CHAPTER XI

THE CERIUM GROUP--CERIUM

The extraction of the rare earth elements from minerals, by which they are obtained in the form of the oxalates, and the methods of bringing these into solution, have already been described. From the solution, before any separation of the rare earths is attempted, thorium should be removed; for this purpose, any of the methods described under estimation of thorium (see p. 286) may be used, the most convenient being the peroxide precipitation of Wyrouboff and Verneuil.

The solution is then treated with potassium sulphate until the absorption bands of didymium (praseodymium and neodymium) can no longer be observed, or appear only very faintly, when a layer of the solution is examined with a spectroscope; the precipitate then consists of the potassium double sulphates of the cerium with some of the terbium elements. If the mixture is very rich in the cerium elements, and correspondingly poor in the yttrium elements--as, for example, the mixture of earths obtained from monazite--Drossbach[195] recommends a preliminary separation by means of the double carbonates; the double sulphate method may then be employed to remove the last of the yttrium and most of the terbium elements. The sparingly soluble double sulphates of the cerium metals may be transformed into the hydroxides by digestion with potassium hydroxide, and these taken into solution, after washing, by hydrochloric or nitric acid.

[195] _Ber._ 1900, ~33~, 3506.

~Cerium~, Ce = 140·25

Of all the rare earth elements, cerium, by virtue of its property of forming ceric salts corresponding to the dioxide CeO₂, is the one most easily separated and obtained in the pure state. In those compounds in which it is tetravalent, cerium functions as a much less strongly electropositive element than in the cerous compounds, and all the methods of separation are based on this fact. Mosander, who first demonstrated that the old ‘ceria’ was a mixture, separated the element by treating a suspension of the hydroxides in potassium hydroxide with chlorine; yellow ceric hydroxide remains undissolved, whilst the other elements go into solution as the chlorides and hypochlorites. This method was extensively used until quite recently; it has the advantage of separating the cerium completely, but the product is very impure, and several repetitions are required to give good results. The basic nitrate method, which is now used on the commercial scale in extracting cerium from monazite (see p. 284), is also due to Mosander, though it has been employed subsequently by many workers.

Several methods take advantage of the ease with which the ceric salts, as compared with salts of the trivalent elements, may be hydrolysed. Brauner[196] dissolves the oxides in nitric acid, and after removal of excess of acid, boils with a large volume of water--basic ceric nitrate is thrown down, the other elements remaining in solution as nitrates. The precipitate is redissolved, and the process repeated until the cerium is found spectroscopically to be free from didymium. The hydrolysis of the ceric salt may be effected more quickly and completely by the addition of ammonium sulphate or magnesium acetate.[197] James[198] boils the solution of the nitrates with potassium bromate, keeping the whole neutral by addition of powdered marble; the cerium is completely and very quickly precipitated as basic nitrate.

[196] _Trans. Chem. Soc._ 1885, ~47~, 879.

[197] Meyer and Koss, _Ber._ 1902, ~35~, 672.

[198] _J. Amer. Chem. Soc._ 1912, ~34~, 757; this paper gives a complete scheme for a full separation of all the elements.

An interesting method is due to Koppel[199]; the oxides are dissolved in a solution of hydrogen chloride in methyl alcohol, and treated with pyridine, when the sparingly soluble double chloride, (C₅H₅NH)₂CeCl₆, separates, and may be obtained pure by recrystallisation from alcohol and ether. The permanganate method of Drossbach, which is used on the commercial scale, is described on p. 285.

[199] _Zeitsch. anorg. Chem._ 1898, ~18~, 305.

The cerium compounds obtained by these methods are purified by transformation into the anhydrous sulphate, which is dissolved in ice-water; when this solution is allowed to come slowly to room temperature, the pure octohydrate separates. Pure cerium salts should show no trace of absorption when concentrated solutions are examined spectroscopically; on ignition, the oxide obtained should be almost colourless, having at most a faint yellow tinge. A reddish or brownish-red shade indicates the presence of praseodymium. An arc spectrum examination will generally show the presence of lanthanum, which occurs in traces even in the most carefully purified cerium preparations.

The preparation and properties of metallic cerium have already been described (see p. 114); for an account of the pyrophoric alloys, see p. 314.

