Chapter I, and under Gadolinite, p. 35). The discovery of Ceria followed

in 1804 (see under Cerite, p. 32). The classical work of Mosander, carried out between 1838 and 1842, showed the complex nature of the new oxides. From ceria he separated three new earths, Ceria proper, Lanthana, and Didymia. Yttria was shown to be a mixture of at least three oxides, for which the names Yttria, Erbia, and Terbia were proposed. These oxides were believed to have the general formula RO, by analogy with the alkaline earths, which they were found to resemble in many respects, notably in their strongly basic character.

The properties of the new oxides were examined during the next twenty years by many chemists, the chief workers being Marignac, Rammelsberg, and Hermann, but the next important advance was the investigation of the absorption spectra of solutions of the rare earth salts, first suggested by Gladstone in 1856, and developed more fully by Bunsen and Kirchhoff in 1860 and the following few years. The introduction of the methods of spectrum analysis furnished a very delicate and valuable method of examining and identifying the various oxides, and so greatly assisted the laborious processes of separation.

Sixteen elements (excluding thorium and zirconium) are at the present time recognised as belonging to the rare earth group. With one or two exceptions, these show the closest resemblance to one another, both in chemical behaviour and in the properties of their compounds, so that the difficulties of separating and purifying them are very great. They may be said to form a series, in which the properties vary continuously but gradually from member to member, so that no sharp differences are anywhere perceptible. The method of division into groups is, therefore, almost entirely one of convenience, and has arisen from the course which the separations have followed.

The elements are divided into two chief families or groups, that of the cerium metals and that of the yttrium metals respectively. The cerium elements are separated by a process depending on the relative insolubility of their alkali double sulphates; in this group are included cerium, lanthanum, praseodymium, neodymium, and samarium. The yttrium family is further divided into four sub-groups: the first consists of scandium and yttrium; the second or terbium group of europium, gadolinium, and terbium; the third or erbium group of dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium group of ytterbium and lutecium--the element celtium, recently discovered by Urbain, will also fall into this sub-group, but the discovery awaits confirmation. Whilst scandium and yttrium fall into somewhat abnormal positions, corresponding to their low atomic weights, the terbium elements occupy an intermediate position between the cerium elements and the remaining yttrium elements, or yttrium group proper, and so are frequently classified as a third or intermediate group.

This list does not include all the names which have been put forward to designate what have been claimed from time to time as new elements; whilst the individuality of some of those included is not yet fully established, and the homogeneity of others has been called in question. The uncertainty is more pronounced among the yttrium elements than among the cerium elements; owing to the opportunities for investigation furnished by the commercial treatment of monazite, the chemistry of the cerium group may be regarded as complete.

In the following table the elements are arranged in order of increasing atomic weight, and it can be seen at once how closely the division into groups follows this order:

ELEMENT ATOMIC WT. COLOUR OF SALTS {Scandium, Sc 44·1 Colourless {Yttrium, Yt 89·0 Colourless

{Lanthanum, La 139·0 Colourless {Cerium, Ce 140·25 Cerous, colourless; ceric, Cerium { orange to red Group. {Praseodymium, Pr 140·6 Green {Neodymium, Nd 144·3 Red to reddish-violet {Samarium, Sa 150·4 Topaz yellow

Terbium {Europium, Eu 152·0 Faint rose Group. {Gadolinium, Gd 157·3 Colourless {Terbium, Tb 159·2 Colourless

{Dysprosium, Dy 162·5 Bright green Erbium {Holmium, Ho 163·5 Yellow to orange Group. {Erbium, Er 167·7 Deep rose {Thulium, Tm 168·5 Bluish-green

Ytterbium {Ytterbium, Yb 172·0 Colourless Group. {Lutecium, Lu 174·0 Colourless

In their chemical relations, the rare earth elements may be placed between the metals of the alkaline earths, and the trivalent metals iron, aluminium, and chromium. With the exceptions of cerium in the ceric salts, and of samarium and europium in the recently discovered dichlorides, they are uniformly trivalent, but the oxides are very strong bases, and the salts very slightly hydrolysed in dilute solutions; generally, therefore, they resemble the calcium family rather than the aluminium group. Among the common salts, the oxalates, phosphates, chromates, iodates, fluorides, carbonates, tartrates, and borates are almost insoluble; the sulphates are only sparingly soluble at ordinary temperatures. Among the double salts, the alkali double sulphates are of great importance from their employment for separations; the tendency to the formation of complex salts is greater among the yttrium than among the cerium elements, increasing with the atomic weight, and with the decrease in basic strength of the oxides.

The great similarity in chemical behaviour of the rare earth elements is apparent not only in the similarity in composition, solubility and chemical properties of the salts--which is so great that the general account of the compounds which follows applies almost in its entirety to each member of the group--but also in the crystallographic relations between corresponding compounds. Many of the salt hydrates form isomorphous series; the sulphate octohydrates, for example, appear to be isomorphous throughout the whole group, and probably the relation would be found to apply even more completely than is generally accepted, if the necessary data were forthcoming. Of great interest and practical importance is the isomorphism between the nitrates and double nitrates of the cerium elements and bismuth, which has been utilised with such valuable results in the processes of fractional crystallisation.

~The Metals.~--The earlier attempts to reduce compounds of the rare earth elements to the metallic condition, by means of metallic sodium or potassium, did not yield pure products; nor did the use of aluminium or magnesium lead to results of practical importance. The metals were first obtained in a coherent physical condition by Hillebrand and Norton,[147] by electrolysis of the fused chlorides. These investigators obtained cerium, lanthanum, and the so-called didymium, and measured their specific heats; their results confirmed the atomic weights assigned to the elements by Mendelejeff, except in the case of lanthanum. Their method has since been elaborated by Muthmann, Hofer and Weiss,[148] who have prepared large quantities of the cerium elements in the pure state. More recently, Hirsch has prepared metallic cerium in large quantities,[149] and has studied its properties.

[147] _Pogg. Ann._ 1875, ~155~, 631; ~156~, 466.

[148] _Annalen_, 1902, ~320~, 231; see also Muthmann and Weiss, _ibid._ 1904, 331, 1.

[149] _Met. Chem. Eng._ 1911, ~9~, 543.

