The Rare Earths: Their Occurrence, Chemistry, and Technology

Spectrum analysis.—Cerous salts show no absorption, ceric salts general absorption of the violet end of the spectrum. Arc spectrum—see Exner and Haschek,[230] Eder and Valenta,[231] and Cooper.[232] The emission spectrum of cerium is especially rich in lines; for identification, the following may be used:

The estimation of cerium cannot be carried out accurately by gravimetric methods in the presence of other earths; volumetric methods, however, will give reasonably accurate results, if the necessary precautions are taken. In Bunsen’s method the ignited oxides are treated with hydrochloric acid in presence of potassium iodide, the iodine set free from the hydriodic acid by reduction of the cerium dioxide being estimated by means of sodium thiosulphate, in the usual way. This method gives very inaccurate results, since in the presence of cerium dioxide, other oxides of the group can be converted into higher oxides which will also liberate iodine under these conditions.

The most reliable method is that of v. Knorre.[233] The solution to be estimated is acidified with sulphuric acid, and oxidised by means of ammonium persulphate. The excess of the oxidising agent having been destroyed by boiling, the cooled solution is treated with a slight excess of hydrogen peroxide, which reduces the ceric salt according to the equation:

2Ce(SO₄)₂ + H₂O₂ = Ce₂(SO₄)₃ + H₂SO₄ + O₂

The excess of hydrogen peroxide is then estimated by means of a dilute permanganate solution. Permanganate is itself reduced by the cerous salt formed, but the action is so slow in acid solution at the ordinary temperature that the excess of peroxide can be accurately determined without unduly hurrying the titration. In this form the method is generally employed for the estimation of cerium in monazite sands, and in the incandescent mantle industry. The greatest difficulty is the adjustment of the concentration of the sulphuric acid required. If this be too low, basic ceric sulphate separates on boiling, and the estimation fails; if it be too high, oxidation to the ceric salt is hindered, and may even be inhibited. This difficulty disappears in the modified method of Waegner and Muller,[234] in which the oxidation to the ceric condition is effected by means of bismuth tetroxide in nitric acid solution. A similar method, in which reduction to the cerous state is effected by a ferrous salt, in place of hydrogen peroxide, has been employed by Metzger.[235]

Many attempts have been made to estimate cerium compounds by means of permanganate, which in alkaline solution oxidises cerous salts to the ceric condition, but the autoxidation of cerous hydroxide in the air introduces errors, unless suitable precautions are taken. Meyer and Schweitzer[236] show that if the solution of the cerous salt be added, with constant shaking, to a known volume of a standard permanganate solution, in presence of excess of magnesia, the liquid being kept warm, this difficulty is overcome; the results are usually a little high, however, probably by reason of the oxidising action of the cerium dioxide on the other oxides present.

Good results have also been obtained by the use of potassium ferricyanide in alkaline solution,[237] oxidation taking place according to the equation:

Ce₂O₃ + 2K₃Fe(CN)₆ + 2KOH = 2K₄Fe(CN)₆ + 2CeO₂ + H₂O

The ceric hydroxide is filtered off, and the ferrocyanide formed estimated by means of permanganate in acid solution.

CHAPTER XII
CERIUM GROUP (continued)

Lanthanum, Praseodymium, Neodymium, and Samarium

In his examination of the ceria earths in 1839, Mosander discovered a new constituent, which he called Lanthana; the new oxide was removed in solution when the ignited mixture was extracted with dilute nitric acid, which leaves cerium dioxide undissolved. On examination, the new oxide was found to be heterogeneous; by fractional precipitation with ammonia, and subsequent recrystallisation of the sulphates, he obtained two oxides, which he called respectively Lanthana (λανθανειν, to be hidden), from the absence of colour and specific reactions, and Didymia, (διδυμοι, twins) from their similarity and the occurrence of the two together.