THE CEROUS COMPOUNDS

The salts of trivalent cerium are very similar to those of the other rare earth elements, and a detailed description of them is therefore unnecessary. The _sesquioxide_, Ce₂O₃, cannot be obtained by ignition of the oxalate, nitrate, or other similar salt, since these decompose at high temperatures with formation of the dioxide, CeO₂. It has been prepared by the reduction of the dioxide with calcium;[200] it has a great affinity for oxygen, and readily absorbs the gas when exposed to moist air. _Cerous hydroxide_, Ce(OH)₃, obtained by addition of alkali to solutions of cerous salts, has also strong reducing properties,[201] and can only be prepared and preserved when oxygen is carefully excluded. It has been obtained as a perfectly white solid by the action of water on the carbide;[202] when dried in an inert atmosphere, it yields a perfectly white oxide. In presence of air, it darkens, assuming a reddish-violet colour, which passes into yellow as the oxidation becomes complete. The oxidation proceeds more quickly in presence of potash or soda, ceric hydroxide, Ce(OH)₄, being formed; in presence of potassium carbonate, however, a dark-coloured peroxyhydrate is formed by autoxidation. The colour so produced disappears on shaking if an ‘acceptor’ is present, ceric hydroxide being left; if the acceptor cannot reduce this, the solution after shaking loses the power of re-forming the dark peroxide, but if the acceptor can reduce the ceric compound to cerous hydroxide, the solution after shaking regains the power of forming the peroxide which is a property of the lower hydroxide.

[200] Burger, _Ber._ 1907, ~40~, 1652.

[201] Dennis and Magee, _J. Amer. Chem. Soc._ 1894, ~16~, 649; also Biltz and Zimmerman, _Ber._ 1907, ~40~, 4979.

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

_Cerous nitride_, CeN, has been prepared by Moissan[203] by the action of ammonia on the heated carbide; it can also be obtained by heating the hydride in a stream of nitrogen.[204] Muthmann and Kraft also state[205] that it can be prepared by heating metallic cerium in the gas, the metal burning with the liberation of much energy in the form of heat and light; but Dafert and Miklanz[206] deny that it can be obtained in this way. Cerium nitride is a lustrous, brass yellow to bronze coloured solid, stable in dry air, but at once attacked by moist air, with evolution of ammonia, and formation of the dioxide. When moistened in air with a few drops of water, the substance reacts violently, becoming heated to redness. Alkalies and acids decompose it, with formation of cerous compounds.

[203] _Compt. rend._ 1900, ~131~, 865.

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

[205] _Annalen_, 1902, ~325~, 261.

[206] _Loc. cit._

The _sulphide_, Ce₂S₃, has been prepared by Biltz[207] by heating the sulphate to a red heat in a current of sulphuretted hydrogen; he describes it as a red powder. The _chloride_, CeCl₃, combines with ammonia with evolution of heat even at a temperature of -80°. Five additive compounds are described;[208] they are white powders, decomposed by water.

[207] _Ber._ 1908, ~41~, 3341.

[208] Barre, _Compt. rend._ 1913, ~156~, 1017.

The solubility curve of the various _sulphate hydrates_ has already been given (see p. 125). Various _double sulphates_ with ammonium sulphate, and the sulphates of sodium, potassium, thallium and cadmium are known. The cadmium double compound has the composition Ce₂(SO₄)₃,CdSO₄,6H₂O, and is prepared by mixing solutions of the simple salts in presence of sulphuric acid. Many _double nitrates_ have been prepared; these are for the most part stable, highly crystalline compounds, easily soluble in water and alcohol. With the nitrates of the common divalent metals, cerous nitrate forms a series of double salts of the general formula 2Ce(NO₃)₃,3R(NO₃)₂,24H₂O, where R = Mg, Mn, Co, Ni, or Zn; these form an isomorphous series, crystallising in the hexagonal system. The _acetylacetone compound_ melts at 131°-132°.

In the presence of hydrogen peroxide in the cold, ammonia throws down from solutions of cerous salts a reddish-brown peroxyhydrate, Ce(OOH)(OH)₃,[209] which on heating loses oxygen, and yields ceric hydroxide. The reaction is very delicate, and may be used as a test for cerium. If the precipitate be treated with acids in the cold, ceric salts are first obtained, but these are at once reduced, in the acid solution, by the hydrogen peroxide formed, so that cerous salts remain; ceric salts may be obtained by first boiling the suspension of the peroxyhydrate and treating the ceric hydroxide so obtained with acids.