By electrolytic reduction of the mixed chlorides of the cerium elements, a mixture known as ‘Misch metal’ is obtained; this has powerful reducing properties, and, like aluminium, reduces the oxides of iron, chromium, etc., with great development of heat.[150] The yttrium metals have not yet been obtained in the pure state, the electrolytic method giving unsatisfactory results on account of the high melting-points of the metals, and the volatile nature of their chlorides.

[150] A full account of the properties and preparation of the cerium metals and their alloys will be found in the monograph of Kellermann, ‘_Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp, Halle, 1912.

The cerium metals are white or slightly yellowish in colour, and are moderately stable in dry air. In moist air they tarnish slowly, lanthanum, as the most positive, being most readily oxidised. The melting-points and specific gravities are as follows:

Element Melting-point Specific Gravity

Cerium 623° 7·0242 Lanthanum 810° 6·1545 Praseodymium 940° 6·4754 Neodymium 840° 6·9563 Samarium 1300°-1400° 7·7-7·8

The metals decompose water slowly in the cold, but rapidly at the boiling-point, with evolution of hydrogen. They have a great affinity for oxygen, the heats of formation of the oxides being of the order of those of alumina and magnesia:

Heat of Formation per Equivalent Weight of Oxide[151]

¹⁄₃La₂O₃ 74·1 K ¹⁄₃Nd₂O₃ 72·5 „ ¹⁄₃Pr₂O₃ 68·7 „ ¹⁄₄CeO₂ 56·1 „ ¹⁄₃Al₂O₃ 64·3 „ ¹⁄₂MgO 71·9 „

[151] Muthmann and Weiss, _loc. cit._; K = 1 kilogram-calorie, or 1000 cal.

In consequence of the high values of the heats of combustion, the metals have powerful reducing properties.

The cerium metals form alloys with magnesium, zinc, aluminium, and iron, and combine with boron and silicon. The alloys of cerium, and the metal itself, are remarkable for their property of emitting brilliant sparks when scratched (see Chapter XXI). Cerium also forms an amalgam with mercury.

The metals burn brilliantly when heated in oxygen, and dissolve readily in dilute mineral acids. When heated to a temperature of 200°-300° in a current of hydrogen, they absorb the gas very readily, forming the _hydrides_. These compounds are also obtained by heating the oxides with magnesium in a current of hydrogen. They were first prepared by Winkler,[152] who deduced from his analyses the general formula RH₂; the more recent work of Muthmann and Beck,[153] however, points to the formula RH₃.

[152] _Ber._ 1890, ~23~, 2642; 1891, ~24~, 873.

[153] _Annalen_, 1904, ~331~, 58.

If nitrogen be substituted for hydrogen in either of the above methods of preparation, _nitrides_ of the general formula RN are obtained; cerium nitride, however, cannot be obtained by heating the element in the gas.[154] These compounds are also obtained when the carbides are heated in ammonia. They are amorphous solids, which yield ammonia when acted upon by water.

[154] Dafert and Miklanz, _Monats._ 1912, ~33~, 911.

~Hydroxides.~--The hydroxides are thrown down as gelatinous precipitates on the addition of alkalies to hot dilute solutions of the salts; precipitation in the cold, or in strong solution, usually gives a basic salt, or an hydroxide mixed with a large quantity of basic salt. The hydroxides are insoluble in excess of precipitant, but the precipitation is inhibited by the presence of some organic hydroxy-acids.[155]

[155] For effect of tartaric acid, see p. 133.

The hydroxides are insoluble in water, but dissolve very readily in acids. The most basic of them absorb carbon dioxide from the air; lanthanum hydroxide is exceptional in that it colours litmus blue.

Whilst hydrogen peroxide in neutral solution does not react with rare earth salts,[156] alkalies in presence of this reagent precipitate gelatinous hydrated peroxides, which are very unstable, decomposing on standing, or on treatment with acids, with evolution of oxygen. The general formula R₄O₉ + _x_H₂O was proposed for these compounds by Cleve, but more recently the formula R(OOH)(OH)₂ has been advanced.[157]

[156] Compare behaviour of thorium and zirconium, Ch. XVI.

[157] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~, 70; Melikoff and Klimento, _Chem. Zentr._ 1902, ~1~, 172.

~Oxides.~--In their most stable state of oxidation, the rare earth elements are generally trivalent. In the case of cerium, the dioxide, CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are unstable, and are very readily reduced to cerous compounds, corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty among the other elements only in the cases of praseodymium and terbium, but these do not give rise to salts.

The oxides R₂O₃ are fairly strong bases, being comparable in strength to the alkaline earths, and far more strongly basic than alumina and oxides of other trivalent elements; thus they liberate ammonia from ammonium compounds, whilst the salts they form with strong acids are not easily hydrolysed. Their relative strengths as bases are expressed in the following series, in which the elements are placed in order of diminishing electropositive character:[158]

La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Ce^{iv}.

[158] The position of yttrium in this series is not known with certainty; it is probably as positive as neodymium. It is usually stated (see Meyer and Hauser, pp. 32-33) that the terbia oxides are intermediate in basic strength between the ceria and yttria earths, though the arrangement into two series, consisting of the cerium and yttrium groups respectively, is generally adopted; the electropositive character of the elements in each series then weakens as the atomic weight rises, scandium being of course exceptional.

It will be seen that, with the exception of scandium and yttrium, the metals of the cerium and yttrium groups become less electropositive as the atomic weight increases.

This arrangement is obtained by ascertaining the order in which the various hydroxides are precipitated from a solution by gradual addition of a dilute solution of a strong base. The weakest base is precipitated first, and the strongest last; those intermediate in strength are thrown down in ascending order of strength. Similar results may be obtained by the fractional decomposition of the nitrates by heat; in this case the nitrate of the weakest base is decomposed at the lowest temperature. This order is also confirmed, as far as the data are available, by measurements of the equivalent conductivities of solutions of the salts (see, for example, p. 122).

