GENERAL METHODS OF SEPARATION
The chemist who sets out to prepare a pure compound of a rare earth
element is faced by a great difficulty. The rare earth compounds occur
in nature, as one might expect from their great similarity, as mixtures
of very complex composition. After the relatively simple separation from
foreign elements has been accomplished, the enormously greater
difficulty of separating the elements from one another has to be
encountered. So great is this difficulty, by reason of the fact that,
with the sole exception of cerium, the elements show no variation in
property sufficient to allow of the use of ordinary analytical methods,
that even at the present day it is extremely doubtful if all the
elements in the yttrium group are known to us.
The methods which can be adopted in attempting a separation are of two
kinds. The first includes those processes which take advantage of the
gradual variation in basic strength of the hydroxides as the atomic
weight changes; the most important of these are fractional precipitation
of the hydroxides, and fractional decomposition of the nitrates.
Fractional precipitation of the hydroxides is generally effected by
gradual addition of ammonia, soda, magnesia, or other base, to a
solution of the mixed salts; such a solution may also be digested with
the oxides obtained by ignition of another fraction of the rare earth
compounds. If the digestion be sufficiently complete, the precipitate in
each case will be richer in the less basic hydroxides, whilst the
solution will be richer in the salts of the more electropositive
elements.
The fractional decomposition of the nitrates is based on the fact that
when a mixture of the salts is heated gradually, the nitrate of the
least positive element begins to decompose first. The temperature is
maintained for some time at the point at which decomposition begins;
when nitrous fumes cease to be evolved the mixture is cooled, and
extracted with water or dilute acids. The insoluble portion--basic or
superbasic nitrate (see p. 128)--will then be richer in the less
electropositive elements; the solution is evaporated, and the solid so
obtained subjected to a somewhat higher temperature, and the process
repeated several times. In this way, a series of fractions is obtained,
in which the elements tend to distribute themselves in order of
electropositive character. By a sufficient number of systematic
repetitions of such steps, the elements may eventually be obtained in
the form of compounds of approximate purity, which may then be refined
by one of the methods of the second kind described below. Experience has
shown, however, that a quicker and more complete separation may
generally be effected by combining two or more methods of separation;
one method will give the best separation up to certain limits, but then
becomes much less valuable; the separation at this point is therefore
taken up by another process. A process depending on differences of basic
strength of the hydroxides is generally supplemented by a method of the
second class, i.e. a process of fractional crystallisation; where the
basicity method is not used (as, for example, in most of the recent
processes for separation of the cerium elements), two or more different
methods of fractional crystallisation will supplement one another.
The methods of the second class, which are processes of fractional
crystallisation, depend on the differences in solubility which are
observed in analogous compounds in passing from one member of the group
to another. The value of these methods, as opposed to the methods
depending on differences in basic strength, was clearly shown by Auer
von Welsbach, who in 1885 succeeded in resolving Mosander’s ‘Didymium’
into two new elements, praseodymium and neodymium, by fractional
crystallisation of the ammonium double nitrates; since that date, much
attention has been devoted to the task of finding rare earth compounds
which will lend themselves to such processes. The method is extremely
laborious, and may involve several thousand recrystallisations, in
consequence of the generally very slight differences of solubility, and
the ease with which the rare earth compounds, being almost always
isomorphous with one another, form mixed crystals.
Whilst the method of fractional crystallisation has come into general
use for the separation of one element from another only within the last
thirty years, processes for the separation of the cerium group from the
yttrium elements, depending on differences of solubility, have long been
known and used. The most important of these, the double sulphate method,
depends on the fact that the potassium double sulphates of the cerium
metals are almost insoluble, whilst those of the terbium group are
sparingly, and of the yttrium group readily soluble in a concentrated
solution of potassium sulphate. The cerium elements may be thus
completely removed from a solution of mixed salts by addition of a crust
of potassium sulphate crystals, or of an hot concentrated solution of
the same reagent. In other cases, _e.g._ in the double carbonate and
double oxalate processes, separation is effected by taking advantage of
the greater tendency to the formation of double salts possessed by the
yttrium metals.
In effecting a separation of closely related bodies by fractional
processes, in which a large number of repetitions of the same operation
are necessary, only the most careful and systematic procedure can avoid
much waste of valuable material; in these processes, the object of the
chemist is to obtain pure end fractions, whilst keeping the middle
fractions as small as possible. One method of procedure generally
adopted is illustrated in Fig. 7, which represents a fractional
crystallisation of a mixture of four or five substances, α, β, ... φ;
the separations being usually conducted in such a way that subgroups of
three, four or five elements are first obtained, these being then
further fractionated to obtain the pure elements. In the diagram, crops
of crystals are represented by crosses, the mother-liquors by circles;
for the sake of illustration, the process is made to appear as simple
as possible.