Samaria was isolated by Lecoq de Boisbaudran, in 1879, from a specimen of didymia extracted from the mineral samarskite. Two years previously, Delafontaine had shown that the didymia separated from this mineral was not spectroscopically identical with the oxide obtained from other sources, and in 1878 had isolated an oxide which he called Decipia; this was shown later, however, to be a mixture of which samaria was one component. The samaria obtained by de Boisbaudran was by no means pure, being associated with terbia earths; several investigators claimed to have separated from it new oxides, most of these being proved afterwards to have been more or less impure specimens of Europia.

In 1885, Auer von Welsbach[238] employed for the first time the method which has now become of paramount importance for the separation of the cerium group, viz. the fractional crystallisation of the double nitrates. By this method he succeeded in resolving Mosander’s didymia into two new oxides, for which he proposed the names Praseodidymia (πρασινος, leek-green), from the colour of the salts, and Neodidymia respectively; the shorter names praseodymia and neodymia are, however, now generally adopted.

GROUP A
Mixed Double Nitrates.

Separation

The modern methods for the separation of these elements are based almost entirely on the differences in solubility of the various double nitrates.[239] The mixed double sulphates separated by saturation of a solution of the chlorides with sodium sulphate, which contain the cerium and most of the terbium elements, are transformed into nitrates, and the neutral solution boiled with potassium bromate, in presence of powdered marble, till all the cerium is precipitated as basic ceric nitrate. From the filtered solution the other elements are thrown down as oxalates, transformed into the magnesium double nitrates (A in Fig. 8), and fractionated from nitric acid solution[240] until a rough separation has been effected (fractions 1, 2, 3, and 4). The separation, which is somewhat long and tedious, is followed by means of the absorption spectra, and by the colour changes of the fractions. Fraction 1, containing lanthanum and some praseodymium, should be faint green to colourless; fraction 2 is colourless by the complementary action of the coloured salts of neodymium and praseodymium; fraction 3, which should contain the crude neodymium salt, is amethyst; and fraction 4, the mother-liquor, is yellow from the presence of the samarium compound.

Fraction 1 is now converted to the double ammonium nitrates, which allow of a readier separation at this stage; two fractions are obtained, of which the less soluble, fraction 5, is the fairly pure lanthanum compound, whilst the more soluble, fraction 6, contains the praseodymium with a little lanthanum. The lanthanum ammonium nitrate, fraction 5, is converted into the anhydrous sulphate, which is dissolved in ice-water; when the solution is gradually warmed, the enneahydrate, La₂(SO₄)₃,9H₂O, separates, and may be obtained perfectly pure by recrystallisation. It is of interest that the radioactive element actinium is chemically very similar to lanthanum, and follows it closely through the process of separation.

The mixed praseodymium and neodymium magnesium nitrates which constitute fraction 2 are transformed into the double manganese nitrates, and the crystallisation from nitric acid continued.[241] The less soluble part, fraction 7, is fairly free from neodymium, and the separation is continued with that of fraction 6, until both lanthanum and neodymium have been completely removed. The more soluble part, fraction 8, yields the pure neodymium compound, as does also the crude neodymium magnesium nitrate which constitutes fraction 3, if the crystallisation be continued.

The mother-liquors, fraction 4, are treated with bismuth magnesium nitrate,[242] which is intermediate in solubility between the analogous compounds of samarium and europium, and the crystallisation continued. The less soluble fraction contains the samarium compound, in which bismuth is the only impurity; this is easily removed by treatment with sulphuretted hydrogen. The remaining fractions are used as a source of the terbium elements (see p. 186).

The double carbonate method[243] is very suitable for the preparation of pure lanthanum compounds after the removal of cerium. The mixture of salts is added to a warm 50% solution of potassium carbonate, and to the clear liquid, water is added gradually, with constant stirring. The double carbonates of the most positive elements are the least soluble, and are first thrown down, so that the precipitate is rich in lanthanum; it is collected and washed with a 25% potassium carbonate solution, and the process repeated. A few repetitions suffice to separate lanthanum completely from the other members of the group. The method may also be used for the purification of praseodymium salts.

