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.