[Illustration: FIG. 7]
The mixture is dissolved up, and allowed to crystallise; the crystals
are filtered off, the filtrate concentrated, and a second crop obtained;
this is repeated until five or six crops of crystals have been obtained.
These, with the mother-liquor, constitute series A. The first fraction
is now recrystallised; it yields a crop of crystals, fraction 1 of
series B, and a mother-liquor, which is added to fraction 2 of series A,
as indicated by the dotted arrow and circle; on recrystallisation of
this mixture, a crop of crystals, fraction 2 of series B, is obtained,
together with a mother-liquor, which is recrystallised with fraction 3
of series A. In this way, by continued repetition, series are obtained,
of which each contains one fraction more than its predecessor; the least
soluble constituent is thus concentrated in the fractions represented on
the left of the diagram, whilst the most soluble accumulates in the
mother-liquors. After a greater or smaller number of series have been
traversed, according to the differences in solubility, the end fractions
in each series will be pure. These are no longer fractionated, and the
number of fractions in each series begins to diminish, as shown on the
diagram. The middle fractions will contain the compounds of intermediate
solubility; these may be separated by further fractionation on the same
lines, or may perhaps be better treated by a different or modified
process.
In a modification of the method, each fraction of series A is
recrystallised separately, yielding a crop of crystals, and a
mother-liquor; series B is then built up by adding to the crystals from
fraction 2 the mother-liquor from fraction 1, to the crystals from
fraction 3 the mother-liquor from fraction 2, and so on; the fractions
in this series are then recrystallised separately, and the third series
built up by the similar combination of the crystals and mother-liquors.
Similar systematic methods of procedure must be adopted in working out
any method of fractional separation; it can at once be seen that where,
as in the rare earth group, only small variations in properties exist,
much time and care must be expended, if pure products are required.
Since the development of the methods of spectrum analysis, the
difficulty of testing the efficiency of a method of separation, and of
examining the purity of the products obtained, has been greatly
lessened. The only reliable test at the disposal of the earlier chemists
was the determination of the equivalent weight, which still constitutes
an important check on the modern methods. Some account of the methods
available for the control of the methods of separation is essential in a
general account of the rare earths; but before describing these, it will
be convenient to give a short description of the methods used in the
extraction of the elements from the rare earth minerals.
EXTRACTION OF THE RARE EARTHS FROM MINERALS
With the exception of those containing large proportions of columbium,
tantalum, and titanium, the rare earth minerals are easily decomposed
by acids. The silicates, as a general rule, can be satisfactorily
treated with hydrochloric acid in the ordinary way, but for large
quantities, the use of sulphuric acid is more desirable. The more
refractory minerals are completely decomposed by fused alkali hydrogen
sulphate; sodium bisulphate is more suitable for this purpose than the
potassium compound, the sodium double sulphates of the rare earth
elements being more soluble than the potassium salts. Hydrofluoric acid
also attacks the refractory minerals very readily; the rare earths, in
this case, are left as the insoluble fluorides.
After decomposition with sulphuric acid or bisulphate, the cold residue
is extracted with water, the rare earth sulphates or double sulphates
being removed in solution. Digestion with nitric acid may be necessary
at this stage, if titanium, columbium, etc., are present; after
filtration, the solution is evaporated to dryness, and the residue
extracted with dilute hydrochloric acid. The solution is saturated with
sulphuretted hydrogen to remove lead, copper, bismuth, molybdenum, etc.,
and treated in the usual way with ammonium chloride and ammonia. The
precipitate is washed, and dissolved in hydrochloric acid, the solution
heated to about 60°, and the rare earths precipitated by addition of
excess of oxalic acid, which holds in solution any zirconium which may
be present. In the presence of phosphates, _e.g._ in the treatment of
monazite or xenotime, the precipitate of oxalates should be ignited to
the oxides, these dissolved in acid, and a second precipitation with
oxalic acid effected; this treatment is necessary to remove phosphoric
acid completely.