Lanthanum, La = 139·0

As the most electropositive element of the rare earth group, lanthanum is the most similar in its chemical properties to the metals of the alkaline earths. The metal itself (see p. 115) oxidises even in dry air, and in moist air rapidly becomes coated with a white layer of hydroxide; it attacks water, and burns vigorously when heated in the air. An alloy with aluminium, of the formula LaAl₄, has been prepared by Muthmann and Beck[244]; it forms lustrous white crystals, very stable in the air and very resistant towards acids.

The hydroxide is of interest from the fact that, if precipitated under suitable conditions, it has the power of taking up solid iodine to form a deep blue adsorption compound[245]; colloidal solutions of basic lanthanum acetate are also coloured blue by addition of a few drops of iodine solution. If precipitation with alkali be carried out in presence of hydrogen peroxide, an hydrated peroxide of the composition La₂O₅,nH₂O is obtained.[246] This compound partially decomposes with evolution of oxygen at ordinary temperatures; towards carbon dioxide and acids it acts as a true peroxide, with formation of hydrogen peroxide.

The oxide is colourless, and forms colourless salts with those acids in which the anion is not coloured. The oxide is distinguished from the other rare earth oxides in that it turns moistened litmus paper blue; it resembles lime, in hissing when slaked, absorbing carbon dioxide from the air, and liberating ammonia from ammonium salts. By fusion with alkali carbonates, and by digestion with concentrated alkali hydroxides, Baskerville and Catlett[247] claim to have obtained lanthanates and metalanthanates, but their work has not yet been confirmed.

The sulphate, La₂(SO₄)₃,9H₂O, is the least soluble of all the rare earth sulphates. The enneahydrate is the only form stable at ordinary temperatures,[248] though under special conditions, hydrates with 6 and with 16 molecules of water of crystallisation have been obtained. It separates in needles belonging to the hexagonal system; 100 parts of water dissolve at 0°, 3·01, and at 100°, 0·69 parts of the salt. The acetylacetone compound melts at 185°.

A large number of other lanthanum compounds have been prepared, but these are so typical of the rare earth salts generally that no detailed treatment is required; for a full account of them, the reader is referred to Abegg’s classical handbook.

Atomic Weight.

—A large number of determinations of this constant have been made, but the results even of recent investigations do not agree so closely as might be desired. The value adopted by the International Committee, 139·0, is based on the work of Brauner and Pavliček,[249] carried out in 1902. These authors give an account of all the determinations made up to that date, with critical discussion of the methods employed and the possible sources of error. The more important investigations have been based on the ratio La₂O₃ : La₂(SO₄)₃, for the determination of which the most stringent precautions must be taken. The synthetic method has generally been employed, on account of the tenacity with which the oxide clings to traces of sulphuric anhydride. In this method, the total decomposition of the acid sulphate, and the protection of the very hygroscopic sulphate, La₂(SO₄)₃, from atmospheric moisture, constitute the chief difficulties. By this method, H. C. Jones[250] in 1902 obtained a result (138·76) considerably lower than the value found by Brauner and Pavliček (loc. cit.) A later research by Brill,[251] who carried out a synthetic sulphate determination on a minute scale, using a Nernst microbalance, gave the value 139·5, which, whilst considerably higher than either of the other figures, shows that Brauner and Pavliček’s number can hardly be too high.

Praseodymium, Pr = 140·6

This element occurs only in small quantities in the commoner rare earth minerals, and its separation in the pure state is in consequence a matter of very great difficulty. The salts and their solutions have a characteristic green colour. The salts are derived from the sesquioxide, Pr₂O₃, but a dioxide, PrO₂, and an intermediate oxide of uncertain composition are known. The absorption spectrum has five absorption bands, one of which coincides with a band in the absorption spectrum of neodymium; this fact has been interpreted as an indication of the non-elementary nature of both metals.[253] Difference in the absorption spectra have been put forward by several workers as indicating the complex nature of praseodymium, but an exhaustive examination by Stahl[254] in 1909 showed that there is no reason to doubt that the metal is really an element.

The metal is prepared by electrolysis of the fused chloride; in order to attain the temperature required to fuse the element, a very thin cathode is employed; if too powerful a current be used, the dioxide is formed. The metal is purified by remelting it in crucibles of magnesia, under a layer of anhydrous barium chloride. It has a yellowish shade, and is more stable in the air than lanthanum and cerium. For physical properties, see p. 115. No alloys have been prepared.

