—The sulphates of the rare earth elements are obtained by dissolving the oxides or hydroxides in sulphuric acid. From the solutions so obtained, various hydrated salts separate according to the temperature of crystallisation. By heating the hydrated salts to a temperature of 300°-400°, the anhydrous salts are prepared. These are extremely soluble in water at 0°, having a great tendency, which is indeed to be observed in the hydrated forms also, to form supersaturated solutions. When the temperature of such a solution is allowed to rise, larger or smaller quantities of an hydrated form separate out, the differences of solubility among the sulphate hydrates of the various elements being sometimes considerable.
The hydrated sulphates of the cerium elements have been very closely studied in connection with the purification of thorium. Cerium sulphate itself forms hydrates with 12, 9, 8, 5, and 4 molecules of water, but sulphates of the other elements generally form fewer hydrates; the commonest have 12, 8, or 4 molecules of water, and numerous cases of isomorphism are known among them. The solubility curve of the cerium sulphate hydrates is shown in the diagram. [Fig. 3]. The sulphates of the yttrium elements have not yet been systematically investigated, and in most cases only the octohydrates are known. Scandium sulphate is notably different from the other sulphates, in that it is considerably more soluble, and crystallises with six molecules of water.
Fig. 3.
It is an important characteristic of the rare earth elements that the solubility of the sulphates diminishes rapidly as the temperature rises. The study of the various equilibrium conditions is greatly complicated by the tendency to form supersaturated solutions, and the fact that many hydrates can exist throughout considerable ranges of temperature in the metastable condition; in consequence of this, also, the solubilities of many hydrates are known for temperatures far beyond the transition points. Foreign elements may be separated by taking advantage of the very great solubility of the anhydrous sulphates at 0°, and the rapid decrease in solubility with rise of temperature. For this purpose, a solution of the anhydrous sulphates saturated at 0° is prepared, and after filtration is slowly allowed to come to room temperature; the hydrated rare earth sulphates then separate, leaving in solution the foreign sulphates. This method may indeed be used instead of the oxalate separation (see [p. 147]).
In presence of excess of sulphuric acid, acid sulphates of the general formula R(HSO₄)₃ are formed. These are fairly stable, and must be heated to a temperature of 400°-500° to decompose them completely to the normal salts; even at that temperature, traces of acid are tenaciously retained, a fact which renders the determination of the equivalents by the sulphate method unreliable, unless special precautions are taken. On further heating, the normal sulphates pass into basic salts, R₂O₃,SO₃, and finally, at the temperature of the blowpipe flame, into the oxides. The temperatures at which these decompositions occur vary with the positive character of the elements; the most basic oxide clings most tenaciously to sulphuric anhydride, and forms the most stable acid salt. Lanthanum sulphate, for example, requires to be heated for a considerable time at a white heat if the pure oxide is required, whilst the sulphates of the less positive elements are easily decomposed at a red heat. The order of basic strength of the oxides, as determined by the ease with which the sulphates are decomposed, seems, however, to be very different from the order determined by decomposition of the nitrates (see [p. 118]).
With the alkali sulphates, the sulphates of the rare earth elements readily form double salts, which are of great importance in separation, on account of the great differences in solubility. The double sulphates of the cerium group are almost insoluble in excess of alkali sulphate, whereas the yttrium double sulphates, with the exception of those of the terbium metals, which occupy an intermediate position, are very easily soluble. This method of separating the elements into the two main groups was first employed by Berzelius, and though a century has elapsed, it remains to-day the most efficient method of effecting the separation.
The ethylsulphates have been employed by Urbain and others in effecting separations, especially in the erbium and terbium groups. The solubilities of these salts are in the same general order as those of the alkali double sulphates, and they are especially convenient for separating the metals into the three groups of the cerium, terbium, and yttrium elements respectively. They may be prepared by double decomposition of the rare earth sulphates with barium ethylsulphate, but on account of the ease with which the alkylsulphates are hydrolysed by acids, it is essential that the solutions should be quite neutral. A more convenient method, according to James, is the treatment of the anhydrous chlorides in alcohol solution with sodium ethylsulphate dissolved in the same medium; sodium chloride is precipitated, whilst the ethylsulphates of the rare earth elements remain in solution.
The sulphites of the rare earth elements are sparingly soluble crystalline salts, of the general formula R₂(SO₃)₃,xH₂O. They are obtained by passing sulphur dioxide into a suspension of the hydroxides in water, or by double decomposition of soluble salts with alkali sulphite. They dissolve in excess of sulphurous acid, and on evaporation of the solution are deposited unchanged. They are distinguished from thorium sulphite by the fact that they form no alkali double salts. The strongly electropositive character of the rare earth metals is shown by the fact that they form normal and not basic sulphites.
The thiosulphates are readily soluble, crystalline bodies. With the exception of the ceric and scandium salts, they are not hydrolysed in boiling solution, a fact which allows of a complete separation from the readily hydrolysed thiosulphates of zirconium and thorium.