROCl. The _platinocyanides_ crystallise with 18 molecules of water, and have the characteristic appearance of the analogous compounds of the yttrium elements. The _sulphates_ crystallise at all temperatures as the normal octohydrates, and are moderately easily soluble in water; conductivity measurements show that they are partially hydrolysed in solution. The _nitrates_ crystallise from concentrated aqueous or nitric acid solutions as the tetrahydrates; by evaporation of the aqueous solutions over sulphuric acid, the trihydrates are obtained. These compounds are anomalous among the rare earth nitrates, by reason of their low water content. The neutral _carbonates_ are thrown down by ammonium carbonate as the tetrahydrates; if a stream of carbon dioxide be led into aqueous suspension of the hydroxides, _basic carbonates_ of the formula R(OH)CO₃,H₂O, are obtained. The _oxalates_ are precipitated as the decahydrates; they are readily soluble in excess of alkali oxalate.

Many other salts of the old ytterbium have been prepared.

~Atomic Weights.~--The values determined by Urbain (_loc. cit._) for the fractions obtained by the nitrate method gave the number 170·1 for the least soluble fraction free from terbium, and 173·4 for the most soluble fraction. Auer von Welsbach (_loc. cit._) obtained the values 172·9 and 174·2 for the least soluble and most soluble fractions from the double oxalate crystallisation respectively. More recently[393] he has determined these constants with highly purified material, employing a modified method. The weighed anhydrous sulphates are transformed into the oxalates, which are then ignited to the oxides. He obtained the values Yb = 173·00, Lu = 175·00.

[393] _Monats._ 1913, ~34~, 1713.

The values adopted by the International Committee are Yb = 172·0 and Lu = 174·0.

~Spectra.~--The spark spectra are of more use in distinguishing the two elements than the arc spectra. The spark spectrum of the old ytterbium was mapped by Exner and Haschek,[394] and of the two compounds by both discoverers (_loc. cit._). See also Eder and Valenta.[395]

[394] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1899, ~108~, II_a_, 1123.

[395] _Ibid._ 1910, ~119~, II_a_, 3.

The arc spectra have been mapped by Eder and Valenta (loc. cit.) and by Exner and Haschek; the latter authors give as the most intense lines the following:

Yb Lu ┌────────────┴──────────────┐ 3031·26 2615·50 3397·21 4124·87 3107·99 2911·53 3472·65 4184·40 3289·50 3077·75 3507·57 4518·74 3464·47 3198·27 3508·55 5476·88 3988·16 3254·45 3554·58 5983·92 5556·67 3281·89 3568·00 5984·32 3312·30 3624·10 6222·10 3359·74 3636·41 6463·40 3376·69 3876·80

~Celtium~

The separation of Marignac’s ytterbium into the two elements described above was accomplished by Urbain with the yttria earths extracted from xenotime. In carrying out the same process with the ytterbia earths from gadolinite, that author[396] obtained from the mother-liquor an earth for which the coefficient of magnetisation was found to be 4·1 × 10⁻⁶; lutecia has a coefficient three to four times as great. A spectroscopic examination revealed the presence of lines which did not correspond with those of any known body, and Urbain considered that a new element, for which he proposed the name Celtium, with the symbol Ct, must be present. Lutecia from xenotime shows no trace of the new element.

[396] _Compt. rend._ 1911, ~152~, 141.

Spectroscopic evidence for the existence of a third ytterbium element had previously been brought forward by Auer von Welsbach[397] and also by Exner and Haschek.[398]

[397] _Monats._ 1908, ~29~, 204.

[398] Exner and Haschek, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 771.

The new element appears to be intermediate between lutecium and scandium, and therefore may be expected to have a higher atomic weight than the former element. Its chloride is more volatile than that of lutecium, less volatile than that of scandium; its hydroxide is more feebly basic than that of lutecium, but more strongly basic than that of scandium.