The hydroxide is thrown down by alkalies as a gelatinous green precipitate; in the presence of hydrogen peroxide, an hydrated peroxide, which closely resembles the corresponding lanthanum compound, is thrown down.

The Oxides.—By ignition of salts of volatile acids, Auer von Welsbach[255] obtained an oxide to which he assigned the formula Pr₄O₇. More recent work[256] has shown that the composition of the oxide obtained depends upon the conditions under which the various salts are decomposed. By fusing the nitrate in presence of potassium nitrate at 400-450°C., Meyer obtained the dioxide, PrO₂; at higher temperatures this decomposes, giving the intermediate oxides. The formation of the dioxide is greatly influenced by the presence of other oxides,[257]—ceric oxide, acting as an oxygen carrier, favouring whilst the other oxides hinder. The pure dioxide is a brownish-black powder, which resembles manganese dioxide, but is less stable. It liberates halogens from the halogen acids, and oxidises manganese salts to permanganates, but does not completely oxidise ferrous or stannous salts, losing instead a part of its oxygen in the gaseous form. The dioxide cannot be obtained in the wet way.

When heated in a stream of hydrogen, the dioxide yields the sesquioxide, Pr₂O₃, as a greenish-yellow powder,which readily absorbs oxygen from the air, becoming brown, with formation of the intermediate oxide.

The chloride, PrCl₃,7H₂O, forms large green prisms, very readily soluble in water; 100 parts of the solvent at 13° take up 334·2 parts of the hydrated salt, the solution having the specific gravity 1·687. The anhydrous chloride is a pale green deliquescent powder, which melts at a red heat to a clear green liquid; ebullioscopic measurements show that in alcoholic solution it has the simple molecular formula PrCl₃.

The Bromate, Pr(BrO₃)₃,9H₂O, has been obtained by James and Langelier[258] by dissolving the oxide in aqueous bromic acid, and also by double decomposition. It forms greenish hexagonal prisms, melting at 56·5°, and is readily soluble; 100 parts of water dissolve 190 parts of this salt at 25°. At 100° it loses five molecules of water, forming the tetrahydrate Pr(BrO₃)₃,4H₂O, which loses all its water at 130°. The anhydrous salt begins to decompose at 150°.

The sulphate crystallises with 8 molecules of water of crystallisation at ordinary temperatures, but hydrates with 15¹⁄₂, 12, and 5 molecules of water respectively have been described. The octohydrate is considerably more soluble than lanthanum sulphate enneahydrate. The anhydrous salt is a bright green powder.

Praseodymium acetylacetone melts at 146°.

Atomic Weight.

—The value 140·6, adopted by the International Committee, is based on the work of Jones, v. Scheele, Auer von Welsbach, and Feit and Przibylla; the work of Brauner, however, points consistently to a higher atomic weight. Most of these investigators have used the sulphate method. The first determinations of von Welsbach for the newly discovered element[259] gave the value 140·8 (see p. 179); another series of determinations published in 1903[260] gave the mean value 140·57. Jones[261] obtained the sesquioxide for the synthetic sulphate operation by reduction of the peroxide in a current of hydrogen; according to Brauner, this method gives an oxide which is not perfectly pure, probably by absorption of water vapour and carbon dioxide from the air. Jones’ mean value was 140·466. v. Scheele[262] used the same method, as well as a combined oxalate-sulphate method; his figures vary considerably, the mean value being 140·55. Feit and Przibylla,[263] using their volumetric method, obtained the value 140·54.

Brauner’s earlier work,[264] carried out in 1898, gave the value 140·95. In 1901 this author[265] carried out an extensive research on the atomic weight of praseodymium, employing four different methods with spectroscopically pure material; the mean value of his very concordant results was 140·97, almost the value he obtained in 1901. A further investigation into the value of this constant appears desirable.

Neodymium, Nd = 144·3.