~Preliminary examination of the earth mixture.~--Before a method of
separation can be decided upon, some knowledge of the composition of the
mixture to be treated must be obtained. The nature of the mineral used
for the extraction will, as a rule, afford useful information. It is
known that in some minerals the cerium group, in others the yttrium
group, predominates more or less completely; certain minerals, also, are
known to be rich in elements of one or another subgroup. An approximate
knowledge of the relative proportions of the cerium, terbium, and
yttrium groups will be afforded by a rough double sulphate separation;
thorium, zirconium, and scandium come down with the cerium earths. For
approximate separation, Urbain[185] proposes the use of the
ethylsulphates. The yttrium elements can be quickly separated in an
approximate manner by fractional precipitation of the hydroxides with
magnesia. The successive fractions obtained by these methods are
examined spectroscopically; from the results, the composition of each,
and so of the original mixture, may be roughly deduced.
[185] _Ann. Chim. Phys._ 1900, [vii.], ~19~, 184.
THE SPECTRUM EXAMINATION
In no department of chemistry have the methods of spectrum analysis
proved of more value than in the field of the rare earths. They provide
the chemist with a means of following and controlling his processes of
separation which is far more delicate and decisive than the older method
of determining the equivalent weight. Whilst the examination of emission
spectra, and especially of arc spectra, is of decisive value in every
case, it has the disadvantage of requiring delicate and complicated
apparatus and great experimental skill; wherever possible, therefore,
the examination of the absorption spectra is preferred, though this is
useful only for a few of the elements, and varies considerably with the
conditions employed.
~The Absorption Spectra.~--Absorption in the visible region of the
spectrum is observed only with those rare earth compounds which are
coloured, and is of value, therefore, chiefly for identification in the
case of praseodymium and neodymium among the cerium elements, and of
erbium among the yttrium metals; these give characteristic absorption
bands, even in dilute solution. The absorption spectra of the rare earth
compounds are highly characteristic, the bands being well defined and
sharply bounded, whereas coloured compounds of the common elements show
general absorption, or at best diffuse bands, under the same conditions.
In observing an absorption spectrum, the light from a Nernst lamp, or
incandescent burner, is passed through a layer of a suitable solution of
the coloured compound, of known concentration and thickness, and after
collimation is analysed by a suitable prism; the spectrum is observed by
a telescope in the ordinary way. Where accurate readings are not
required, as, for example, in testing for the presence or absence of a
particular element, the position of the bands may be read to a
sufficient degree of accuracy by means of a scale, the image of which is
adjusted to coincide with the spectrum as seen through the eyepiece; but
in mapping a spectrum accurately, more refined methods must of course be
used. The photographic method, in which a photograph of the spectrum is
taken on a plate which bears, for purposes of measurement, a comparison
spectrum of known lines, is very convenient for examining the absorption
in the violet and ultraviolet regions.
The intensity, and to some extent also the position, of bands in an
absorption spectrum may vary considerably, according to the conditions
employed. Of the various factors which must be considered, the
concentration of the solution, the thickness of the layer used, the
nature of the solvent, and of the acid radicle, and the presence of
other earths are the most important. The concentration of the solution,
and the thickness of the layer, which together constitute the Optical
Density, must be so adjusted that the absorption is neither too strong
nor too weak; in the first case the sharp bands tend to merge into broad
diffusion areas, and details are obscured, whilst in the second case the
presence of coloured compounds which do not show strong absorption bands
may be overlooked.
The nature of the acid radicle has considerable influence on the
position of the absorption maxima, the general rule being that the bands
are shifted towards the red end of the spectrum as the molecular weight
of the compound used increases. Naturally, also, the nature of the
solvent has an important effect, all the usual phenomena which must be
considered in the measurement of the physical properties of substances
in solution coming into play; electrolytic dissociation, hydration,
dissociation and the formation of complexes, for example, are all
important factors. The presence of colourless earths has also been found
to cause important differences. It follows, therefore, that for the
chemist, the absorption spectra can be considered as a valuable aid only
in detecting the presence or absence of the three elements which give
the strongest and most characteristic absorption bands, viz.
praseodymium, neodymium, and erbium, and that conclusions regarding the
quantitative composition of mixtures must be drawn with the utmost
caution.
~The Emission Spectra: Spark Spectra.~--The factors which tend to limit
the value of the absorption spectra for analytical purposes, for the
most part disappear when the emission spectra are employed. In the case
of the spark spectra, indeed, great differences are observed according
to the conditions and method of experiment; but the arc spectra are
practically invariable under all conditions, and hence they constitute
the ultimate test in all cases. The spark spectra are observed when one
terminal--the cathode--of an induction coil is embedded in the oxides to
be examined, and the discharge then passed. The discharge is also
frequently passed between platinum poles partly immersed in a strong
solution of a salt of the element under examination; a form of apparatus
very suitable for this method of observation has been described by Sir
W. Crookes.[186] The spectra so obtained are in a high degree
characteristic, but they vary very considerably with the form and
dimensions of the coil, the length and cross-section of the wires, the
potential difference employed, and so on. An entirely new spectrum also
is obtained in many cases by mere reversal of the current; under these
conditions, a phosphorescent appearance is observed, the spectrum of
which--reversed spark spectrum of de Boisbaudran--has been found in many
cases to resemble the cathode luminescence spectra of Crookes.
