[215] Ber. 1908, 41, 3341.
Halogen salts.—No halogen compounds are known in the free state, except the fluoride, CeF₄,H₂O, which was obtained by Brauner as a yellowish-brown mass, by the action of hydrofluoric acid on the hydroxide. A double fluoride, 2CeF₄,3KF,2H₂O, was prepared by the same author by dissolving the hydroxide in potassium hydrogen fluoride; it is insoluble in water. By dissolving a ceric salt in concentrated hydrochloric acid, a dark red solution is obtained, which is believed to contain the unstable complex acid, H₂CeCl₆; this decomposes slowly in the cold, more quickly on warming, with evolution of chlorine, and formation of cerous chloride. Several double compounds of ceric chloride with hydrochlorides of organic bases have, however, been obtained.
Ceric sulphate, Ce(SO₄)₂, is obtained by the action of concentrated sulphuric acid on the dioxide. It is a deep yellow crystalline powder, dissolving readily in water to a brown solution, which has a strongly acid reaction; on warming or diluting, a basic sulphate separates. The solution slowly evolves oxygen, and therefore always contains cerous compounds. On evaporation, a cero-ceric acid sulphate of the formula HCeiiiCeiv(SO₄)₄,12(13 ?)H₂O first separates; the hydrated sulphate Ce(SO₄)₂,4H₂O, being more soluble, separates on further concentration.[216] The relative amounts of the two compounds obtained depends on the temperature and the concentration of acid in the solution; if both these factors are kept low, the almost pure hydrated sulphate can be at once obtained. This separates in yellow crystals belonging to the rhombic system; it is readily soluble in water. The mixed acid salt is less soluble, and forms orange prisms and needles, which cling tenaciously to sulphuric acid. Other complex and double salts have also been obtained. When, for example, silver nitrate is added to a warm solution of the sulphate in concentrated sulphuric acid, a bright orange-yellow precipitate of the salt 10Ce(SO₄)₂,6Ag₂SO₄ is obtained.[217]
[216] See Meyer and Aufrecht, Ber. 1904, 37, 140; Brauner, Zeitsch. anorg. Chem. 1904, 39, 261.
[217] Pozzi-Escot, Compt. rend. 1913, 156, 1074.
Neutral ceric nitrate is unknown. A basic nitrate, Ce(NO₃)₃OH,3H₂O, is obtained in red crystals by evaporation of a solution of ceric hydroxide in strong nitric acid. The solid is readily soluble in water, forming a yellow, acid solution, which becomes paler by hydrolysis, on warming or on standing. The course of the hydrolysis is also indicated by the action towards acids, and towards hydrogen peroxide.[218] A freshly prepared ceric salt, on addition of acid, becomes immediately much darker in colour, whereas the colour change is very slow, if considerable hydrolysis has occurred. Similarly, hydrogen peroxide at once reduces a freshly prepared solution, forming colourless cerous salts, whilst if much hydrolysis has occurred, deeply coloured higher oxidation products are at first formed, and these lose their colour only slowly.
[218] Meyer and Jacoby, Zeitsch. anorg. Chem. 1901, 27, 359.
The double ceric nitrates[219] are a large and very important class of compounds; they are the most stable of the ceric salts. With nitrates of the monovalent metals, ceric nitrate forms double nitrates of the type R₂Ce(NO₃)₆; these are deep red hygroscopic substances, crystallising in the monoclinic system, readily soluble in water and alcohol, but dissolving only sparingly in nitric acid. The ammonium salt is important for the separation of cerium. A series of double nitrates with the nitrates of manganese, magnesium, zinc, nickel, and cobalt has the general formula RCe(NO₃)₆,8H₂O, but these are much less stable in solution than the alkali double salts.
[219] Meyer and Jacoby, loc. cit.