Urbain (_loc. cit._) gives the following as the principal lines in the spectrum; strong lines are denoted by a single, very strong by a double, asterisk:

2459·4 2469·3 2481·6 * 2536·9 * 2677·7 2685·2 ** 2729·1 * 2737·9 2765·8 ** 2834·3 * 2837·3 * 2845·2 * 2870·2 2885·1 * 2903·9 * 2931·9 2949·5 * 3080·7 ** 3118·6 ** 3171·4 * 3197·9 ** 3326·0 * 3391·5 * 3665·6

~Yttrium~, Yt = 89·0

Since the separation of yttria proper from the old yttria earths by Mosander, in 1842, the individuality of yttrium has been well established. The yttria of the workers of the sixties and seventies, to judge from the atomic weight determinations, must have been very impure, but no doubts were raised as to its homogeneity. By examination of the cathode luminescence spectra, Crookes[399] concluded that the oxide was of a complex nature; Lecoq de Boisbaudran, however, showed that the phenomena observed by Crookes were due to traces of impurity in his material, a conclusion confirmed by the work of Baur and Marc.[400]

[399] _Trans. Chem. Soc._ 1889, ~55~, 255.

[400] _Ber._ 1901, ~34~, 2460.

The oxide is the most strongly basic of all the yttria earths; in the basicity methods of separation, therefore, it collects in the end fractions, and is easily separated from the erbia and ytterbia earths by the nitrate fusion and similar processes. The terbia earths, however, which are comparable to it in basic strength, cannot be easily separated by such methods; processes of fractional crystallisation are very convenient in this case, since yttrium falls, with regard to the solubility of its simple salts, among the erbium group--between holmium and erbium generally--which is easily separated from the less soluble terbium elements. The separation of yttrium, therefore, affords an example of the combination of methods of both kinds.

The methods for the separation and purification of yttrium have recently been exhaustively examined by Meyer and Wuorinen.[401] They consider the chromate method suitable only if the terbium elements have already been removed. The ethylsulphate method is said to be tedious, whilst the ferrocyanide method indeed effects very rapid concentration, but with great loss. For purposes of concentration they find the most suitable method in the fractional hydrolysis of the phthalates; these salts are soluble in cold water, but hydrolyse when the solution is warmed, the most positive elements remaining of course longest in solution. For the final purification, they recommend fractional precipitation of the iodate from nitric acid solution; yttrium iodate being more soluble than the iodates of the erbium and ytterbium group, the latter collect in the first precipitates.

[401] _Zeitsch. anorg. Chem._ 1913, ~80~, 7; Meyer and Weinheber, _Ber._ 1913, ~46~, 2672.

Pure yttria is quite white, and gives rise to colourless salts, which in solution show no absorption spectrum in the visible region. A very large number of yttrium compounds have been prepared, of which sufficiently detailed accounts have been given in the general description of rare earth compounds. For an exhaustive treatment, the reader is referred to Abegg’s ‘Handbuch.’

The _metal_ has probably not been obtained in the pure state; impure yttrium has been obtained by Winkler[402] by the action of magnesium on the oxide, and by Cleve[403] by the action of sodium on a mixture of the chloride with common salt, and by electrolysis of the mixture of fused chlorides. It is described as a greyish metal, resembling iron in appearance; it oxidises in the air and readily decomposes boiling water. The _hydroxide_ is thrown down as a gelatinous precipitate by alkalies; ammonia throws down basic salts, but in presence of hydrogen peroxide an hydrated _peroxide_ is obtained. The _oxide_ absorbs carbon dioxide from the air, and liberates ammonia from ammonium salts.

[402] _Ber._ 1890, ~23~, 772.

[403] _Bull. Soc. Chim._ 1874, [ii.], ~21~, 344; Cleve and Höglund, _ibid._ 1873, [ii.], ~18~, 193; see also Popp, _Annalen_, 1864, ~131~, 359.

The anhydrous _chloride_ has been prepared by many authors; it melts at a relatively low temperature, 680°, and is the most easily volatilised of all the rare earth chlorides. After fusion, it forms a mass of brilliant white lamellæ.[404] It is characterised by the ease with which it dissolves in pyridine. From aqueous solution it separates as the hexahydrate, YtCl₃,6H₂O, which melts at 160°. The _bromide_ separates from solution as the enneahydrate, YtBr₃,9H₂O; the _bromate_[405] also separates with 9 molecules of water of crystallisation.

[404] _Compt. rend._ 1902, ~134~, 1308.

[405] James and Langelier, _J. Amer. Chem. Soc._ 1909, ~31~, 913.