Quite recently, a very different order has been obtained from a consideration of the dissociation tensions, and of the heats of dissociation of the anhydrous sulphates.[159] In the following table the elements are arranged in the order of the increase of the dissociation tension (T) measured at 900°, which is the same as the order of decrease of the heats of dissociation (Q):

Element At. Wt. T. (Mm. Hg.) Q. La 139·0 2 59·8 Yt 89·0 3 58·9 Lu 174·0 3·5 58·5 Yb 172·0 4 58·2 Er 167·7 5 57·6 Pr 140·6 5·5 57·4 Nd 144·3 6 57·2 Gd 157·3 7 56·9 Sa 150·4 8 56·5 Sc 44·1 11 54·5 Ce 140·25 52·4

[159] Wöhler and Grünzweig, _Ber._ 1913, ~46~, 1726.

It will be observed that the order is very different from the order of increase of atomic weight, the positions of lutecium and ytterbium being especially surprising; these elements are generally considered to be among the least electropositive of the whole series. The anomalous position of cerium is probably due to the fact that the sulphate on decomposition leaves the dioxide, and not the sesquioxide, as with the other elements; this would undoubtedly affect the values. The heats of dissociation are the greatest yet observed for the sulphates of trivalent metals, a further evidence of the strongly basic nature of the oxides.

Ignited lanthana resembles quicklime in that it readily absorbs carbon dioxide from the air, and hisses when slaked with water; as the basicity becomes weaker, the affinity for water and carbon dioxide becomes less marked. All the oxides are soluble in dilute acids, even after prolonged ignition; but the ease with which solution occurs is naturally much influenced by the treatment to which the oxide has been subjected, as well as by its strength as a base.

The rare earth oxides are capable of existing in more than one modification, the compounds obtained by ignition of the hydroxides differing in appearance and reactivity from those prepared by ignition of the oxalates or nitrates, and so on; they are probably highly polymerised. Cerium dioxide, CeO₂, is remarkable for its power of combining with the other oxides, R₂O₃, of the rare earth metals. The pure dioxide is insoluble in nitric acid, but mixtures of earths containing up to 50 per cent. of the dioxide dissolve readily. The various colours of mixtures of the ceria earths may sometimes be attributed to a similar combination,[160] and there can be little doubt that the dioxide sometimes functions as an acid in the rare earth minerals.

[160] The brown colour of a mixture of ceria oxides containing praseodymium is generally attributed to the presence of the strongly coloured peroxide of that element.

~Sulphides.~--These compounds cannot be prepared in the wet way, that is, by the action of hydrogen sulphide or ammonium sulphide on the salts in solution; the former reagent gives no precipitate, the latter throws down the hydroxides. In this behaviour, the rare earth elements resemble aluminium and chromium.

The normal sulphides, R₂S₃, are obtained by reduction of the anhydrous sulphates, or from the oxides at high temperatures, by treatment with hydrogen sulphide. They are strongly coloured compounds, fairly stable towards cold water, but readily hydrolysed on boiling.

Disulphides, RS₂, are known in the cases of cerium, lanthanum, and praseodymium; these are to be regarded as polysulphides, since on treatment with dilute acids they yield hydrogen persulphide, H₂S₂.

~Carbides.~--By reduction of the oxides with carbon in the electric furnace, Moissan obtained the carbides in the form of microscopic yellow crystals. They have the general formula RC₂, and are attacked by water and dilute acids, with evolution of very complex mixtures of gases.[161] The principal product is acetylene, with various higher homologues, and in smaller quantities ethylene and ethane and their homologues. No methane is formed,[162] but hydrogen is always present, the olefines and paraffins probably arising from its action on the acetylenic hydrocarbons. The relation of the rare earth elements to the calcium group is here very close; calcium carbide when attacked by water yields pure acetylene, whereas aluminium carbide gives pure methane.

[161] Damiens, _Compt. rend._ 1913, ~157~, 214.

[162] Moissan stated that 24-30 per cent. of methane was formed in this action; compare _Compt. rend._ 1900, ~131~, 595.

~Halogen Salts.~--The halides of the rare earth elements show a close analogy with the corresponding compounds of the alkaline earth elements. The _fluorides_ are insoluble in water and dilute mineral acids, and are obtained as gelatinous precipitates by the addition of hydrofluoric acid, or a soluble fluoride, to solutions of the salts. They may be prepared in the crystalline condition by heating the carbides in a stream of fluorine, or by the action of hydrofluoric acid upon the hydroxides in aqueous suspension. The rare earth elements, as well as thorium, may be separated from zirconium by taking advantage of the insolubility of their fluorides in excess of hydrofluoric acid or alkali fluorides, since zirconium fluoride is readily soluble in excess of the precipitant. The solubility of the fluorides in a large excess of concentrated acid increases with the electropositive character of the metal, the fluorides of the more negative elements being the least soluble. Thorium and scandium may, therefore, be concentrated to a large extent by repeated precipitation with hydrofluoric acid in acid solution.

The _silicofluorides_ of the rare earth elements have been used by R. J. Meyer in the extraction of scandium from wolframite (see Chapter I and under Scandium, p. 215). They are thrown down as gelatinous precipitates on addition of potassium or sodium silicofluoride to boiling, neutral solutions of rare earth salts. In presence of mineral acids, however, they are not thrown down in the cold; on boiling, the cerium metals are precipitated as fluorides, by hydrolysis of the silicofluorides--the yttrium elements, with the exception of scandium, being held in solution by the mineral acid.

With the exception of the fluorides, the halogen salts of the rare earth metals are readily soluble in water, and crystallise from the concentrated solutions in the hydrated form. The bromides and iodides have not been so fully studied as the chlorides; they are hygroscopic salts, and decompose rather easily. The iodides have been obtained by Moissan in the anhydrous state, by the action of iodine vapour on the carbides at high temperature.

The anhydrous _chlorides_ may be obtained by the application of any of the ordinary methods, _e.g._ by heating the oxides with carbon in a stream of chlorine, by heating the carbides in the same gas, by heating the sulphides or hydrated chlorides in hydrogen chloride, or by evaporating the solutions of the hydrated salts to dryness in presence of ammonium chloride, and then igniting till the latter has all been removed. As obtained by any of these methods, they are fusible at a red heat, but only slightly volatile; they are easily soluble in water or alcohol, with disengagement of heat. They are insoluble in most organic solvents, but dissolve to some extent in some bases; the chlorides of the yttrium elements, for example, are readily soluble in pyridine. With such solvents, the chlorides form compounds which may be considered as derived from the hydrated forms, by replacement of the so-called water of crystallisation by the organic base.

