[152] Ber. 1890, 23, 2642; 1891, 24, 873.
[153] Annalen, 1904, 331, 58.
If nitrogen be substituted for hydrogen in either of the above methods of preparation, nitrides of the general formula RN are obtained; cerium nitride, however, cannot be obtained by heating the element in the gas.[154] These compounds are also obtained when the carbides are heated in ammonia. They are amorphous solids, which yield ammonia when acted upon by water.
[154] Dafert and Miklanz, Monats. 1912, 33, 911.
Hydroxides.
—The hydroxides are thrown down as gelatinous precipitates on the addition of alkalies to hot dilute solutions of the salts; precipitation in the cold, or in strong solution, usually gives a basic salt, or an hydroxide mixed with a large quantity of basic salt. The hydroxides are insoluble in excess of precipitant, but the precipitation is inhibited by the presence of some organic hydroxy-acids.[155]
[155] For effect of tartaric acid, see [p. 133].
The hydroxides are insoluble in water, but dissolve very readily in acids. The most basic of them absorb carbon dioxide from the air; lanthanum hydroxide is exceptional in that it colours litmus blue.
Whilst hydrogen peroxide in neutral solution does not react with rare earth salts,[156] alkalies in presence of this reagent precipitate gelatinous hydrated peroxides, which are very unstable, decomposing on standing, or on treatment with acids, with evolution of oxygen. The general formula R₄O₉ + xH₂O was proposed for these compounds by Cleve, but more recently the formula R(OOH)(OH)₂ has been advanced.[157]
[156] Compare behaviour of thorium and zirconium, [Ch. XVI].