The _nitrate_ cannot be obtained anhydrous; the normal hydrate, Yt(NO₃)₃,6H₂O, loses 3 molecules of water at 100°, but further heating converts it into basic salts. A _basic nitrate_, 3Yt₂O₃,4N₂O₅,20H₂O, is described by James and Pratt[406] as stable at ordinary temperatures, and in contact with solutions of the normal nitrate. The _sulphate_ octohydrate is isomorphous with analogous compounds of the rare earth elements, and with the _selenate_, Yt₂(SeO₄)₃,8H₂O; the latter compound can also form an enneahydrate. The _phosphate_, YtPO₄, occurs in nature in the mineral xenotime, and has been obtained in the laboratory in the crystalline form; many other phosphates have been prepared. The _platinocyanide_, Yt₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour with greenish-blue fluorescence.

[406] _J. Amer. Chem. Soc._ 1910, ~32~, 873.

Many _organic yttrium salts_ have been prepared by James and Pratt[407] and by Tanatar and Voljanski.[408]

[407] _J. Amer. Chem. Soc._ 1911, ~33~, 1330.

[408] _Vide Abstr. Chem. Soc._ 1910, ~98~, i. 809.

~Atomic Weight.~--The numbers obtained by the investigators who have determined this constant vary to such an extent that considerable uncertainty attaches to the value, 89·0, at present accepted by the International Committee. The determinations carried out prior to 1870 gave such diverse results that they are of little use in fixing the constant; since that date, all the investigations, with the exception of the most recent, have given values below 90, the sulphate method being generally employed.

Cleve and Höglund,[409] in 1883, carried out six determinations by the synthetic method; their results were concordant, and gave the mean value 89·57. Brauner considers this result if anything too low, as traces of undecomposed acid sulphate may have been present in the anhydrous sulphate. The same method was employed again by Cleve in 1884;[410] the mean of twelve very concordant results gave the number 89·11.

[409] _Loc. cit._

[410] _Compt. rend._ 1883, ~95~, 1225.

Much stress is laid by Brauner[411] on an unpublished determination of Marignac, carried out with material entirely free from terbia, which gave the value 88·88. H. C. Jones in 1895[412] carried out two series of determinations with material purified by Rowland’s method, _i.e._ precipitation with potassium ferrocyanide;[413] the results in both series were very concordant, the synthetic method giving the value 88·95, the analytical method the value 88·97. This work has been taken by the International Committee as the basis for the accepted value. According to Brauner, the ferrocyanide method does not give perfectly pure material.[414]

[411] Abegg’s _Handbuch_, III. i. 328.

[412] _Amer. Chem. J._ 1895, ~17~, 154.

[413] Rowland, _Chem. News_, 1894, ~70~, 68; compare also Crookes, _ibid._ ~70~, 81-82. Bettendorff (see Böhm, _Die Darstellung der seltenen Erden_, I. 480) has also used the method.

[414] See also Meyer and Wuorinen (_loc. cit._).

Egan and Balke[415] have recently found the ratio Yt₂O₃ : 2YtCl₃ to be very suitable as a basis for atomic weight determinations; the oxide is converted into the anhydrous chloride in a quartz flask. In a preliminary experiment, they obtain as a mean of three consistent determinations the provisional value 90·12; the yttria employed was considered to contain not more than one-half per cent. of erbia.

[415] _J. Amer. Chem. Soc._ 1913, ~35~, 365.

Recent work by Meyer and his co-workers[416] indicates that the accepted value is too high. Preliminary work with the synthetic sulphate method gave the values (corrected) 88·71 and 88·73; the mean value of six analytical sulphate determinations, made on material carefully purified by the iodate method, was 88·75, the extreme values being 88·71 and 88·76. They consider that the true atomic weight is 88·7, the value of the second decimal figure being a little uncertain.

[416] Meyer and Wuorinen; Meyer and Weinheber, _loc. cit._

~Detection.~--The spark spectrum of yttrium has been examined by many authors, and the ultraviolet as well as the visible regions have been mapped; _vide_ Exner and Haschek; Eder and Valenta, also Becquerel.[417]

[417] _Compt. rend._ 1908, ~146~, 683.