Conductivity measurements show that the salts are not perceptibly hydrolysed in moderately dilute aqueous solutions, though the values for the equivalent conductivities vary somewhat with the variations in the electropositive character of the elements. In the following table, the equivalent conductivities of the chlorides in solutions of dilution 32 and 1024 at 25°C. are given. It will be seen that the value (λ₁₀₂₄ - λ₃₂) ÷ 10 is in all cases (except for the highly hydrolysed scandium salt) very close to 3, an experimental proof of the trivalent nature of the elements. The values for the chlorides of iron, aluminium and chromium are included; it will be seen that these elements are considerably less positive than the rare earth metals (with the exception, of course, of scandium).

Salt λ₃₂ λ₁₀₂₄ λ₁₀₂₄ - λ₃₂ LaCl₃ 105·8 131·5 25·7 CeCl₃ 107·8 135·2 27·6 PrCl₃ 105·5 135·9 30·4 NdCl₃ 103·8 134·3 30·5 YtCl₃ 98·8 123·4 24·6 YbCl₃ 107·4 140·4 33·0 ScCl₃ 116·9 257·9 141·0

AlCl₃ 99·9 138·0 38·1 CrCl₃ 98·4 152·6 54·2 FeCl₃ 117·2 200·7 83·5

From aqueous solutions the chlorides crystallise with six molecules of water, except praseodymium chloride, which has seven. The hydrated salts, when heated to 120° in the air, form insoluble oxychlorides of the general formula ROCl.

The chlorides do not show a great tendency to form double salts with other metallic chlorides; on the other hand, they readily form complex compounds with the chlorides of the less electropositive metals, e.g. tin, bismuth, gold, and platinum.

Subchlorides of samarium and europium have recently been obtained; in these compounds, for the first time, rare earth metals have been shown to be capable of functioning as divalent elements.

_Cyanides_ of the rare earth elements are not known; addition of potassium cyanide to solutions of the salts throws down the hydroxides. The _platinocyanides_ may be obtained by double decomposition of the sulphates with barium platinocyanide. They are very stable and characteristic bodies, of the general formula R₂[Pt(CN)₄]₃, with 18 or 21 molecules of water. The compounds of the cerium elements are yellow, with a strong blue fluorescence; they crystallise in the monoclinic system. The platinocyanides of the yttrium metals are red or crimson, with a splendid green fluorescence, and crystallise in the rhombic system. Scandium platinocyanide is of great interest from the fact that it exists in two modifications, which show the characteristic appearance of the two groups of compounds respectively.

Potassium ferrocyanide precipitates _potassium earth ferrocyanides_ of the general formula KR(FeC₆N₆),3H₂O, from neutral solutions;[163] the precipitate is somewhat soluble in excess. The ferrocyanides have been proposed for the purification of yttrium; the method is useful where rapid concentration of the element is required, yttrium ferrocyanide being far more soluble than the analogous compounds of the erbium and ytterbium metals, but the precipitates are gelatinous, and very difficult to handle.

[163] Compare Astrid Cleve, _Zeitsch. anorg. Chem._ 1902, ~32~, 129.

~Halogen Oxy-salts.~--_Perchlorates_ and _periodates_ of the rare earth elements, of the general formula R(XO₄)₃,_x_H₂O, have been obtained. The existence of _chlorates_ has been observed only in the yttrium group; yttrium chlorate, Yt(ClO₃)₃,8H₂O, has been prepared by double decomposition of the sulphate with barium chlorate. The _bromates_ are also prepared in this way. They are readily soluble compounds, of which several hydrated forms are known. They are of considerable importance for purposes of separation in the yttrium group.

The _iodates_ are sparingly soluble bodies, precipitated by addition of the alkali compound to solutions of the rare earth salts. The rare earth iodates are soluble in nitric acid, the solubility increasing as the electropositive character of the element becomes stronger. A method for the purification of yttrium has recently been based upon this property of the iodates, whilst the fact that thorium iodate is completely insoluble in nitric acid allows of the easy separation and estimation of thorium in minerals or mixtures containing rare earth elements.

~Sulphates.~--The sulphates of the rare earth elements are obtained by dissolving the oxides or hydroxides in sulphuric acid. From the solutions so obtained, various hydrated salts separate according to the temperature of crystallisation. By heating the hydrated salts to a temperature of 300°-400°, the anhydrous salts are prepared. These are extremely soluble in water at 0°, having a great tendency, which is indeed to be observed in the hydrated forms also, to form supersaturated solutions. When the temperature of such a solution is allowed to rise, larger or smaller quantities of an hydrated form separate out, the differences of solubility among the sulphate hydrates of the various elements being sometimes considerable.

The hydrated sulphates of the cerium elements have been very closely studied in connection with the purification of thorium. Cerium sulphate itself forms hydrates with 12, 9, 8, 5, and 4 molecules of water, but sulphates of the other elements generally form fewer hydrates; the commonest have 12, 8, or 4 molecules of water, and numerous cases of isomorphism are known among them. The solubility curve of the cerium sulphate hydrates is shown in the diagram. Fig. 3. The sulphates of the yttrium elements have not yet been systematically investigated, and in most cases only the octohydrates are known. Scandium sulphate is notably different from the other sulphates, in that it is considerably more soluble, and crystallises with six molecules of water.

It is an important characteristic of the rare earth elements that the solubility of the sulphates diminishes rapidly as the temperature rises. The study of the various equilibrium conditions is greatly complicated by the tendency to form supersaturated solutions, and the fact that many hydrates can exist throughout considerable ranges of temperature in the metastable condition; in consequence of this, also, the solubilities of many hydrates are known for temperatures far beyond the transition points. Foreign elements may be separated by taking advantage of the very great solubility of the anhydrous sulphates at 0°, and the rapid decrease in solubility with rise of temperature. For this purpose, a solution of the anhydrous sulphates saturated at 0° is prepared, and after filtration is slowly allowed to come to room temperature; the hydrated rare earth sulphates then separate, leaving in solution the foreign sulphates. This method may indeed be used instead of the oxalate separation (see p. 147).

