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.