The arc spectrum has been examined by Kayser, Eberhard,[418] and Eder and Valenta;[419] Exner and Haschek give the following as the most intense lines:

[418] _Zeitsch. wiss. Photochem._ 1909, ~7~, 245.

[419] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, IIa, 1.

3216·83 3242·42 3328·02 3600·92 3611·20 3621·10 3633·28 3664·78 3710·47 3774·52 3788·88 3950·52 3982·79 4077·54 4102·57 4128·50 4143·03 4177·74 4302·45 4309·79 4348·93 4375·12 4883·89 6191·91 6435·27

Pure yttrium compounds should be colourless, show no absorption in the visible region, and yield a perfectly white oxide.

~Scandium~, Sc = 44·1

The scandia obtained by Nilson in 1879 was isolated from the minerals gadolinite and euxenite; it consisted very largely of ytterbia, as shown by spectrum examination[420] and by atomic weight determinations, which gave the value 90. In the same year[421] Cleve prepared the oxide in a much purer state, using as his source the minerals gadolinite and keilhauite; he described several salts, carried out atomic weight determinations by the analytical and synthetic sulphate methods, and showed that scandium corresponds with the Eka-boron of which the existence was predicted by Mendelejeff in 1871.[422] Starting from a large quantity of euxenite, Nilson[423] in the following year prepared several grams of approximately pure scandia, which contained only traces of ytterbium.

[420] Thalén, _Compt. rend._ 1879, ~88~, 642; 1880, ~91~, 45.

[421] _Compt. rend._ 1879, ~88~, 419.

[422] See also Mendelejeff, _Ber._ 1881, ~14~, 2821.

[423] _Ber._ 1880, ~13~, 1439.

The investigation of scandium, which occurs only in extremely small quantities in the minerals employed by Nilson and Cleve, and was therefore believed to be exceedingly rare, was not continued until 1908, when Sir William Crookes[424] made a systematic investigation of a large number of minerals in order to find a convenient source of the element. He showed that scandium is present in many rare earth minerals, and selected as the most suitable for the extraction of the element a complex mineral named Wiikite, some specimens of which he found to contain over 1 per cent. of scandia (see p. 70). The mineral was decomposed by fusion with potassium hydrogen sulphate, and scandia extracted from the rare earths by the nitrate fusion. The separation effected on these lines was very thorough, Crookes considering a specimen of scandia unsatisfactory if it showed any trace of the dominant ytterbium line, 3694·344, on an over-exposed plate, or if it gave an atomic weight for the element higher than 44·1.

[424] _Phil. Trans._ 1908, A, ~209~, 15.

A systematic investigation of the common rocks and minerals for scandium was carried out by Eberhard in 1908, as a result of which processes for the extraction of the oxide from wolframite were worked out by R. J. Meyer (see pp. 3, 131). Wolframite is a tungstate of iron and manganese, containing, in addition to other oxides, small quantities of the rare earths, of which considerable proportions are found to be scandia. The mineral is fused with soda in the usual way, and the rare earths concentrated by the oxalate method. Scandium is precipitated as the fluoride by addition of sodium silicofluoride to the boiling acid solution, and purified by precipitation as the double ammonium tartrate.[425]

[425] Meyer and Goldenberg, _Chem. News_, 1912, ~106~, 13.

Whilst the researches of Crookes and Eberhard have shown how widely distributed the element really is, the minerals which they found richest in scandium still contained extremely small quantities of the oxide. The discovery of the mineral Thortveitite (see p. 44), which contains about 37 per cent. of scandia, is therefore of the greatest scientific interest, and will doubtless allow of a very searching examination of the properties of this interesting element.

Whilst the low atomic weights of scandium and yttrium place them, to some extent, apart from the other rare earth elements, the latter element at least is so closely allied in properties to the other members of the group that yttria is one of the typical oxides of the family. Scandium and its compounds, however, present many peculiarities of behaviour when compared with the typical members, on the grounds of which Urbain[426] has contended that scandia should not be classed among the rare earths at all. Whilst this contention is perhaps rather extreme, especially in view of the fact that in nature scandia always occurs with other yttria oxides, it must be admitted that in many respects the element is anomalous. The oxide is the weakest base of the whole group, yet the oxalate is comparatively readily soluble in mineral acids (compare p. 132), and the potassium double sulphate is almost insoluble in potassium sulphate. The sulphate is altogether exceptional in that it is very easily soluble in water, and crystallises out with 6 molecules of water of crystallisation. The fluoride and the carbonate both dissolve readily in excess of precipitant, whilst sodium thiosulphate precipitates a basic salt from neutral solutions.

