—By reduction of the oxides with carbon in the electric furnace, Moissan obtained the carbides in the form of microscopic yellow crystals. They have the general formula RC₂, and are attacked by water and dilute acids, with evolution of very complex mixtures of gases.[161] The principal product is acetylene, with various higher homologues, and in smaller quantities ethylene and ethane and their homologues. No methane is formed,[162] but hydrogen is always present, the olefines and paraffins probably arising from its action on the acetylenic hydrocarbons. The relation of the rare earth elements to the calcium group is here very close; calcium carbide when attacked by water yields pure acetylene, whereas aluminium carbide gives pure methane.
[161] Damiens, Compt. rend. 1913, 157, 214.
[162] Moissan stated that 24-30 per cent. of methane was formed in this action; compare Compt. rend. 1900, 131, 595.
Halogen Salts.
—The halides of the rare earth elements show a close analogy with the corresponding compounds of the alkaline earth elements. The fluorides are insoluble in water and dilute mineral acids, and are obtained as gelatinous precipitates by the addition of hydrofluoric acid, or a soluble fluoride, to solutions of the salts. They may be prepared in the crystalline condition by heating the carbides in a stream of fluorine, or by the action of hydrofluoric acid upon the hydroxides in aqueous suspension. The rare earth elements, as well as thorium, may be separated from zirconium by taking advantage of the insolubility of their fluorides in excess of hydrofluoric acid or alkali fluorides, since zirconium fluoride is readily soluble in excess of the precipitant. The solubility of the fluorides in a large excess of concentrated acid increases with the electropositive character of the metal, the fluorides of the more negative elements being the least soluble. Thorium and scandium may, therefore, be concentrated to a large extent by repeated precipitation with hydrofluoric acid in acid solution.
The silicofluorides of the rare earth elements have been used by R. J. Meyer in the extraction of scandium from wolframite (see [Chapter I] and [under Scandium], p. 215). They are thrown down as gelatinous precipitates on addition of potassium or sodium silicofluoride to boiling, neutral solutions of rare earth salts. In presence of mineral acids, however, they are not thrown down in the cold; on boiling, the cerium metals are precipitated as fluorides, by hydrolysis of the silicofluorides—the yttrium elements, with the exception of scandium, being held in solution by the mineral acid.
With the exception of the fluorides, the halogen salts of the rare earth metals are readily soluble in water, and crystallise from the concentrated solutions in the hydrated form. The bromides and iodides have not been so fully studied as the chlorides; they are hygroscopic salts, and decompose rather easily. The iodides have been obtained by Moissan in the anhydrous state, by the action of iodine vapour on the carbides at high temperature.
The anhydrous chlorides may be obtained by the application of any of the ordinary methods, e.g. by heating the oxides with carbon in a stream of chlorine, by heating the carbides in the same gas, by heating the sulphides or hydrated chlorides in hydrogen chloride, or by evaporating the solutions of the hydrated salts to dryness in presence of ammonium chloride, and then igniting till the latter has all been removed. As obtained by any of these methods, they are fusible at a red heat, but only slightly volatile; they are easily soluble in water or alcohol, with disengagement of heat. They are insoluble in most organic solvents, but dissolve to some extent in some bases; the chlorides of the yttrium elements, for example, are readily soluble in pyridine. With such solvents, the chlorides form compounds which may be considered as derived from the hydrated forms, by replacement of the so-called water of crystallisation by the organic base.
Conductivity measurements show that the salts are not perceptibly hydrolysed in moderately dilute aqueous solutions, though the values for the equivalent conductivities vary somewhat with the variations in the electropositive character of the elements. In the following table, the equivalent conductivities of the chlorides in solutions of dilution 32 and 1024 at 25°C. are given. It will be seen that the value (λ₁₀₂₄ - λ₃₂) ÷ 10 is in all cases (except for the highly hydrolysed scandium salt) very close to 3, an experimental proof of the trivalent nature of the elements. The values for the chlorides of iron, aluminium and chromium are included; it will be seen that these elements are considerably less positive than the rare earth metals (with the exception, of course, of scandium).
| Salt | λ₃₂ | λ₁₀₂₄ | λ₁₀₂₄ - λ₃₂ |
|---|---|---|---|
| LaCl₃ | 105·8 | 131·5 | 25·7 |
| CeCl₃ | 107·8 | 135·2 | 27·6 |
| PrCl₃ | 105·5 | 135·9 | 30·4 |
| NdCl₃ | 103·8 | 134·3 | 30·5 |
| YtCl₃ | 98·8 | 123·4 | 24·6 |
| YbCl₃ | 107·4 | 140·4 | 33·0 |
| ScCl₃ | 116·9 | 257·9 | 141·0 |
| AlCl₃ | 99·9 | 138·0 | 38·1 |
| CrCl₃ | 98·4 | 152·6 | 54·2 |
| FeCl₃ | 117·2 | 200·7 | 83·5 |