THE TERBIUM GROUP
In his examination of the yttria earths in 1842, Mosander described two
new oxides isolated from the old yttria. To one of these, an
orange-yellow earth which yielded colourless salts, he gave the name
Erbia; the second earth, which was colourless and gave rose-coloured
salts, he called Terbia. Bahr and Bunsen examined the yttria oxides in
1866, and obtained only the latter earth, which gave rose-coloured
salts; to this they applied Mosander’s name Erbia, and stated that the
earth to which Mosander had given that name had no existence.
Delafontaine, however, confirmed Mosander’s work, showing that the
orange-yellow earth which yielded colourless salts (Mosander’s Erbia)
had been fractionated out of their material by Bahr and Bunsen in the
double sulphate separation of the cerium group; to avoid further
confusion, however, he proposed to give to this oxide (Mosander’s Erbia)
the name Terbia, leaving for the colourless oxide, which forms
rose-coloured salts (which Mosander had called Terbia) the name Erbia
applied to it by Bahr and Bunsen. This reversed nomenclature has been
generally accepted.
Delafontaine,[300] continuing his work on the earths from samarskite
(see p. 168) announced in 1878 the discovery of a new oxide, Philippia,
intermediate between terbia and yttria; but this was subsequently shown
to be a mixture of yttria and terbia (see p. 133). In the same year,
Lawrence Smith[301] announced the discovery of another oxide, Mosandria,
from the samarskite earths; this was afterwards shown by Lecoq de
Boisbaudran to be a mixture of terbia with gadolinia.[302] In 1880
Marignac[303] announced the discovery of two more new oxides, Y_{α} and
Y_{β} from the same mineral; Y_{β} was afterwards found to be identical
with samaria, whilst Y_{α} was subsequently separated from the old
terbia earths by Lecoq de Boisbaudran, who proposed, with the assent of
Marignac, the name Gadolinium.[304] The terbia left after removal of the
erbia earths and gadolinia was believed by that author to be still a
mixture, a conclusion supported by the work of Hofmann and Kruss in
1893.[305]
[300] _Compt. rend._ 1878, ~87~, 559.
[301] _Ibid._ 1878, ~87~, 146.
[302] _Ibid._ 1886, ~102~, 647.
[303] _Compt. rend._ 1880, ~90~, 899.
[304] _Loc. cit._
[305] _Zeitsch. anorg. Chem._ 1893, ~4~, 27.
In 1886 Demarçay[306] isolated from samaria a new oxide, which he
designated S₁. From his work on this oxide in 1892-1893, de
Boisbaudran[307] concluded that samaria consisted of at least three
oxides, samaria proper, and two new oxides Z_{ξ} and Z_{ε}. In 1896,
Demarçay[308] separated an earth Σ, which showed the spark-spectrum of
Z_{ε} and the reversal spectrum of Z_{ξ}, and finally in 1901[309] he
obtained the new oxide in a fairly pure condition, and gave it the name
Europia.
[306] _Compt. rend._ 1886, ~102~, 1551.
[307] _Ibid._ 1892, ~114~, 575; _ibid._ 1893, ~116~, 611 and 674.
[308] _Ibid._ 1896, ~122~, 728.
[309] _Ibid._ 1901, ~132~, 1484.
The complicated history of the terbium group has been entirely cleared
up by the work of Urbain and his co-workers during the early years of
the present century, and processes have been devised by which the
separation of the three members of the group from one another, and from
the related elements of the erbium group on the one side, and samarium
on the other, can be satisfactorily accomplished. The chemistry of this
group, therefore, may be regarded as satisfactorily settled, though
relatively little is known of the properties of the elements and their
compounds.
