[483] Zeitsch. anorg. Chem. 1912, 79, 260.
By precipitation with potassium sulphate the double salt, Th(SO₄)₂,2K₂SO₄,2H₂O, is formed; this is soluble in water but insoluble in potassium sulphate solution. The analogous sodium and ammonium salts are soluble both in water and excess of the corresponding alkali sulphate.
The sulphite, Th(SO₃)₂,H₂O, is obtained as a white amorphous precipitate by warming a solution of a thorium salt with sulphurous acid. Basic sulphites and double sulphites are also known; the precipitates obtained by addition of alkali sulphite dissolve readily in excess. The hydroxide is almost insoluble in sulphurous acid, behaviour which distinguishes thorium (and zirconium) from all the trivalent metals. No thiosulphate is known, the hydroxide being thrown down from boiling solution by addition of sodium thiosulphate: this method of precipitation was formerly much used for purposes of estimation, but it is more tedious and less accurate than the modern methods.
Thorium nitrate, Th(NO₃)₄,12H₂O, crystallises at ordinary temperatures in large hygroscopic tablets, very soluble in water and alcohol. The hydrates, Th(NO₃)₄,6H₂O and Th(NO₃)₄,5H₂O, have been obtained from hot solution and from nitric acid solution respectively. Thorium is employed in commerce almost entirely in the form of this salt, which is dehydrated until it contains about 48 per cent. ThO₂, which approximates to the formula Th(NO₃)₄,4H₂O; the commercial product, however, is not a definite hydrate. Kolbe[484] has described the additive product with antipyrine, 2Th(NO₃)₄,5C₁₁H₁₂ON₂, which melts at 168°. The extent to which thorium salts are hydrolysed in solution is very considerable, as is evident from the fact that the nitrate may be titrated with standard potash in presence of phenolphthalein as indicator; the solution first becomes alkaline to this reagent when 3·5 molecules of potash have been added for each molecule of thorium nitrate present.[485] Of the large number of double nitrates which have been prepared, the general types R´₂Th(NO₃)₆, where R´ = NH₄,K,Rb,Cs, and R´´Th(NO₃)₆,8H₂O, where R´´ = Mg,Mn,Zn,Ni,Co, are the most important.
[484] Zeitsch. anorg. Chem. 1913, 83, 143.
[485] Halla, loc. cit.
Thorium phosphates.—The precipitates obtained by addition of phosphoric acid or alkali phosphates to solutions of thorium salts are gelatinous solids of doubtful composition; they dissolve in mineral acids and in alkali carbonates, and their behaviour is of great importance in the technical treatment of monazite. Various phosphates and double phosphates are obtained by fusion methods, but none of these are important. The phosphite, Th(HPO₃)₂,3H₂O, and hypophosphite, Th(H₂PO₂)₄, are insoluble solids obtained by double decomposition. The hypophosphate ThP₂O₆,11H₂O, is of great importance for purposes of detection and estimation, since it is thrown down quantitatively from strongly acid solutions; under these conditions the rare earths remain in solution.
No neutral carbonate of thorium is known. Alkali carbonates precipitate a basic salt, which dissolves readily in excess; this fact is of very great importance in the commercial extraction of thorium, the sodium and ammonium double carbonates of the cerium elements being almost insoluble in alkali carbonates. Addition of alcohol to the solution throws down double carbonates, which can be washed with ice water. The salts K₆Th(CO₃)₅,10H₂O, Na₆Th(CO₃)₅,12H₂O, and (NH₄)₂Th(CO₃)₃,6H₂O have been obtained in this way; they dissolve readily in water or dilute alkali carbonate, though on warming or diluting the solution, the hydroxide separates. The thallium compound, Tl₆Th(CO₃)₅, is sparingly soluble, and is thrown down from a solution of the ammonium compound on addition of a thallium salt; it has been proposed for the microchemical detection of thorium. The quantitative separation of thorium by means of pure moist lead carbonate has been proposed for the purpose of estimation (see [p. 288]).
Thorium oxalate, Th(C₂O₄)₂,6H₂O, is precipitated quantitatively by means of oxalic acid, even in presence of considerable quantities of mineral acids. It is less soluble in sulphuric acid than any of the rare earth oxalates,[486] and is not attacked, as are the latter compounds, by concentrated nitric acid. In hydrochloric acid the solubility first increases rapidly with the concentration of the acid, and then suddenly decreases; this behaviour is due to the formation of an oxalochloride, 3Th(C₂O₄)₂,ThCl₄,20H₂O. When the amorphous oxalate obtained by precipitation is allowed to remain for a considerable time in contact with acids, it forms characteristic tetragonal prisms of the more stable form. The dihydrate, Th(C₂O₄)₂,2H₂O, is obtained when the hexahydrate is dried over sulphuric acid, or heated to 100°. The salt dissolves easily in excess of alkali oxalate, but is precipitated from the solutions by mineral acids, a fact which allows of another means of separation from zirconium, the double oxalates of which are much more stable towards acids. The solubility of the oxalate in alkali oxalate allows of separation from the rare earth elements, whilst its insolubility in excess of oxalic acid can be used for the separation from zirconium.
[486] Hauser and Wirth, Zeitsch. anorg. Chem. 1912, 78, 75.