[186] _Proc. Roy. Soc._, 1903, ~72~, 295.
~The Arc Spectra.~--The final criterion of purity in the examination of
a rare earth element is in almost all cases the arc spectrum. Since for
some of the elements, especially in the yttrium group, the entire
spectrum has not yet been accurately mapped out, spectra are generally
observed frequently throughout the course of a fractionation; by this
means, the separation can be followed by the disappearance of some
lines, and the appearing or strengthening of others, and such
examinations have led occasionally to the discovery of new elements
(see, for example, under Separation of ytterbium earths, p. 205). Such
determinations, however, require much time and extensive and complicated
apparatus.
Carbon electrodes are generally employed, and it is immaterial in this
case which is the anode, and which the cathode. The lower carbon is
hollowed out, and the space filled with the oxide or sulphate of the
element or mixture to be examined; or the electrode may be impregnated
with a concentrated solution of a salt. The light is examined by means
of a diffraction grating, and the spectrum photographed on a plate which
bears a comparison spectrum for measurement. The lines are most numerous
in the violet and ultraviolet regions, and the most characteristic
spectra are given by the colourless earths. The method is naturally more
delicate for some elements than for others; the great persistency of the
scandium line 3613·984, for example, was found very valuable by Crookes
and by Eberhard in the examination of various rocks and minerals for
that element, whilst other intense and persistent lines have served for
the detection of various rare earth elements in the sun and many stars.
~The Cathode Luminescence Spectra.~--The phenomenon of cathode
luminescence, which was observed and very fully investigated by Sir
William Crookes, and which led that author to his theory of
Meta-elements, is one of the greatest scientific interest. Crookes
observed that certain of the rare earths, when subjected to the action
of cathode rays in a vacuum tube, exhibit a brilliant phosphorescence,
which, when examined by the spectroscope, show characteristic spectra,
which differ greatly for fractions of apparently identical chemical
composition, and are otherwise distinguishable by physical properties.
The researches of Lecoq de Boisbaudran, and the more recent work of
Baur and Marc,[187] have shown that this luminescence is observed when a
small quantity of a coloured earth is present with a very large quantity
of a colourless earth, the maximum phosphorescence being produced by
about 1 per cent. of the coloured earth, or ‘phosphorogen.’ The question
has recently been very fully examined by Urbain.[188] He shows that the
sensitiveness of the phenomenon is so great that it cannot be employed
for the ordinary purposes of chemical analysis, one part in a million of
the phosphorogen being sufficient to cause a clearly perceptible
luminescence in a pure colourless oxide.
[187] _Ber._ 1901, ~34~, 878.
[188] _Ann. Chim. Phys._ 1909, [viii.], ~18~, 222; see also
_Introduction à l’étude de la Spectrochimie_, pp. 145 _et seq._
~The Magnetic Susceptibility.~--The fact that the rare earths differ
very considerably from one another in their magnetic properties has been
known for several years,[189] and has recently been employed by Urbain
and Jantsch[190] as a means of identification, and a test of purity, and
for following processes of fractionation. The magnetic susceptibility
reaches a minimum at samarium, and rises very sharply on either side of
that element, so that the presence of the closely related elements,
neodymium on the one side, and europium and gadolinium on the other,
which differ only very slightly from samarium in atomic weight and
solubility, can easily be detected by this means. The property is highly
additive, and can be used, therefore, to estimate the relative
proportions of two oxides in a mixture; the determinations are said to
be easily and quickly carried out.
[189] See Meyer, _Monats._ 1898, ~20~, 369 and 793.
[190] _Compt. rend._ 1908, ~147~, 1286; see also Urbain, _ibid._,
1910, ~150~, 913.
When the elements are considered in order of atomic weight, the
coefficient reaches a maximum at neodymium in the cerium group, and
again at dysprosium (or holmium) in the yttrium group:--[191]
Coefficient of magnetisation
Element. Atomic Weight. for the oxide.