In their general chemical relations, elements of the terbium group
occupy an intermediate position between the cerium group and the
elements of the yttrium group in the narrower sense. In the solubility
relations of the double salts, they are bounded on the one side by
samarium and the less soluble cerium group, on the other by dysprosium
and holmium and the more soluble yttrium group. They show only very
slight differences in electropositive character, and methods based on
differences in basic strength of the oxides, therefore, are of very
little use for separating them from one another. Fractional
precipitation with ammonia separates them in the order terbium,
samarium, gadolinium, and europium--samaria being less strongly basic
than the oxides of gadolinium and europium; this constitutes an
exception to the general rule regarding the solubilities of the double
nitrates and sulphates with increasing electropositive character.[310]
The difficulties of separation are greatly increased by the very small
proportions in which the elements are usually found in rare earth
minerals. Gadolinium usually occurs in the largest quantities; in
consequence of this, there is little doubt that most of the material
described by the earlier workers as terbia consisted very largely of
gadolinia.
[310] See Lecoq de Boisbaudran, _Compt. rend._ 1890, ~111~, 394.
The group is not characterised by well-marked absorption spectra;
europium and terbium show weak absorption in the blue region. Terbium,
of which the salts are colourless, forms a very strongly coloured
peroxide, analogous to that of praseodymium; small quantities of this
give to the mixed oxides obtained by ignition the characteristic yellow
colour, whilst mixtures richer in the peroxide become correspondingly
darker and darker.
SEPARATION
In the double sulphate separation of the yttrium and cerium groups, the
terbium elements divide themselves between the soluble and the insoluble
portions; if the separation is made as complete as possible by addition
of a large excess of alkali sulphate under suitable conditions, the
larger part of the compounds of the group will be precipitated with the
cerium elements. In the separation of the cerium elements the terbium
elements collect in the most soluble fractions, and the mother-liquors
of the double nitrate crystallisations therefore form a very convenient
source of these elements. A considerable proportion, however, will
usually remain in solution with the double sulphates of the yttrium
group; in the bromate separation of these (see p. 198), the terbium
elements collect in the least soluble fractions. By careful
fractionation under suitable conditions, the double sulphate method may
be used to separate the terbium group completely from the cerium and
yttrium elements. A very convenient method of separating the terbium
group from a rare earth mixture is the ethylsulphate process of Urbain.
By fractional crystallisation of these salts from alcohol or water, the
separation into three groups can be satisfactorily accomplished.
For the separation of the terbium elements from one another, the nitrate
and double nitrate methods are most suitable. Samarium can readily be
separated by crystallisation of the double magnesium nitrates in
presence of bismuth magnesium nitrate; by continuing the fractionation,
europium magnesium nitrate can be separated in a pure state, as there is
a considerable difference between the solubility of this salt and the
corresponding compound of gadolinium;[311] the process, however, is
somewhat long and tedious. For the separation of gadolinium and terbium,
the double nitrates are converted into the simple nitrates, and these
fractionated from nitric acid in presence of bismuth nitrate. The
gadolinium nitrate separates before the bismuth nitrate, and may be
obtained fairly pure in this way, though the process is extremely
tedious, and several thousand recrystallisations are required.[312]
Terbium nitrate has almost the same solubility as bismuth nitrate, and
the two separate together in the middle fractions. The more soluble
nitrates of the erbia earths collect in the mother-liquors.
[311] James (_J. Amer. Chem. Soc._ 1912, ~34~, 757) employs at this
stage the fractional crystallisation of the double nickel nitrates.
[312] See Urbain, _Compt. rend._ 1904, ~139~, 736.
~Europium~, Eu = 152·0
This element is one of the rarest of the whole group, and occurs only in
extremely small quantities. Monazite sand is said to contain about 0·002
per cent. of the oxide, though on account of the remarkable intensity of
some of the stronger lines in the arc spectrum, Eberhard[313] was able
to detect europium with ease in a mixture of rare earth oxides from
that mineral, after the separation of cerium. The _oxide_ has a pale
rose colour; the salts are also faintly coloured, and in solution show
weak absorption bands.
[313] _Zeitsch. anorg. Chem._ 1905, ~45~, 378.
_Europium sulphate_, Eu₂(SO₄)₃,8H₂O, separates in pink crystals, which
are completely dehydrated at 375°; _europic chloride_, EuCl₃, in the
anhydrous state forms fine yellow needles; _europium oxychloride_,
EuOCl, prepared by heating europic chloride in dry air to 600°, is a
white solid, insoluble in water, but soluble in strong acids; _europous
chloride_, EuCl₂, prepared by reduction of the higher chloride in
hydrogen, is a white amorphous solid, soluble in water to a neutral
solution, which on boiling throws down the oxide, Eu₂O₃.[314] Several
organic salts have been prepared by James and Robinson.[315]
[314] Urbain and Bourion, _Compt. rend._ 1911, ~153~, 1155.