In presence of excess of sulphuric acid, _acid sulphates_ of the general formula R(HSO₄)₃ are formed. These are fairly stable, and must be heated to a temperature of 400°-500° to decompose them completely to the normal salts; even at that temperature, traces of acid are tenaciously retained, a fact which renders the determination of the equivalents by the sulphate method unreliable, unless special precautions are taken. On further heating, the normal sulphates pass into _basic salts_, R₂O₃,SO₃, and finally, at the temperature of the blowpipe flame, into the oxides. The temperatures at which these decompositions occur vary with the positive character of the elements; the most basic oxide clings most tenaciously to sulphuric anhydride, and forms the most stable acid salt. Lanthanum sulphate, for example, requires to be heated for a considerable time at a white heat if the pure oxide is required, whilst the sulphates of the less positive elements are easily decomposed at a red heat. The order of basic strength of the oxides, as determined by the ease with which the sulphates are decomposed, seems, however, to be very different from the order determined by decomposition of the nitrates (see p. 118).

With the alkali sulphates, the sulphates of the rare earth elements readily form _double salts_, which are of great importance in separation, on account of the great differences in solubility. The double sulphates of the cerium group are almost insoluble in excess of alkali sulphate, whereas the yttrium double sulphates, with the exception of those of the terbium metals, which occupy an intermediate position, are very easily soluble. This method of separating the elements into the two main groups was first employed by Berzelius, and though a century has elapsed, it remains to-day the most efficient method of effecting the separation.

The _ethylsulphates_ have been employed by Urbain and others in effecting separations, especially in the erbium and terbium groups. The solubilities of these salts are in the same general order as those of the alkali double sulphates, and they are especially convenient for separating the metals into the three groups of the cerium, terbium, and yttrium elements respectively. They may be prepared by double decomposition of the rare earth sulphates with barium ethylsulphate, but on account of the ease with which the alkylsulphates are hydrolysed by acids, it is essential that the solutions should be quite neutral. A more convenient method, according to James, is the treatment of the anhydrous chlorides in alcohol solution with sodium ethylsulphate dissolved in the same medium; sodium chloride is precipitated, whilst the ethylsulphates of the rare earth elements remain in solution.

The _sulphites_ of the rare earth elements are sparingly soluble crystalline salts, of the general formula R₂(SO₃)₃,_x_H₂O. They are obtained by passing sulphur dioxide into a suspension of the hydroxides in water, or by double decomposition of soluble salts with alkali sulphite. They dissolve in excess of sulphurous acid, and on evaporation of the solution are deposited unchanged. They are distinguished from thorium sulphite by the fact that they form no alkali double salts. The strongly electropositive character of the rare earth metals is shown by the fact that they form normal and not basic sulphites.

The _thiosulphates_ are readily soluble, crystalline bodies. With the exception of the ceric and scandium salts, they are not hydrolysed in boiling solution, a fact which allows of a complete separation from the readily hydrolysed thiosulphates of zirconium and thorium.

_Dithionates_ of the commoner rare earth elements, of the general formula R₂(S₂O₆)₃,_x_H₂O, have been prepared by double decomposition of the sulphates with barium dithionate. They are readily soluble, crystalline salts.

The _selenates_ are soluble, crystalline salts, which separate from aqueous solutions in various hydrated forms. They resemble the sulphates in being less soluble in hot than in cold water, and numerous cases of isomorphism have been observed among the corresponding sulphate and selenate hydrates. Several alkali double selenates have been described; they show a close resemblance to the analogous double sulphates.

The _selenites_ are amorphous, insoluble compounds, obtained by the action of selenious acid on the carbonates, or on solutions of neutral salts. Basic and acid selenites are also known.

~Nitrates.~--The nitrates are crystalline, deliquescent compounds, readily soluble in water and alcohol, but less easily in nitric acid, a fact which has been of considerable importance for purposes of separation. The solubility is greatest in the case of lanthanum nitrate, diminishing through the cerium group to a minimum in gadolinium nitrate, and then increasing again. They separate from aqueous solution in the form of crystalline hydrates; in the cerium group, these have commonly the formula R(NO₃)₃,6H₂O, whilst the nitrates of the yttrium elements usually crystallise with 3 or 5 molecules of water. By carefully heating the hydrated salts, basic nitrates may be obtained, which in the yttrium group are soluble in water, and may be obtained crystalline; in the cerium group, the basic nitrates are insoluble. By further heating, insoluble ‘superbasic salts,’ and finally the oxides, are obtained in all cases. The temperatures at which these basic and superbasic compounds are formed vary with the electropositive character of the element; this fact affords a method of separation which has been very frequently employed.

An interesting series of addition compounds of the rare earth nitrates with antipyrine (dimethylphenylpyrazolone, C₁₁H₁₂ON₂) has been described recently by Kolbe.[164] Those of the cerium metals have the general formula R(NO₃)₃,3C₁₁H₁₂ON₂; the yttrium nitrates appear to combine with four molecules of the base.

[164] _Zeitsch. anorg. Chem._ 1913, ~83~, 143

The tendency to form double nitrates with nitrates of the metals of Group IA and Group IIA also varies with the basic strength of the hydroxides. In the most positive elements of the cerium group, the tendency is very pronounced, and there are a large number of stable, crystalline double salts; but the stability decreases rapidly as the atomic weight of the element rises, and in the terbium and yttrium groups crystallised double nitrates cannot be obtained. The solubility of these double salts increases rapidly in the same direction, the lanthanum double nitrates being the least soluble. For this reason, these compounds are of great importance for the purpose of separation, especially in the cerium group. Bismuth nitrate and the various bismuth double nitrates are isomorphous with the corresponding compounds of the cerium group, and the double bismuth ammonium and bismuth magnesium salts have been largely used by Urbain in the separation of samarium and the elements of the terbium group.

~Phosphates.~--Addition of phosphoric acid, or an alkali phosphate to solutions of rare earth salts throws down the phosphates as gelatinous precipitates, which slowly become crystalline on standing. The precipitate is soluble in excess of phosphoric acid, and in other mineral acids, a fact of great importance in the commercial treatment of monazite. The composition of the precipitate is not known with certainty; both neutral and acid phosphates can probably be obtained according to the conditions. Double salts with the alkali phosphates can be prepared by fusion methods. The naturally occurring phosphates, monazite and xenotime, are mixtures of the orthophosphates of the cerium and yttrium elements respectively.

