Quite recently, methods have been proposed by which the thorium can be separated in a fairly pure condition from the acid solution obtained from the sulphuric acid treatment. Rosenheim, Meyer and Koppel[499] protect the use of hydrofluosilicic acid (H₂SiF₆), and its salts, for this purpose. The sodium salt, added to the hot acid liquid, produces a quantitative separation of thorium silicofluoride; the precipitate is washed by decantation, and treated with sulphuric acid, the thorium sulphate being then purified directly by the sulphate method described below. A second method proposes to make use of the insolubility of thorium hypophosphate, ThP₂O₆,11H₂O, which was found by Kaufmann in 1899 to be insoluble in water, and in acids and alkalies. This method has already been in use for some years for analytical work;[500] it appears to be readily susceptible of adaptation for the technical extraction,[501] the sodium hypophosphate, Na₂H₂P₂O₆,6H₂O required as the precipitating agent being obtainable in large quantities by the electrolytic oxidation of copper phosphide, employed as the anode in an electrolytic cell.[502] This method also gives a thorium compound sufficiently free from other earths to be subjected at once to the refining process; the hypophosphate has in fact been suggested as a very suitable compound for the impregnation of artificial silk mantles directly. The thorium nitrate of commerce, however, is still prepared almost entirely from the crude product obtained by one or other of the two methods of fractional precipitation first described, so that it becomes necessary to outline the method generally employed for separating from this a compound pure enough to be suitable for the final refining process. [499] _D. R. P._ 214886, October, 1909. [500] Rosenheim, _Chem. Zeitg._ 1912, ~36~, 821; also Koss, _ibid._ 686 [501] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678. [502] Rosenheim and Pinsker, _Ber._ 1910, ~43~, 2003. The crude oxalate or hydroxide is thoroughly digested with a concentrated solution of sodium carbonate. The carbonates of the cerium elements are much less soluble in sodium carbonate solution than is thorium carbonate. After thorough digestion the liquid is filtered from the undissolved carbonates. The thorium is reprecipitated from the filtrate, either as oxalate, by the addition of hydrochloric acid (if the crude material was in the form of oxalate), or as hydroxide, by the addition of sodium hydroxide. The process is again repeated, and a final digestion is then made with ammonium carbonate; addition of an alkali to the clear filtrate now gives thorium hydroxide sufficiently pure to be used for the last refining.