[206] Loc. cit.
The sulphide, Ce₂S₃, has been prepared by Biltz[207] by heating the sulphate to a red heat in a current of sulphuretted hydrogen; he describes it as a red powder. The chloride, CeCl₃, combines with ammonia with evolution of heat even at a temperature of -80°. Five additive compounds are described;[208] they are white powders, decomposed by water.
[207] Ber. 1908, 41, 3341.
[208] Barre, Compt. rend. 1913, 156, 1017.
The solubility curve of the various sulphate hydrates has already been given (see [p. 125]). Various double sulphates with ammonium sulphate, and the sulphates of sodium, potassium, thallium and cadmium are known. The cadmium double compound has the composition Ce₂(SO₄)₃,CdSO₄,6H₂O, and is prepared by mixing solutions of the simple salts in presence of sulphuric acid. Many double nitrates have been prepared; these are for the most part stable, highly crystalline compounds, easily soluble in water and alcohol. With the nitrates of the common divalent metals, cerous nitrate forms a series of double salts of the general formula 2Ce(NO₃)₃,3R(NO₃)₂,24H₂O, where R = Mg, Mn, Co, Ni, or Zn; these form an isomorphous series, crystallising in the hexagonal system. The acetylacetone compound melts at 131°-132°.
In the presence of hydrogen peroxide in the cold, ammonia throws down from solutions of cerous salts a reddish-brown peroxyhydrate, Ce(OOH)(OH)₃,[209] which on heating loses oxygen, and yields ceric hydroxide. The reaction is very delicate, and may be used as a test for cerium. If the precipitate be treated with acids in the cold, ceric salts are first obtained, but these are at once reduced, in the acid solution, by the hydrogen peroxide formed, so that cerous salts remain; ceric salts may be obtained by first boiling the suspension of the peroxyhydrate and treating the ceric hydroxide so obtained with acids.
[209] Pissarjewski, Zeitsch. anorg. Chem. 1902, 31, 359.
The Ceric Compounds
The ceric salts are much more readily hydrolysed than the cerous salts, and show a great tendency, in dilute solution, to pass over into the latter. So great is this tendency that a solution of a ceric salt acts as if it were supersaturated with oxygen; ceric sulphate, for example, in dilute solution slowly evolves oxygen, whilst the chloride evolves chlorine. In consequence of this behaviour, ceric compounds have a very powerful oxidising action. The ceric salts are yellow to red in colour; their solutions are strongly acid, owing to the ease with which the salts hydrolyse, and on boiling deposit insoluble basic salts.
Beside the methods which have already been mentioned, ceric compounds may be prepared from cerous by oxidation with sodium peroxide, bismuth tetroxide, ammonium persulphate, etc. In electrolysis of cerous salts, also, ceric compounds are obtained at the anode.