[315] _J. Amer. Chem. Soc._ 1913, ~35~, 754.
~Atomic Weight.~--Using the material isolated from samaria,
Demarçay[316] in 1900, by the synthetic sulphate method, found the
atomic weight of europium to be about 151. Urbain and Lacombe[317]
determined the value in 1904, with material free from gadolinium and
samarium, using the three ratios which they employed in the case of the
latter element (see p. 182); their values, corrected by Brauner, were
152·00, 151·93 and 151·94 respectively. Another series of determinations
was carried out by Jantsch[318] in 1908, the same method being employed;
he obtained the mean value 152·03, with an error of ±·02. The
International Committee have adopted the value 152·0.
[316] _Compt. rend._ 1900, ~130~, 1469.
[317] _Ibid._ 1904, ~138~, 627.
[318] _Ibid._ 1908, ~146~, 473.
~Detection.~--The absorption spectrum was determined by Demarçay,[319]
but is not sufficiently intense or characteristic for ordinary
purposes of detection. The spark spectrum has been investigated by the
same author (_loc. cit._); it is very bright, and shows the three blue
rays which characterised Lecoq de Boisbaudran’s Z_{ε}. The reversal
spectrum shows the characteristic band of Z_{ξ}.
The pure oxide, according to Urbain,[320] shows no luminescence under
the influence of cathode rays, but when impure, or very largely
diluted with lime or gypsum, it gives very bright and characteristic
spectra.
[319] _Ibid._ 1900, ~130~, 469.
[320] _Ibid._, 1906, ~142~, 205, 1518.
The arc spectrum[321] is very characteristic, and contains some
exceedingly intense lines, by means of which Lunt[322] has detected
europium in the sun and in many stars. The lines most suited for
identification of the element are the following:
[321] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl.
Akad. Wiss. Wien_, 1910, ~119~, II_a_, 31.
[322] _Proc. Roy. Soc._ 1907, ~79~; A, 118.
3688·57
3725·10
3819·80
3907·28
3930·66
3972·16
4129·90
4205·20
4435·75
4522·76
4594·27
4627·47
4662·10
6645·44
~Gadolinium~, Gd = 157·3.
Gadolinia is the commonest of the terbia oxides, and occurs in
considerable quantities in some of the rare earth minerals, notably in
samarskite and gadolinite; its separation from the neighbouring oxides,
europia and terbia, is, however, exceedingly difficult, and has only
been satisfactorily accomplished in recent times. The gadolinium
compounds prepared and examined by the earlier workers, as appears from
the atomic weight determinations, must have been associated with earths
of lower atomic weight, and undoubtedly also with small quantities of
terbium. After the isolation of Marignac’s Y_{α}, and the examination of
the element by Lecoq de Boisbaudran, to whom the name gadolinium is due,
further investigations were carried out by Bettendorff[323] and by
Benedicts.[324] Pure gadolinia was probably first obtained by
Demarçay,[325] by fractional crystallisation of the magnesium double
nitrate; the oxide obtained by Urbain and Lacombe[326] by
crystallisation of the nitrates in presence of bismuth nitrate, was
proved to be spectroscopically pure by Eberhard.[327]
[323] _Annalen_, 1892, ~270~, 376.
[324] _Zeitsch. anorg. Chem._ 1900, ~22~, 393.
[325] _Compt. rend._ 1900, ~131~, 343; _ibid._ 1901, ~132~, 1484.
[326] _Ibid._ 1905, ~140~, 583, etc.
[327] _Zeitsch. anorg. Chem._ 1905, ~54~, 374.
The gadolinia obtained by ignition of the salts of volatile acids should
be perfectly white; presence of terbia causes it to assume a yellow
colour.[328] The salts are colourless, and their solutions show no
absorption in the visible region, though Urbain[329] has shown that
there are four strong bands in the ultraviolet.