[426] _Chem. News_, 1905, ~90~, 319.

Meyer has pointed out the close resemblance between beryllium and scandium. The oxide and salts are colourless; the latter have a peculiar sweet astringent taste, and readily yield basic salts.

The _hydroxide_, Sc(OH)₃, is thrown down by alkalies as a bulky white gelatinous mass; the _oxide_ is a white powder, less readily soluble in dilute acids than most of the rare earths. The _fluoride_ is important on account of its insolubility in mineral acids, which exceeds that of all the other rare earth fluorides, and approaches that of thorium. It is thrown down from neutral or acid solutions by addition of hydrofluoric acid or a soluble fluoride; if the solution be boiled, a soluble silicofluoride will also precipitate scandium fluoride, though no precipitate is obtained in the cold. This behaviour is due to the ease with which the silicofluoride is hydrolysed at high temperatures, according to the equation:

Sc₂(SiF₆)₃ + 6H₂O = 2ScF₃ + 3SiO₂ + 6H₂F₂

and is of great value in separating scandium from the other earths. The fluoride is extremely resistant to acids, being completely decomposed only by fused bisulphate. In the absence of acids, the freshly precipitated fluoride dissolves in excess of concentrated alkali fluoride, forming double salts; in this behaviour, scandium resembles zirconium, but differs from thorium and the cerium and yttrium elements.

The _chloride_ separates from solution at ordinary temperatures as the dodecahydrate, Sc₂Cl₆,12H₂O, which loses 9 molecules of water when kept for six hours at 100°. The trihydrate Sc₂Cl₆,3H₂O, is converted into scandia at a red heat, with the loss of 6 molecules of hydrogen chloride. The _iodate_, Sc(IO₃)₃,18H₂O, is obtained as an almost insoluble white crystalline powder by addition of ammonium iodate to a salt in solution; hydrates with 15, 13, and 10 molecules of water are known, and at 250° the anhydrous compound is obtained. It resembles the iodates of the cerium and yttrium group in being soluble in strong nitric acid, but the separation of thoria and scandia by this method is tedious and unsatisfactory.[427]

[427] Meyer, Winter and Speter, _Zeitsch. anorg. Chem._ 1911, ~71~, 65.

The _platinocyanide_, Sc₂[Pt(CN)₄]₃,21H₂O, was obtained by Crookes[428] by double decomposition of the sulphate with barium platinocyanide, in crimson monoclinic prisms, with a green fluorescence. It dissolves in water to a colourless solution. Orlov[429] shows that it can occur also in a second form, stable at higher temperatures; this is yellow, with a blue fluorescence and crystallises with 18 molecules of water. The two modifications resemble respectively the platinocyanides of the yttrium and of the cerium elements; in this respect, therefore, scandium occupies an intermediate position between the two groups.

[428] _Phil. Trans._ 1910, A, ~210~, 359.

[429] _Abstr. Chem. Soc._ 1913, ~104~, i. 27.

The _sulphate_, Sc₂(SO₄)₃, is obtained anhydrous by evaporating the excess of acid from a solution of the oxide in the concentrated acid, care being taken to avoid too high a temperature. The compound dissolves very easily in water, and slowly hydrates itself with evolution of heat; no crystals can be obtained from the solution until it has been concentrated to the consistency of a syrup, when on cooling it slowly deposits the hexahydrate. This effloresces in a dry atmosphere, forming the pentahydrate, which appears to be the most stable hydrate at ordinary temperatures. According to Nilson, the hexahydrate loses 4 molecules of water when maintained at 100°. At 250° it becomes anhydrous; above that temperature, basic salts are formed. The _potassium double sulphate_, 3K₂SO₄,Sc₂(SO₄)₃, was shown by Nilson to resemble the analogous cerium compounds in being insoluble in a saturated solution of potassium sulphate. The _nitrate_, Sc(NO₃)₃,4H₂O, separates from concentrated solutions over sulphuric acid as the tetrahydrate; it is very soluble in water and alcohol, and extremely deliquescent.