_Phosphites_ are known in a few cases only; _arsenates_ and _arsenites_ of lanthanum have been prepared. _Vanadates_ of some of the rare earth elements have been described.

~Chromates.~--The rare earth chromates are, as a rule, sparingly soluble in water, and show considerable differences of solubility amongst themselves; for this reason, they have been of some use in the separation of the cerium elements.[165] They are obtained by addition of potassium chromate to neutral solutions of rare earth salts as crystalline precipitates, of the general formula R₂(CrO₄)₃,8H₂O; with a large excess of alkali chromate, double chromates are obtained, which are more readily formed, and more soluble, in the yttrium series than in the cerium group. Addition of chromic acid or alkali bichromate to solutions of the soluble salts gives no precipitate, a fact which allows of the separation of zirconium and thorium, and of cerium in the tetravalent state, since the tetravalent elements are precipitated by both these reagents.

[165] Muthmann and Böhm, _Ber._ 1900, ~33~, 42; Böhm, _Zeitsch. angew. Chem._ 1904, ~15~, 372 and 1282.

Ammonium molybdate throws down from neutral solution of rare earth salts gelatinous precipitates of the _molybdates_; the formula La₂2(HMoO₄)₆ is assigned to the lanthanum compound obtained in this way. No precipitation occurs if the solution be strongly acid; on this fact a process has recently been based for the volumetric estimation of thorium, in presence of rare earth salts, by means of ammonium molybdate (see p. 289).

Various _silicotungstates_ and _double tungstates_ have been described.

~Carbonates.~--The more pronounced electropositive character of the rare earth elements, as contrasted with other trivalent metals, is well illustrated by the fact that they form stable neutral carbonates of the formula R₂(CO₃)₃,_x_H₂O. These may be obtained by passing a current of carbon dioxide through an aqueous suspension of the hydroxides, or by addition of an alkali carbonate to neutral solutions of the salts. Basic carbonates are known in the case of the less positive yttrium elements only; both these and the neutral carbonates are insoluble in water.

In presence of a large excess of alkali carbonate, double carbonates are formed. The stability as well as the solubility of these compounds increases in passing from the cerium to the yttrium group, _i.e._ as the electropositive character becomes weaker. The double carbonates of the cerium elements are sparingly soluble, and are decomposed by water, especially on warming; they may, however, be recrystallised from alkali carbonate solution. The sodium and ammonium double salts are less soluble than the potassium compounds. The latter have the general formula R₂(CO₃)₃,K₂CO₃,12H₂O, and are of considerable importance in many processes of separation. The yttrium elements can be separated from the cerium metals, and the latter from one another, by taking advantage of the differences of solubility shown by the potassium double carbonates. If a concentrated solution of the salts in potassium carbonate solution be fractionally diluted with water, the cerium elements separate in the order: lanthanum, praseodymium, cerium, neodymium, and samarium; the more soluble yttrium compounds remain in the solution. Thorium forms double alkali carbonates which are very readily soluble in excess of alkali carbonate; this property is of great importance for the technical separation of the element.

~Oxalates.~--The oxalates of the rare earth elements are of the greatest importance, on account of the fact that they are not only insoluble in water, but are also very sparingly soluble in dilute mineral acids, and in excess of oxalic acid. They can be completely precipitated even from strongly acid solutions by addition of sufficient excess of oxalic acid, or alkali oxalate, and thus afford a means of easily and completely separating the rare earth group from the commoner elements.

They are thrown down by addition of oxalic acid, or alkali oxalate, as amorphous precipitates, which rapidly become crystalline, especially if the solution is warmed. From water at normal temperatures they usually separate as the decahydrates, R₂(C₂O₄)₃,10H₂O, but hydrates with 7, 9, and 11 molecules of water of crystallisation are also known. From strongly acid solutions, mixed oxalo-salts of the general formula R(C₂O₄)X, where X = Cl, NO₃, HSO₄, etc., may be obtained. These mixed salts may also be prepared by dissolving the oxalates in concentrated solutions of the chlorides, nitrates, etc., whilst nitro-sulphates, R(SO₄)NO₃, have been obtained by recrystallising the sulphates from strong nitric acid. The tendency to form salts with mixed acid radicles appears to be general.[166]

[166] See Meyer and Marckwald, _Ber._ 1900, ~33~, 1003; also Matignon, _Ann. Chim. Phys._ 1906, [viii.], ~8~, 243.

The solubilities of the oxalates in mineral acids of various concentrations have been examined by Hauser and Wirth.[167] Whilst the solubilities in water are exceedingly slight, and increase with increasing atomic weight of the elements, _i.e._ from the cerium to the yttrium group, in mineral acids of concentration 3-4N the solubility becomes noticeable, and is greatest for the oxalates of the most positive elements. The solubility is greatly lessened, however, if considerable excess of oxalic acid be present.

[167] _Zeitsch. anal. Chem._ 1908, ~47~, 389.

Double oxalates with the alkali oxalates can be obtained with the salts of the yttrium elements only, the oxalates of the cerium elements being almost insoluble in excess of alkali oxalate in the cold. Of the alkali double oxalates, the potassium compounds are the most soluble, but the ammonium compounds show the greatest differences in solubility; von Welsbach has employed the method of fractional crystallisation of these salts from a saturated solution of ammonium oxalate for separations in the yttrium group. The sodium double oxalates are the least soluble of these double salts.

Since the rare earth elements are almost always separated in the form of the oxalates, the methods for transforming these into soluble compounds become important. They may be ignited to oxides, and these dissolved in nitric acid; if the content of ceria is very high, the oxide mixture may become insoluble, but this difficulty may be overcome by addition of a reducing agent--hydrogen peroxide is very convenient for this purpose. The oxalates may also be dissolved directly in fuming nitric acid, care being taken to avoid loss; if the mixture contains cerium, the oxidation is hastened, ceric salts having the property of acting as oxygen carriers. By boiling for a short time with potash, the oxalates may be easily transformed into the hydroxides, which can be dissolved in dilute acids.