[328] Eberhard (_loc. cit._) has shown that even in the perfectly
white oxide, traces of terbia can be distinguished by spectroscopic
examination.
[329] _Compt. rend._ 1905, ~140~, 1233.
The _hydroxide_, Gd(OH)₃, is a gelatinous precipitate with strongly
basic properties, rapidly absorbing carbon dioxide from the air. The
_oxide_, Gd₂O₃, also absorbs carbonic anhydride from the air, and is
easily soluble in acids, even after strong ignition. The element is
therefore strongly electropositive. Its position among the yttrium
elements, however, is justified by the properties of the
_platinocyanide_, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red
crystals, with a green metallic lustre, belonging to the rhombic system,
and isomorphous with the corresponding yttrium and erbium salts; the
cerium elements, on the other hand, give yellow platinocyanides, with a
blue metallic lustre, which crystallise in the monoclinic system.
The _nitrate_, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the
ordinary temperatures in large crystals belonging to the anorthic
system, and is isomorphous with the corresponding compounds of
praseodymium and neodymium.[330] From solutions in strong nitric acid, a
pentahydrate is obtained, which melts at 92°; the hexahydrate melts at
91°. The _sulphate_ separates from aqueous solution as the octohydrate,
Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups.
The anhydrous sulphate is much less soluble in water at 0° than the
corresponding compounds of the cerium elements. The _selenate_ forms
hydrates with 10 and 8 molecules of water of crystallisation
respectively; these are isomorphous with the corresponding selenates of
yttrium and the erbium metals.
[330] Lang and Haitinger, _Annalen_, 1907, ~351~, 450.
~Atomic Weight.~--The determinations of this constant made by the
earlier workers were all carried out with impure material and gave
results which were considerably too low. The International Committee
have adopted the value 157·3, which is based on the work of Urbain.[331]
In employing the analytical sulphate method, that author observed that
the anhydrous sulphate did not remain constant in weight when allowed to
remain in a desiccator, and that it could not be accurately weighed. He
therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the
octohydrate directly to oxide, and obtained the mean value 157·24.
[331] _Compt. rend._ 1905, ~140~, 583.
~Detection.~--Pure gadolinium compounds show no absorption in the
visible spectrum, but there are four strong bands[332] in the
ultraviolet, viz. 311·6-310·5; 306·0-305·7; 305·6-305·5; and
305·4-305·0. The arc spectrum[333] is very rich in lines, of which the
most intense are the following:
[332] Urbain, _ibid._ 1905, ~140~, 1233.
[333] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl.
Akad. Wiss. Wien_, 1910, ~119~, II_a_, 21.
3082·15
3100·66
3422·62
3545·94
3549·52
3585·12
3646·36
3671·39
3719·63
3743·68
3768·60
3796·62
3814·18
3852·65
3916·70
4037·49
4050·05
4063·62
4070·51
4073·99
4085·73
4098·80
4130·59
4184·48
4251·90
4262·24
4325·83
4327·29
4342·35
6114·26
The spark spectra have been examined by Demarçay,[334] Baur and
Marc,[335] Urbain[336] and Crookes.[337]
[334] _Compt. rend._ 1900, ~131~, 343.
[335] _Ber._ 1901, ~34~, 2460.
[336] _Loc. cit._
[337] _Proc. Roy. Soc._ 1905, ~74~, 420.
~Terbium~, Tb = 159·2
Terbia occurs among the rare earth oxides in exceedingly small
quantities, and its separation has in consequence presented such great
difficulties that only within the last few years have terbium compounds
been completely freed from gadolinium and neighbouring elements. In 1886
Lecoq de Boisbaudran,[338] by fractional precipitation of the hydroxides
with ammonia, and subsequent fractional crystallisation of the double
sulphates, obtained an oxide much richer in terbia than any specimen
previously prepared; it was dark yellow in colour. In 1902 Marc[339]
obtained from monazite a very dark oxide containing about 15 per cent.
of terbia, whilst Feit[340] in 1905 obtained a dark brown oxide
consisting of gadolinia with about 13 per cent. of terbia. Pure terbium
compounds were obtained by Urbain in 1904,[341] by fractional
crystallisation of the nitrate from nitric acid, in presence of bismuth
nitrate, and by crystallisation of the double nickel nitrates, and
precipitation with ammonia; he showed that the element was identical
with the Z_{δ} and Z_{β} of de Boisbaudran,[342] with the Γ of
Demarçay,[343] and with the G_{β} and possibly the G_{ζ} of Crookes[344]
(see p. 193).