The _carbonate_, Sc₂(CO₃)₃,12H₂O, is thrown down by addition of ammonium carbonate as a bulky white precipitate, easily soluble in a hot solution of the precipitant; the solubility in excess may be used in the separation of scandia from yttria. Addition of water to such solutions causes separation of a basic carbonate, but crystalline _double carbonates_ may be obtained by evaporation of concentrated solutions containing a large excess of alkali carbonate. The sodium compound, Sc₂(CO₃)₃,4Na₂CO₃,6H₂O, is very sparingly soluble, and has been used in the separation from thorium. The _oxalate_, Sc₂(C₂O₄)₃,5H₂O, differs from other oxalates of the group, which generally separate with 10 molecules of water of crystallisation, not only in its water content, and in its solubility in acids, but also in the ease with which it forms double oxalates soluble in excess of alkali oxalate; in this latter property it shows a further resemblance to zirconium and thorium. The _formate_ and _acetate_ have the formulæ Sc(OH)(HCOO)₂,H₂O and Sc(OH)(CH₃COO)₂,2H₂O, respectively. A large number of organic salts have been described by Sir William Crookes.[430]

[430] _Loc. cit._; see also Meyer, _Zeitsch. anorg. Chem._ 1908, ~60~, 134; Meyer and Winter, _ibid._ 1910, ~67~, 398.

~Atomic Weight.~--The mean values obtained by Cleve[431] in 1879 were 44·96 and 45·20 by the analytical and synthetic sulphate methods respectively. In the following year Nilson,[432] using purer material, obtained the value 44·13 by the synthetic method. Meyer and others (_loc. cit._) have criticised Nilson’s estimation on the ground of his empirical method of obtaining the neutral anhydrous sulphate. Determinations made with material purified from thorium by the iodic acid method gave the values 44·11, 44·11, 44·20; material purified by the double ammonium tartrate method gave the atomic weight 43·90. Meyer has shown that small quantities of thoria in the oxide cannot be detected spectroscopically; the value of the magnetisation coefficient, however, showed the oxide obtained by the last method to be free from thoria, and he considers another determination of the atomic weight to be necessary.

[431] _Loc. cit._

[432] _Loc. cit._

The value accepted by the International Committee is 44·1.

~Detection.~--Scandium gives no absorption spectrum in the visible region. The spark spectrum has been examined by Thalèn (_loc. cit._) and Nilson;[433] see also Exner and Haschek, Lockyer and Baxendall,[434] and Crookes (_loc. cit._). The arc spectrum has been examined by Fowler,[435] Eder and Valenta,[436] and Exner and Haschek.

[433] _Compt. rend._ 1880, ~91~, 56, 118.

[434] _Proc. Roy. Soc._ 1905, ~74~, 538.

[435] _Phil. Trans._ 1908, A, ~209~, 47.

[436] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 576.

The most intense lines of the arc spectrum are the following:

3353·90 3372·33 3558·69 3567·89 3572·73 3576·53 3614·00 3630·93 3642·99 3907·69 3912·03 4020·60 4023·88 4247·02 4314·31 4320·98 4325·22 4374·69 4400·63 4415·78 6305·94

Fowler (_loc. cit._) examined the arc spectrum with reference to solar spectra. For detection of the element in minerals see Crookes (_loc. cit._) and Eberhard (_loc. cit._).

The purity of scandium preparations may be determined by the following tests:

(1) Precipitation with thiosulphate in boiling solution should remove all the rare earth content from solution.

(2) The iodate test for thorium should give no result.

(3) The oxide must be perfectly white, and salt solutions show no absorption.

(4) R. J. Meyer has found that whilst 0·5 per cent. of thoria cannot be detected spectroscopically in scandia, the magnetisation coefficient affords an exceedingly delicate test. The value for pure scandia is -0·12 × 10⁻⁶, the oxide being diamagnetic; for scandia with 0·5 per cent. thoria the coefficient was found to be +0·04 × 10⁻⁶, the mixture being paramagnetic.