The preparation and properties of metallic cerium have already been described (see [p. 114]); for an account of the pyrophoric alloys, see [p. 314].
The Cerous Compounds
The salts of trivalent cerium are very similar to those of the other rare earth elements, and a detailed description of them is therefore unnecessary. The sesquioxide, Ce₂O₃, cannot be obtained by ignition of the oxalate, nitrate, or other similar salt, since these decompose at high temperatures with formation of the dioxide, CeO₂. It has been prepared by the reduction of the dioxide with calcium;[200] it has a great affinity for oxygen, and readily absorbs the gas when exposed to moist air. Cerous hydroxide, Ce(OH)₃, obtained by addition of alkali to solutions of cerous salts, has also strong reducing properties,[201] and can only be prepared and preserved when oxygen is carefully excluded. It has been obtained as a perfectly white solid by the action of water on the carbide;[202] when dried in an inert atmosphere, it yields a perfectly white oxide. In presence of air, it darkens, assuming a reddish-violet colour, which passes into yellow as the oxidation becomes complete. The oxidation proceeds more quickly in presence of potash or soda, ceric hydroxide, Ce(OH)₄, being formed; in presence of potassium carbonate, however, a dark-coloured peroxyhydrate is formed by autoxidation. The colour so produced disappears on shaking if an ‘acceptor’ is present, ceric hydroxide being left; if the acceptor cannot reduce this, the solution after shaking loses the power of re-forming the dark peroxide, but if the acceptor can reduce the ceric compound to cerous hydroxide, the solution after shaking regains the power of forming the peroxide which is a property of the lower hydroxide.
[200] Burger, Ber. 1907, 40, 1652.
[201] Dennis and Magee, J. Amer. Chem. Soc. 1894, 16, 649; also Biltz and Zimmerman, Ber. 1907, 40, 4979.
[202] Damiens, Compt. rend. 1913, 157, 214.
Cerous nitride, CeN, has been prepared by Moissan[203] by the action of ammonia on the heated carbide; it can also be obtained by heating the hydride in a stream of nitrogen.[204] Muthmann and Kraft also state[205] that it can be prepared by heating metallic cerium in the gas, the metal burning with the liberation of much energy in the form of heat and light; but Dafert and Miklanz[206] deny that it can be obtained in this way. Cerium nitride is a lustrous, brass yellow to bronze coloured solid, stable in dry air, but at once attacked by moist air, with evolution of ammonia, and formation of the dioxide. When moistened in air with a few drops of water, the substance reacts violently, becoming heated to redness. Alkalies and acids decompose it, with formation of cerous compounds.
[203] Compt. rend. 1900, 131, 865.
[204] Dafert and Miklanz, Monats. 1912, 33, 911.
[205] Annalen, 1902, 325, 261.