Ceric hydroxide, Ce(OH)₄, is obtained as a gelatinous yellow precipitate on the addition of alkali to a solution of a ceric salt, or by the oxidation of cerous hydroxide. The freshly prepared precipitate dissolves in nitric acid with a reddish colour; hydrochloric acid reduces it, with evolution of chlorine, and formation of cerous chloride, whilst sulphuric acid dissolves it with partial reduction, oxygen being evolved. If a solution of a ceric compound be dialysed for some days, a clear neutral solution is obtained, which contains the hydroxide in the colloidal condition; by evaporation of the solution, a gummy mass is obtained, which dissolves again in water to a clear solution. Electrolytes rapidly cause coagulation.
Cerium dioxide, CeO₂, is obtained by the ignition of any salt of cerium with a volatile acid, or by burning the element in oxygen; the latter reaction produces a very intense and blinding light, on account of which cerium compounds are often suggested for use in flashlight powders (see [p. 319]). The pure oxide should be almost white, or at most a very faint yellow, but the exact shade and appearance vary according to the method and temperature employed in preparation, doubtless by reason of the possibility of different degrees of polymerisation.[210] The oxide can act as an oxygen carrier towards other substances, notably towards other oxides of the rare earth group,[211] but the phenomena have not been fully elucidated. In virtue of this property, the dioxide has been proposed as a substitute for platinised asbestos in Dennstedt’s method for the combustion of organic bodies.[212]
[210] See in this connection Wyrouboff and Verneuil, Compt. rend. 1898, 127, 863; ibid. 1899, 128, 501; and in La chimie des terres rares, ‘Conférences de la Société chimique de Paris,’ Paris, 1903.
[211] See Meyer and Koss, Ber. 1902, 35, 3740.
[212] Bekk, Ber. 1913, 46, 2574.
The ignited oxide is soluble in nitric or hydrochloric acid only in presence of a reducing agent. Concentrated sulphuric acid converts it into ceric sulphate; fused bisulphate attacks it more readily. In the crystalline form, obtained by fusing the amorphous form with borax, or a suitable salt,[213] it is extremely resistant to acids and to alkalies.
[213] See, e.g. Sterba, Ann. Chim. Phys. 1904, [viii.], 2, 193.
By heating the dioxide in a stream of hydrogen, care being taken to exclude air, a dark blue oxide, of which the composition corresponds approximately to that required by the formula Ce₄O₇, is obtained.[214] This substance has strong reducing properties; when warmed in air, it glows, forming the dioxide, and reduces carbon dioxide when heated in a current of that gas. This intermediate oxide is said to correspond in composition to the violet hydroxide which is obtained as an intermediate product in the oxidation of cerous to ceric hydroxide, and which is said to yield the blue oxide, Ce₄O₇, when dried in vacuo.
[214] Sterba, Compt. rend. 1901, 133, 221; Meyer, Zeitsch. anorg. Chem. 1903, 37, 378.
The disulphide, CeS₂, has been obtained by Biltz[215] by prolonged heating of anhydrous cerous sulphate in a current of sulphuretted hydrogen at a dull red heat; it is a dark, yellowish-brown, crystalline solid, which on treatment with hydrochloric acid yields hydrogen persulphide.