~Formates.~--On account of the considerable differences in solubility by which they are characterised, these salts have been employed for separations. The formates of the cerium group are considerably less soluble than those of the yttrium group. They may be partly precipitated from solutions of rare earth salts by addition of alkali formate--formic acid itself causes precipitation only with salts of weak acids, _e.g._ the acetates--but are best prepared by dissolving the oxides in formic acid; on concentration of the solution, the formates of the cerium and terbium elements successively separate, the salts of the yttrium group remaining in solution. The separation of the terbium earths by this method was attempted by Delafontaine; his ‘new’ element, Philippium, obtained from the mother-liquors, was in reality a mixture of the terbium and yttrium elements, which cannot be completely separated by the formate method.[168]

[168] See Urbain, _Ann. Chim. Phys._ 1900, [vii.], ~19~, 184.

The _acetates_ are readily soluble in water, the yttrium salts being rather less easily soluble than those of the cerium group. They are therefore obtained by dissolving the oxides in acetic acid; addition of alkali acetate to a solution of a rare earth salt gives no precipitate, even on boiling, behaviour which is in marked contrast to the ease with which the salts of other trivalent metals are hydrolysed under these conditions. In this respect the rare earth elements differ also from the tetravalent elements zirconium and thorium (and from cerium in the tetravalent state); soluble salts of the latter, on boiling with sodium acetate, give insoluble basic acetates. Even sparingly soluble compounds of the rare earth elements are as a rule taken into solution by digestion with ammonium acetate.

_Tartrates._--Addition of ammonium tartrate to a neutral solution of rare earth salts throws down an amorphous precipitate, which dissolves easily in acids, and in excess of the precipitant. In the presence of tartaric acid, precipitation of the earths by addition of sodium hydroxide is completely inhibited. Potassium hydroxide under these conditions gives a precipitate in the case of the yttrium elements, though only on boiling; ammonia gives a crystalline precipitate even in the cold with this group. These precipitates are alkali double tartrates of the yttrium metals; the cerium elements give no precipitate at all. In all cases, therefore, the precipitation of the hydroxides is inhibited by the presence of tartaric acid.

A very large number of organic salts of the rare earth elements has been prepared and examined during the past two decades, in the endeavour to find some class of compounds which will allow of an easy separation of the group. The _benzoates_, _succinates_, _hippurates_, _citrates_ and similar relatively simple salts first received attention, but less common acids, as _e.g._ the hydroxynaphthalenesulphonic acids, have also been employed.[169] The use of various organic acids for the separation and estimation of thorium in presence of the rare earths is outlined in that connection (see p. 288). More recently, the glycollates and cacodylates have been prepared. The _glycollates_[170] of the cerium elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts; they are more soluble than the yttrium compounds, which have the formula R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The _cacodylates_,[171] R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have similar solubility relations.

[169] Erdmann and Wirth, _Annalen_, 1908, ~361~, 190; see also Pratt and James, _J. Amer. Chem. Soc._ 1911, ~33~, 1330; Baskerville and Turrentine, _ibid._, 1904, ~26~, 46; James, Hoben and Robinson, _ibid._, 1912, ~34~, 276, etc.

[170] Jantsch and Grünkraut, _Zeitsch. anorg. Chem._ 1913, ~79~, 305.

[171] Whittlemore and James, J. _Amer. Chem. Soc._ 1913, ~35~, 627.

The _phthalates_ of the yttrium group have been found to be very valuable for purposes of separation by Meyer and Wuorinen.[172] The salts are readily obtained in solution by shaking together cold aqueous suspensions of the rare earth hydroxides, and phthalic acid; the clear solutions when warmed become cloudy, the organic salts hydrolysing very easily, with separation of the hydroxides. The most positive elements naturally remain longest in the solution, the weakly basic oxides accumulating in the first precipitates.

[172] _Zeitsch. anorg. Chem._ 1913, ~80~, 7.

An organic compound which has proved very useful in the treatment of the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic form, this substance forms salts with metals, which in the case of the rare earth elements are especially characterised by the ease with which they may be obtained, and their high crystallising power. They may be prepared by double decomposition of neutral solutions of rare earth salts with ammonium acetylacetone, and crystallise readily from dilute alcohol. They have been used by Urbain in the fractionation of the yttrium group, and for determination of molecular weights by the boiling point method; Biltz[174] has shown that in solution they generally have the double formula R₂(C₅H₇O₂)₆.

[173] Urbain, _Bull. Soc. chim._ 1897, [iii.], ~17~, 98; Urbain and Budischofsky, _Compt. rend._ 1897, ~124~, 618; Biltz and Clinch, _Zeitsch. anorg. Chem._ 1904, ~40~, 218.

[174] _Annalen_, 1904, ~331~, 334.

THE RARE EARTH ELEMENTS, AND THE PERIODIC CLASSIFICATION

At the time of the introduction of the periodic classification the rare earth elements were generally believed to be divalent. This belief, which has persisted until quite recently,[175] was based chiefly on the electropositive character of the metals, and their general chemical resemblance to the elements of the alkaline earths; the isomorphism of the tungstates of calcium and the cerium elements, and of the molybdates of lead and the cerium elements, also supports this view. The physical evidence in favour of Mendelejeff’s view, however, is quite overwhelming; the specific heats of the metals, the equivalent conductivities of the chlorides, and molecular weight determinations by means of vapour densities and the boiling point method, prove beyond doubt that the elements are in fact trivalent.

[175] See Wyrouboff, _Bull. Soc. franc. Min._ 1896, ~19~, 219; Wyrouboff and Verneuil, _Compt. rend._ 1897, ~124~, 1230 and 1300; _ibid._, 1899, ~128~, 1573; etc.

In deciding in favour of the trivalent nature of the rare earth metals, Mendelejeff was influenced chiefly by the fact that there was no room in the table for divalent elements with the equivalent weights then assigned to the cerium and yttrium elements. At that time, only the six oxides obtained by Mosander were known; of these the accepted equivalents and atomic weights were as follows:

Element. Equivalent. Atomic Weight. Lanthanum 46 92 Cerium 46 92 Didymium 48 96 Yttrium 31 62 Erbium 56 112

the values for terbium being uncertain. If cerium be considered trivalent in the cerous salts, its atomic weight becomes 138, that of barium being 136. Mendelejeff placed cerium in Group IV, series 8, in the position which it still occupies; he pointed out that the accepted equivalent must be too low, and suggested that the atomic weight should be at least 140, almost exactly the value accepted to-day.

