CHAPTER X

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.

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.