[209] Pissarjewski, _Zeitsch. anorg. Chem._ 1902, ~31~, 359.

THE CERIC COMPOUNDS

The ceric salts are much more readily hydrolysed than the cerous salts, and show a great tendency, in dilute solution, to pass over into the latter. So great is this tendency that a solution of a ceric salt acts as if it were supersaturated with oxygen; ceric sulphate, for example, in dilute solution slowly evolves oxygen, whilst the chloride evolves chlorine. In consequence of this behaviour, ceric compounds have a very powerful oxidising action. The ceric salts are yellow to red in colour; their solutions are strongly acid, owing to the ease with which the salts hydrolyse, and on boiling deposit insoluble basic salts.

Beside the methods which have already been mentioned, ceric compounds may be prepared from cerous by oxidation with sodium peroxide, bismuth tetroxide, ammonium persulphate, etc. In electrolysis of cerous salts, also, ceric compounds are obtained at the anode.

_Ceric hydroxide_, Ce(OH)₄, is obtained as a gelatinous yellow precipitate on the addition of alkali to a solution of a ceric salt, or by the oxidation of cerous hydroxide. The freshly prepared precipitate dissolves in nitric acid with a reddish colour; hydrochloric acid reduces it, with evolution of chlorine, and formation of cerous chloride, whilst sulphuric acid dissolves it with partial reduction, oxygen being evolved. If a solution of a ceric compound be dialysed for some days, a clear neutral solution is obtained, which contains the hydroxide in the colloidal condition; by evaporation of the solution, a gummy mass is obtained, which dissolves again in water to a clear solution. Electrolytes rapidly cause coagulation.

_Cerium dioxide_, CeO₂, is obtained by the ignition of any salt of cerium with a volatile acid, or by burning the element in oxygen; the latter reaction produces a very intense and blinding light, on account of which cerium compounds are often suggested for use in flashlight powders (see p. 319). The pure oxide should be almost white, or at most a very faint yellow, but the exact shade and appearance vary according to the method and temperature employed in preparation, doubtless by reason of the possibility of different degrees of polymerisation.[210] The oxide can act as an oxygen carrier towards other substances, notably towards other oxides of the rare earth group,[211] but the phenomena have not been fully elucidated. In virtue of this property, the dioxide has been proposed as a substitute for platinised asbestos in Dennstedt’s method for the combustion of organic bodies.[212]

[210] See in this connection Wyrouboff and Verneuil, _Compt. rend._ 1898, ~127~, 863; _ibid._ 1899, ~128~, 501; and in _La chimie des terres rares_, ‘Conférences de la Société chimique de Paris,’ Paris, 1903.

[211] See Meyer and Koss, _Ber._ 1902, ~35~, 3740.

[212] Bekk, _Ber._ 1913, ~46~, 2574.

The ignited oxide is soluble in nitric or hydrochloric acid only in presence of a reducing agent. Concentrated sulphuric acid converts it into ceric sulphate; fused bisulphate attacks it more readily. In the crystalline form, obtained by fusing the amorphous form with borax, or a suitable salt,[213] it is extremely resistant to acids and to alkalies.

[213] See, _e.g._ Sterba, _Ann. Chim. Phys._ 1904, [viii.], ~2~, 193.

By heating the dioxide in a stream of hydrogen, care being taken to exclude air, a dark blue oxide, of which the composition corresponds approximately to that required by the formula Ce₄O₇, is obtained.[214] This substance has strong reducing properties; when warmed in air, it glows, forming the dioxide, and reduces carbon dioxide when heated in a current of that gas. This _intermediate oxide_ is said to correspond in composition to the violet hydroxide which is obtained as an intermediate product in the oxidation of cerous to ceric hydroxide, and which is said to yield the blue oxide, Ce₄O₇, when dried _in vacuo_.

[214] Sterba, _Compt. rend._ 1901, ~133~, 221; Meyer, _Zeitsch. anorg. Chem._ 1903, ~37~, 378.