_x_ × 10⁻⁶
Scandium 44·1 -0·05
Yttrium 89·0 -0·14
Lanthanum 139·0 -0·18
Neodymium 144·3 33·5
Samarium 150·4 6·5
Europium 152·0 33·5
Gadolinium 157·3 161
Terbium 159·2 237
Dysprosium 162·5 290
[191] See Urbain and Jantsch, _loc. cit._; the values for lanthana,
scandia, and yttria were determined by Wedekind (see Meyer and
Wuorinen, _Zeitsch. anorg. Chem._ 1913, ~80~, 7).
Erbium, thulium, ytterbium, and lutecium appear in descending order at
the end of the series, but no figures are given.
The most interesting application of the property has been Urbain’s
discovery of the new element Celtium (see p. 207).
THE EQUIVALENT WEIGHT DETERMINATION
The determination of the mean equivalent weight, which was for the
earlier chemists the only reliable method of controlling their
fractionations, is still of considerable importance for this purpose,
especially in the yttrium group, in which the differences in atomic
weights are more considerable than among the cerium metals. Great
importance, moreover, still attaches to these determinations, since they
serve to fix the atomic weights; save that the methods used in an atomic
weight determination are somewhat more elaborate and refined than those
used when it is desired merely to test a fractionation, the same
processes apply in both cases.
The methods which have been most commonly used are those based on a
determination of the ratio R₂O₃ : R₂(SO₄)₃, and these are of two kinds,
the synthetic and the analytical. The first, in which a known weight of
the oxide is converted into the sulphate, has been most used for the
most strongly basic oxides, since with these it is difficult to remove
the last traces of sulphuric anhydride from the oxide by heat. The
oxides are best obtained from the oxalates, which are precipitated from
an acid solution of the nitrates, washed thoroughly with water, alcohol
and ether in succession, dried, and ignited in a tarred platinum
crucible. The oxide is best dissolved in dilute hydrochloric or nitric
acid on the waterbath, a slight excess of sulphuric acid being added
only when a clear solution has been obtained; the liquid is then heated
gradually to 300°, and finally in the electric furnace at 450°-550°
until constant in weight. If sulphuric acid be added directly to the
weighed oxide, particles of the latter may become completely coated with
the insoluble sulphate, and so escape the action of the acid.
In the analytical method, a known weight of sulphate is ignited to the
oxide, and weighed as such. This method is most suitable for the less
basic members of the yttria earths, of which the sulphates can be
completely decomposed without difficulty at a red heat. By the use of
the microbalance, a sufficiently accurate determination can be carried
out by either of these methods in little more than half an hour, as the
chemical changes are exceedingly rapid where only small quantities are
employed, and no time is required to allow the vessels and solids to
cool. Using the microbalance, Brill[192] has carried out a series of
experiments to determine the limits of temperature within which the
various steps of the process should be carried out. He finds that a
temperature of 400°-550° is required to decompose the last traces of
acid sulphate, and give the pure neutral sulphate. Between the
temperatures of 850° and 950°, basic salts are formed, from which the
last trace of sulphuric anhydride is expelled at 900°-1150°; the precise
temperature required in each case depends, of course, on the basic
strength of the oxide in question.
[192] _Zeitsch. anorg. Chem._ 1905, ~47~, 464.
The determination of equivalents by means of the ratio R₂O₃ : R₂(C₂O₄)₃,
has been brought to a high degree of accuracy by Brauner.[193] A weighed
quantity of the carefully prepared oxalate is ignited, with suitable
precautions, to the oxide, in a tarred platinum crucible. A second
weighed specimen of the same oxalate preparation is dissolved in dilute
sulphuric acid, and titrated at 60° with permanganate, which is
standardised against pure ammonium oxalate.
[193] _Ibid._ 1903, ~34~, 103, 207.
Of the methods of volumetric analysis which have been proposed, that put
forward by Feit and Przibylla appears to be the most suitable. A
convenient quantity of oxide, which has been ignited until constant in
weight, is dissolved by gently heating with a known excess of N/2
sulphuric acid, in a conical flask of Jena glass. The excess of acid is
titrated with N/10 sodium hydroxide, using methyl orange as indicator.
This method, which has the advantages of ease and quickness, is very
reliable, if suitable precautions are taken, in the case of the more
strongly basic oxides; but with the least strongly basic members of the
yttria group, the erbia and ytterbia oxides, the end point is not very
sharp, whilst with the weakly basic scandia, the method breaks down
entirely.[194]
[194] _Zeitsch. anorg. Chem._ 1905, ~43~, 202; 1906, ~50~, 249.