[338] _Compt. rend._ 1886, ~102~, 395, 483.
[339] _Ber._ 1902, ~35~, 2382.
[340] _Zeitsch. anorg. Chem._ 1905, ~43~, 267.
[341] _Compt. rend._ 1904, ~139~, 736; 1905, ~141~, 521; 1909, ~149~,
37.
[342] _Ibid._ 1895, ~121~, 709; 1904, ~139~, 1015.
[343] _Ibid._ 1900, ~131~, 343.
[344] _Trans. Chem. Soc._ 1889, ~55~, 258.
The element gives the white _sesquioxide_, Tb₂O₃, and colourless
salts.[345] The _peroxide_, of which the composition corresponds
approximately to the formula Tb₄O₇, is obtained as a brownish-black
powder by ignition of suitable salts. Its presence, even in small
quantities, gives so deep a colouration to the other earths that some
kind of salt formation seems probable. It is insoluble in cold acids; it
dissolves in hot nitric acid with evolution of oxygen, forming a
solution from which the _nitrate_, Tb(NO₃)₃,6H₂O, melting at 89·3°,
separates on cooling. In hot hydrochloric acid, the peroxide dissolves
with evolution of chlorine, forming solutions from which the _chloride_,
TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely
deliquescent, and easily forms supersaturated solutions. The _sulphate_,
Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of
the oxide by addition of considerable quantities of alcohol; it is
isomorphous with the other sulphate octohydrates, and is completely
dehydrated at 360°.
[345] The terbium compounds here described have been prepared by
Urbain (_loc. cit._) from carefully purified material; other compounds
have been described by Potratz (_Chem. News_, 1905, ~92~, 3), but her
material contained a large proportion of gadolinium.
~Atomic Weight.~--The value adopted by the International Committee is
159·2, which was obtained by Urbain in 1905 (_loc. cit._) from the
ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on
which reliance can be placed, as the material of the earlier workers was
seldom even approximately pure.
~Detection.~--Solutions of terbium salts show only one band in the
visible spectrum, at 487·7 in the blue. This band was observed by
Lecoq de Boisbaudran in a specimen of terbia containing dysprosia, and
assumed by him to belong to a new element, Z_{δ} (_loc. cit._) In the
ultraviolet nine absorption bands have been observed (Urbain, _loc.
cit._)
The spark spectrum shows the lines observed by Demarçay in 1900, and
attributed by him to the new element Γ. Lecoq de Boisbaudran’s element
Z_{β} showed a green fluorescence with the reversed spark, a
phenomenon which Urbain has found to be exhibited by pure terbium
compounds.
The arc spectrum of Urbain’s pure terbia was examined by
Eberhard[346]--see also Exner and Haschek, and Eder and Valenta.[347]
The element may be detected in minerals and earth mixtures by the
following lines:
3523·82
3676·52
3703·05
3704·01
4005·62
4278·71
The chief lines in the arc spectrum (Exner and Haschek) are the
following:
3324·53
3509·34
3531·86
3561·90
3568·69
3600·60
3628·53
3650·60
3659·02
3704·10
3711·91
3848·90
3874·33
3899·34
3925·60
3939·75
3977·01
3982·07
4005·70
4012·99
4278·70
4752·69
Pure terbia does not exhibit the phenomenon of cathode luminescence,
but gadolinia containing a trace of terbia shows a marked green
fluorescence, which was attributed by Crookes to a new Meta-element,
G_{β}. A trace of terbia in aluminium oxide causes the latter to
exhibit a highly characteristic intense white luminescence.
[346] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1906, ~18~, 385.
[347] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
14.