The _disulphide_, CeS₂, has been obtained by Biltz[215] by prolonged heating of anhydrous cerous sulphate in a current of sulphuretted hydrogen at a dull red heat; it is a dark, yellowish-brown, crystalline solid, which on treatment with hydrochloric acid yields hydrogen persulphide.

[215] _Ber._ 1908, ~41~, 3341.

_Halogen salts._--No halogen compounds are known in the free state, except the _fluoride_, CeF₄,H₂O, which was obtained by Brauner as a yellowish-brown mass, by the action of hydrofluoric acid on the hydroxide. A _double fluoride_, 2CeF₄,3KF,2H₂O, was prepared by the same author by dissolving the hydroxide in potassium hydrogen fluoride; it is insoluble in water. By dissolving a ceric salt in concentrated hydrochloric acid, a dark red solution is obtained, which is believed to contain the unstable complex acid, H₂CeCl₆; this decomposes slowly in the cold, more quickly on warming, with evolution of chlorine, and formation of cerous chloride. Several double compounds of ceric chloride with hydrochlorides of organic bases have, however, been obtained.

_Ceric sulphate_, Ce(SO₄)₂, is obtained by the action of concentrated sulphuric acid on the dioxide. It is a deep yellow crystalline powder, dissolving readily in water to a brown solution, which has a strongly acid reaction; on warming or diluting, a basic sulphate separates. The solution slowly evolves oxygen, and therefore always contains cerous compounds. On evaporation, _a cero-ceric acid sulphate_ of the formula HCe^{iii}Ce^{iv}(SO₄)₄,12(13 ?)H₂O first separates; the hydrated sulphate Ce(SO₄)₂,4H₂O, being more soluble, separates on further concentration.[216] The relative amounts of the two compounds obtained depends on the temperature and the concentration of acid in the solution; if both these factors are kept low, the almost pure hydrated sulphate can be at once obtained. This separates in yellow crystals belonging to the rhombic system; it is readily soluble in water. The mixed acid salt is less soluble, and forms orange prisms and needles, which cling tenaciously to sulphuric acid. Other complex and double salts have also been obtained. When, for example, silver nitrate is added to a warm solution of the sulphate in concentrated sulphuric acid, a bright orange-yellow precipitate of the salt 10Ce(SO₄)₂,6Ag₂SO₄ is obtained.[217]

[216] See Meyer and Aufrecht, _Ber._ 1904, ~37~, 140; Brauner, _Zeitsch. anorg. Chem._ 1904, ~39~, 261.

[217] Pozzi-Escot, _Compt. rend._ 1913, ~156~, 1074.

Neutral ceric nitrate is unknown. A _basic nitrate_, Ce(NO₃)₃OH,3H₂O, is obtained in red crystals by evaporation of a solution of ceric hydroxide in strong nitric acid. The solid is readily soluble in water, forming a yellow, acid solution, which becomes paler by hydrolysis, on warming or on standing. The course of the hydrolysis is also indicated by the action towards acids, and towards hydrogen peroxide.[218] A freshly prepared ceric salt, on addition of acid, becomes immediately much darker in colour, whereas the colour change is very slow, if considerable hydrolysis has occurred. Similarly, hydrogen peroxide at once reduces a freshly prepared solution, forming colourless cerous salts, whilst if much hydrolysis has occurred, deeply coloured higher oxidation products are at first formed, and these lose their colour only slowly.

[218] Meyer and Jacoby, _Zeitsch. anorg. Chem._ 1901, ~27~, 359.

The _double ceric nitrates_[219] are a large and very important class of compounds; they are the most stable of the ceric salts. With nitrates of the monovalent metals, ceric nitrate forms double nitrates of the type R₂Ce(NO₃)₆; these are deep red hygroscopic substances, crystallising in the monoclinic system, readily soluble in water and alcohol, but dissolving only sparingly in nitric acid. The ammonium salt is important for the separation of cerium. A series of double nitrates with the nitrates of manganese, magnesium, zinc, nickel, and cobalt has the general formula RCe(NO₃)₆,8H₂O, but these are much less stable in solution than the alkali double salts.

[219] Meyer and Jacoby, _loc. cit._

ATOMIC WEIGHT OF CERIUM

No less than twenty-eight separate determinations of the atomic weight of cerium have been carried out. The earlier determinations are rendered unreliable by the almost certain presence of other elements, and Brauner[220] has shown that some of the methods employed in later work give erroneous results.

