1911 Encyclopædia Britannica/Thorium
THORIUM (symbol Th, atomic weight 232.42 [O=16]), a metallic chemical element. It belongs to the group of metals whose oxides are generally denominated “ rare earths,” and its history is bound up in the history of the group, which is especially interesting from the fact that it supplies the material for the manufacture of the mantles used in incandescent gas-lighting, and also that the radio-active substances are almost invariably associated with these oxides. The name thoria (after the Scandinavian god Thor) was first given in 1815 by Berzelius to a supposed new earth which he had extracted from several rare Swedish minerals. This “ new earth ” turned out to be nothing more nor less than a basis yttrium phosphate. In 1828 he gave the name thoria to an earth which he extracted from a mineral found at Lërön. This mineral is the modern thorite. Thorium has proved to be very widely, although extremely sparingly, distributed: pyrochlor, orangite, monazite, euxenite, gadolonite, orthite, and in fact most of the rare minerals of this type contain it (see B. Szilard, Le Radium, 1909, 6, p. 233). The extraction of thorium salts from these minerals is a matter of much tedium. Metallic thorium is obtained by heating potassium thorium chloride or the tetrachloride with sodium (see W. von Bolton, R. J. Meyer and H. Karstens, Journ. Chem. Soc., 1909, vol. 96). It forms microscopic hexagonal plates having a silver-white streak. It is very heavy, its density being about 11; it inflames when heated in air and is not attacked by alkalis; it readily dissolves in nitric acid and aqua regia, but with difficulty in hydrochloric acid.
In its salts, thorium is tetravalent, and in the periodic classification it occurs in the same sub-group as titanium, cerium and zirconium.
Thorium dioxide or thoria, ThO2, is the most important compound, being manufactured commercially in comparatively large quantities from monazite sands, with a view to its utilization for gas mantles (see Lighting, Gas). It is an amorphous white powder; but it may also be obtained in crystals isomorphous with cassiterite by heating the amorphous form with borax to a very high temperature. An oxide Th3O5 is formed by heating the oxalate.
Thorium fluoride, ThF4, is obtained as a heavy white insoluble powder by dissolving the hydrate in hydrofluoric acid and evaporating. By precipitating a thorium salt with a fluoride, a gelatinous hydrate, ThF4·4H2O, is obtained. Acid potassium fluoride precipitates K2ThF5·4ThF4·H2O from a solution of thorium chloride. Potassium thorofluoride, K2ThF6·4H2O, is a heavy black powder formed by boiling the hydroxide with potassium fluoride and hydrofluoric acid. Thorium chloride, ThCl4, is obtained as white shining crystals by heating a mixture of carbon and thoria in a current of chlorine. Baskerville (Journ. Amer. Chem. Soc., 1904, 26, p. 922) divided the product into three fractions according to their volatility. He concluded that the first contained the chloride of berzelium, having an atomic weight of 212, the second contained thorium chloride, and the third the chloride of carolinium, having an atomic weight of 255.6. E. Chauvenet (Compt. rend., 1908, 147, p. 1046) obtains it by heating thoria in a current of carbonyl chloride. Thorium chloride readily deliquesces on exposure and forms double salts with alkaline chlorides.
Thorium sulphate, Th(SO4)2, is obtained by dissolving the oxide in sulphuric acid. It forms several crystalline hydrates. Evaporation of a solution at ordinary temperatures gives colourless monoclinic prisms of Th(SO4)2·9H2O, which is isomorphous with uranium sulphate, U(SO4)2·9H2O. Above 43° Th(SO4)2·4H2O is deposited. B. Roozeboom (Zeit. phys. Chem., 1890, 5, p. 198) has described several other hydrates. Thorium sulphate forms double salts with the alkaline sulphates. Thorium nitrate, Th(NO3)4·12H2O, forms white deliquescent tables very soluble in water. It forms double salts such as MgTh(NO3)6·8H2O, which are isomorphous with the corresponding cerium compounds. Thorium sulphide, ThS2, is obtained by burning the metal in sulphur. It cannot be produced by precipitation.
The atomic weight has been variously given. Berzelius found 235.5; Delafontaine, 229.7; Cleve, 232.6 by analyses of the sulphate, and 232.2 by analyses of the oxalate. Krüss and Nilson derived the value 230.7 (H=1) from analyses of the carefully purified sulphate.
For the so-called “ disintegration of the thorium atom ” and the relation of this element to the general subject of radio-active emanations, see Radio-Activity.
A number of salts of thorium have been prepared for therapeutic use, including the hydroxide, nitrate, salicylate, oleate and lactate. The oleate has been used in chronic eczema and psoriasis and locally in cancer. Inhalations of thorium emanations produced from thorium nitrate through a wash-bottle inhaler are said to have a bactericidal action in diseases of the lungs. F. Soddy has used them in phthisis, and Louisa Chesney speaks favourably of the emanations in chronic and acute laryngitis and in tuberculous laryngeal ulcerations.