[482] Lebeau and Damiens, Compt. rend. 1913, 156, 1987.
Thorium fluoride, ThF₄, is obtained anhydrous by passing hydrogen fluoride over the anhydrous chloride or bromide at a temperature of 350°-400°. The tetrahydrate, ThF₄,4H₂O, is precipitated by addition of hydrofluoric acid to a solution of a thorium salt, or by the action of the acid on the hydroxide. Hydrofluosilicic acid also throws down the fluoride, even in the cold, from solutions of thorium salts. The fluoride is insoluble in water and mineral acids, as well as in excess of precipitant; this behaviour allows of a complete and easy separation of thorium from titanium and zirconium. The rare earth fluorides are also much more easily soluble in concentrated mineral acids than thorium fluoride, so that this compound may also be used in the separation from the rare earths. When heated in a stream of the acid to 800°, the hydrated salt yields the oxyfluoride, ThOF₂; ignited in the air, it leaves the dioxide. Precipitation with potassium fluoride gives the double fluoride, KThF₅,H₂O, which may be obtained anhydrous by fusion of the mixed fluorides; it is insoluble. An amorphous insoluble compound, K₂ThF₆,4H₂O, is obtained by boiling the hydroxide with a mixture of potassium hydrogen fluoride and hydrofluoric acid. Sodium and ammonium fluorides throw down the simple fluoride.
Thorium chloride, ThCl₄, is obtained in the anhydrous form by all the usual methods, the most convenient being perhaps the action of chlorine and sulphur monochloride on the heated dioxide. It almost invariably contains small quantities of oxychloride. When pure, it forms colourless needles fairly stable in dry air; the impure product gradually darkens in colour. It dissolves in water with considerable evolution of heat, and is soluble also in alcohol and moist ether. It melts at about 820°, and sublimes unchanged at somewhat higher temperatures; the vapour begins to dissociate at about 1050°, the dissociation increasing rapidly as the temperature rises. It resembles zirconium chloride in the ease with which it forms additive compounds with ammonia and organic bases, and addition and condensation products with organic oxygen-compounds; many double and complex chlorides are also known, among which the platinum compounds ThPtCl₈,12H₂O and Th₂Pt₃Cl₁₄,24H₂O, and the pyridine salt (C₅H₅NH)₂ThCl₆ may be mentioned.
From aqueous solution the octohydrate, ThCl₄,8H₂O, separates at ordinary temperatures; a heptahydrate and an enneahydrate have been described as precipitated from the alcoholic solution by addition of water. The basic salts, Th(OH)Cl₃,7H₂O and Th(OH)₂Cl₂,5H₂O, have been obtained by addition of the hydroxide to alcoholic hydrogen chloride. The oxychloride, ThOCl₂, may be obtained by the carefully regulated action of carbon tetrachloride on the dioxide, according to the equation:
ThO₂ + CCl₄ = ThOCl₂ + COCl₂
It is a colourless crystalline solid, which takes up moisture from the air, forming the hexahydrate.
Thorium bromide, ThBr₄, is a volatile solid which boils at 725°; it closely resembles the chloride. The iodide and a basic iodide, Th(OH)I₃,10H₂O, are known.
No cyanide of thorium is known, addition of potassium cyanide merely causing separation of the hydroxide. A ferrocyanide, Th[Fe(CN)₆],4H₂O, is thrown down as a white powder by potassium ferrocyanide; with potassium ferricyanide no precipitate is obtained. The platinocyanide, Th[Pt(CN)₄]₂,16H₂O, is obtained by double decomposition in yellowish-brown prisms.
Among the halogen oxysalts, the perchlorate, chlorate, bromate, and iodate were prepared by Cleve. The iodate is of great importance for purposes of detection and estimation, from the fact that, in presence of a large excess of alkali iodate, it is insoluble in strong nitric acid, whilst the analogous compounds of the rare earth elements dissolve readily in that solvent.
The sulphate, Th(SO₄)₂, is obtained anhydrous by evaporating the excess of acid from a solution of the dioxide in oil of vitriol, or by heating the hydrates. It resembles the sulphates of the rare earth elements, in that it dissolves in water at 0° to form a highly supersaturated solution, from which the hydrated forms separate out almost quantitatively when the temperature is allowed to rise. The solubility relations of the various hydrates, on account of their commercial importance, are somewhat fully treated in [Chapter XVIII]. A dihydrate, Th(SO₄)₂,2H₂O, is obtained by keeping the tetrahydrate at 110°. The ennea- and octohydrates are isomorphous with the corresponding thorium selenate hydrates, and the ennea- and tetrahydrates with the analogous uranous sulphate hydrates. The hydrates yield the anhydrous salt when heated to 400°; the anhydrous sulphate has already a considerable dissociation tension (15 mm.) at 575°. By treatment with excess of acid, and subsequent heating to 130° in vacuo, the acid sulphate, Th(SO₄),H₂SO₄, is obtained. An insoluble basic salt, ThOSO₄,2H₂O, is formed by continued boiling of the tetrahydrate in dilute solution, or more quickly by heating the solution in a closed tube to 120°-125°; a monohydrate, ThOSO₄,H₂O, is also known. Halla[483] has recently obtained the hydrate, ThOSO₄,5H₂O, by boiling a solution of the neutral sulphate with magnesium sulphate, and also by treating the anhydrous sulphate with a little water in presence of magnesium carbonate.