[220] _Trans. Chem. Soc._ 1885, ~47~, 879; also _Zeitsch. anorg. Chem._ 1903, ~34~, 207.

A very careful determination was made by Robinson in 1884.[221] Cerium oxalate was heated in a stream of dry hydrogen chloride, mixed with carbon dioxide, and the anhydrous chloride freed from traces of acid in a vacuum over chalk. The weighed chloride was then dissolved in water, and titrated with silver nitrate. He obtained the value 140·26; recalculation from his data with the modern values for silver and chlorine give 140·19. Brauner points out that this result is too low, since no account was taken of the solubility of silver chloride in water. In the following year, Brauner[222] determined the ratio Ce₂(SO₄)₃ : 2CeO₂, and obtained the atomic weight 140·22. Wyrouboff and Verneuil[223] in 1897 disputed Brauner’s work, and as a result of several determinations gave the values 139·21, 139·43, and 139·50; their determinations, however, varied very considerably, and the work has been severely criticised by Brauner. In 1903, the latter author and Batěk[224] obtained the values 140·21 and 140·27 by the sulphate and oxalate methods respectively; whilst in the same year, using the same methods, Brauner[225] obtained from three independent series of determinations the values 140·25, 140·24, and 140·25.

[221] _Proc. Roy. Soc._ 1884, ~37~, 150.

[222] _Loc. cit._

[223] _Compt. rend._ 1897, ~124~, 1300.

[224] _Zeitsch. anorg. Chem._ 1903, ~34~, 103.

[225] _Zeitsch. anorg. Chem._ 1903, ~34~, 207.

The International Atomic Weight Committee have accepted the value 140·25 since 1904.

DETECTION AND ESTIMATION OF CERIUM

The detection of cerium in a mixture of earths is a comparatively simple matter, as it has several distinctive reactions. The brown colour of the peroxy-compounds has been suggested as a convenient test by several authors. This may be observed when ammonia is added to a cerous salt in presence of hydrogen peroxide. In the presence of a large excess of foreign earths, very dilute ammonia should be added, drop by drop, with continuous shaking, until a small permanent precipitate remains; this will be rich in the weakly basic ceric hydroxide, and on addition of the peroxide solution will show the colour clearly.[226] For very small quantities of cerium, the neutral solution is added to warm concentrated potassium carbonate solution, and one or two drops of dilute hydrogen peroxide added to the clear liquid; the yellow colour is then very characteristic.[227]

[226] Marc, _Ber._ 1902, ~35~, 2370.

[227] Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94.

Biltz and Zimmerman[228] employ the reducing powers of cerous hydroxide; ammoniacal silver nitrate is added to the neutral solution of the cerous salt, and the mixture warmed. Dilute solutions (1-2 mgms. per litre) give a brown colour, concentrated solutions a black precipitate. The oxidation of an ammoniacal solution of the tartrate by air or hydrogen peroxide, by which an intense yellowish brown colour is developed, has been recently suggested by Wirth[229] as a very delicate test for the element.

[228] _Ber._ 1907, ~40~, 4979.

[229] _Abstr. Chem. Soc._ 1913, ~104~, ii. 712.

_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:

[230] _Die Spektren der Elemente, etc._, Leipzig and Vienna, 1911.

[231] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 531.

[232] _Astrophys. J._ 1909, ~29~, 352.

4150·11 4186·78 4222·78 4296·88 4337·96 4382·32 4386·95 4460·40 4479·52 4487·06 4527·51 4528·64 4539·90 4562·52 4572·45 4594·11 4628·33 5512·72

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]

[233] _Ber._ 1900, ~33~, 1924.

[234] _Ber._ 1903, ~36~, 282 and 1732.

[235] _J. Amer. Chem. Soc._ 1909, ~31~, 523; see also Metzger and Heideberger, _ibid._ 1910, ~32~, 642.

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.

[236] _Zeitsch. anorg. Chem._ 1907, ~54~, 104; see also Roberts, _ibid._ 1911, ~71~, 305.

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.

[237] Browning and Palmer, _Zeitsch. anorg. Chem._ 1908, ~59~, 71.