This choice left the positions in Group III, series 8, horizontally before cerium, and in Group IV, series 10, vertically below it (see figure), to be filled by the two elements, lanthanum and didymium. No chemical evidence being available to decide the choice, he provisionally assigned didymium to the first (Group III, series 8), and lanthanum to the second (Group IV, series 10) position, at the same time expressing the opinion that didymium was probably a mixture of closely related elements. Yttrium then fell into place in Group III, series 6, above didymium, and erbium in Group III, series 10, below it. To the vacant space above yttrium in Group III, series 4, he assigned the hypothetical element Eka-boron, with atomic weight 44; this space is now occupied by scandium, which corresponds almost exactly in properties to the metal described by the Russian chemist. A part of the table illustrating these positions is shown in Fig. 4.

The determination of the specific heats of the metals by Hillebrand and Norton in 1875, whilst confirming the trivalency of the elements, rendered it necessary to alter the position of lanthanum, which was placed in Group III, series 8, instead of didymium, which was thus left without a place. This first indication that all the rare earth elements could not be fitted into the table without difficulties was soon followed by the discovery of several other members of the group, for which places could not easily be found.

It was first pointed out by Brauner in 1881 that, with the exception of scandium (44·1) and yttrium (89·0), the rare earth elements form a zone of increasing atomic weight between barium (137·37) and tantalum (181·5). In 1902 he proposed[176] to consider the rare earth metals as a kind of zone or belt among the elements, comparable to the asteroids in the solar system, extending from cerium in Group IV to tantalum in Group V in a continuous series. The suggestion seems at first sight contrary to the whole principle of periodic classification, but it accords very well with the anomalous position of the rare earth group among the other elements; it is very well illustrated in the accompanying Fig. 5, which shows an helical or space representation of the table.

[176] _Zeitsch. anorg. Chem._ 1902, ~32~, 1.

Brauner’s conception is also in accord with the physical properties of the elements and their compounds. These vary continuously throughout the group, and show nowhere the sudden transitions which are characteristic of other series in the table. Benedicts[177] has collected all the data bearing on the atomic volumes, and finds that those also vary continuously, with rise in the atomic weights, within quite small limits, all lying between the values for barium and tantalum. In face of all the evidence furnished by physical and chemical properties, however, Brauner[178] has recently reverted to an idea which he put forward in 1881, according to which lanthanum and cerium are placed as usual in Groups III and IV, series 8, whilst the other elements are distributed in order throughout the remaining groups, as shown in Fig. 6.

[177] _Zeitsch. anorg. Chem._ 1904, ~39~, 41.

[178] _Monats._ 1881, ~3~, 1; _Zeitsch. Elektrochem._ 1908, ~14~, 525.

In support of this arrangement, he quotes the fact that some of the elements appear to be able to form higher oxides in the presence of other oxides, which act as oxygen carriers (see pp. 174, 177-8), though these higher oxides are certainly not salt-forming. He also deduces, from the rates of hydrolysis of the sulphates, that the elements fall into two parallel series, according to the strengths of the hydroxides as bases, on which ground he justifies the distribution throughout series 8 and 9. There can be no doubt, however, that this disposition is far less in accordance with the behaviour and properties of the rare earth elements than is the first arrangement, which places them in a transition zone between barium and tantalum; it is impossible, for example, to reconcile the properties of praseodymium with those of columbium and tantalum, or to find the slightest analogy between neodymium and molybdenum or tungsten, as the second arrangement requires.

The analogy of the rare earth group to the elements of Group VIII has been pointed out by many authors.[179] On the ground that the rare earth elements cannot be spread over the table in series 8-10, Steele[180] favours the early classification of Thomsen, according to which the elements are divided into three groups. The first, corresponding to Groups I and II of Mendelejeff’s table, consists of two sub-groups, each containing seven elements[181]; the second, corresponding to the first two long series of the periodic table, has two sub-groups, each of seventeen elements, of which the first and last seven are analogous--these elements fall into the same groups in the periodic table--whilst the middle three are interperiodic. These interperiodic elements are those which Mendelejeff places in Group VIII. The third division consists of one (or two) group(s) of thirty-one elements; here again, the first and last seven are analogous, whilst the interperiodic elements, which are seventeen in number, include the rare earth metals.

[179] Compare Biltz, _Ber._ 1902, ~35~, 562.

[180] _Chem. News_, 1901, ~84~, 345.

[181] The inert gases are not included.

Steele’s idea has been extended by Werner,[182] who has drawn up a table to illustrate it. In this classification, the elements are arranged in order of atomic weight, but arbitrary gaps are left in such a way that similar elements may fall into the same vertical columns, as in the periodic table. The arrangement has the advantage that the interperiodic elements, consisting of the rare earth elements and the elements placed in Group VIII of the periodic table, here do fall in the middle of their respective periods, but it has several drawbacks, and does not represent the transition of properties from element to element so well as the helical representation of the periodic table, which brings out most clearly the true relations between the elements, and the anomalous position of the rare earth metals.

[182] _Ber._ 1905, ~38~, 914.

Mention must be made at this point of the theory of ‘Meta-elements’ put forward in 1888 by Sir William Crookes.[183] From his work on the cathode luminescence of some of the oxides (see next chapter), that author was led to the conclusion that several of the then-accepted rare earth elements, notably samarium and yttrium, were in reality heterogeneous, consisting of large numbers of very closely related bodies, differing so very slightly in properties that only the most refined methods could perceive the variations; for these he proposed the name Meta-elements. Though it has been proved that the differences observed by Crookes in the luminescence spectra were really due to the presence of very small quantities of impurities, his paper is of great interest, in that it contains a theory of evolution of the elements, and postulates the possibility of their decay. Modern developments in radioactivity have not only lent a curious force to these speculations, but even support his contention that a chemical element, in the ordinary sense of the word, is not necessarily homogeneous.[184] In the field of the rare earths, also, the homogeneity of elements is even now continually being called into question (see Thulium, p. 204). In any case, we have in the rare earth elements a series of bodies in which the change of properties from one member to another--and the consequent possibility of easy separation--is so very slight, and so far without parallel in the whole field of chemistry, that we are at least justified in asking whether some extension of our ordinary conception of an element is not required.

[183] _Trans. Chem. Soc._ 1888, ~53~, 487.

[184] See Soddy, _The Chemistry of the Radio-Elements_, Part II., Introduction.