Neodymium is, after cerium, the commonest constituent of the cerium group in the more important rare earth minerals, and its separation is therefore by no means so difficult as that of praseodymium. The compounds of the element obtained by von Welsbach in 1885 were not pure, being admixed with samarium compounds which had not been completely separated. Neodymium salts were first prepared free from samarium by Demarçay[268] in 1898; they are of a violet-rose colour, and show in solution a well-marked and characteristic absorption spectrum, the bands being very numerous and sharply defined, and extending over the whole optical region. In chemical as well as in physical and crystallographic properties, they show an extremely close resemblance to the compounds of praseodymium.

On account of the high melting-point, the preparation of the metal presents the same difficulties as that of praseodymium. A current of 90-100 ampères is employed at a potential difference of 15-22 volts; this suffices to raise the thin carbon cathode to a bright white heat, and to fuse the liberated metal. For the properties of the element, see p. 115.

The sesquioxide, Nd₂O₃, when perfectly pure, has a light blue or lilac colour, with a faint reddish fluorescence; the shade varies somewhat according to the method of and temperature employed for the preparation. A bluish or violet-red fluorescence is highly characteristic of the salts, and is particularly noticeable if the powdered recrystallised oxalate be viewed in a good light. The greyish or brownish colour of the oxide observed by some authors is probably due to traces of impurity.[269] The existence of higher oxides of the formulæ Nd₂O₄ and Nd₂O₅ respectively, which Brauner[270] put forward, has been disputed by other writers, though it is found[271] that in the presence of ceria and praseodymia, the sesquioxide can take up more oxygen. Waegner[272] claimed to have obtained the compound Nd₄O₇ by heating the oxalate in a stream of oxygen, though his material, as well as that of Brauner, contained praseodymia. More recently, Joye and Garnier[273] have shown that the spectrum attributed by Waegner to the hypothetical Nd₄O₇ was in reality that of an hydrated oxide, 2Nd₂O₃,2H₂O; these authors have also prepared a second hydrated oxide of the formula 2Nd₂O₃,3H₂O.

The chloride, NdCl₃,6H₂O, is obtained by crystallisation from aqueous solutions; it is also precipitated by addition of water to an alcoholic solution. It forms large deliquescent rose-coloured crystals; 100 parts of water at 13° dissolve 246·2 parts of the salt, the saturated solution having the density 1·741; at 100°, 511·6 parts are dissolved. The solution resembles those of the other chlorides of the group in that it readily dissolves the rare earth oxalates. When heated in a current of hydrogen chloride of 130°, the hexahydrate yields a monohydrate, NdCl₃,H₂O; at 160° the anhydrous chloride is obtained as a very deliquescent rose-coloured powder, which melts at a red heat to a clear red liquid. The anhydrous chloride forms an additive compound NdCl₃,12NH₃, when exposed to the action of ammonia at low temperatures;[274] by gradually heating this, a large number of other additive compounds are formed, containing smaller quantities of ammonia.

The anhydrous iodide, NdI₃, has been obtained[275] by passing hydrogen iodide over the heated anhydrous chloride, and also by heating the carbide in iodine vapour. It fuses to a black liquid, which at a higher temperature suddenly becomes transparent.

The bromate, Nd(BrO₃)₃,9H₂O, which is exactly similar to the analogous compound of praseodymium, forms rose-coloured hexagonal prisms, melting at 66·7°.

The sulphate, Nd₂(SO₄)₃,8H₂O, is isomorphous with the corresponding salt of praseodymium, but is considerably less soluble. Only the one hydrate is known.

The nitrates show an interesting case of isomorphism with the corresponding bismuth nitrate hydrates.[276] The stable form of the neodymium salt is the hexahydrate, Nd(NO₃)₃,6H₂O, whilst the pentahydrate, Nd(NO₃)₃,5H₂O, is labile. Of the bismuth salts, on the other hand, the pentahydrate is stable whilst the hexahydrate is labile; but mixed crystals of both pairs may be obtained, the stable neodymium hexahydrate with the unstable bismuth compound, and the stable bismuth pentahydrate with the labile neodymium salt.