Even more tedious and unsatisfactory is the method based on the solubility of thorium oxalate in excess of ammonium oxalate in neutral solution. The solution is boiled, ammonium oxalate added, and after some moments a small quantity of ammonium acetate solution. On cooling, the oxalates of the cerium metals separate, and can be collected; thoria is precipitated from the filtrate by addition of ammonia. The process must be repeated two or three times, the solution being allowed to stand for one or two days each time, and finally the thoria must be precipitated by thiosulphate to remove traces of the other bases before it can be weighed. Benz (_loc. cit._) gives a complete account of this method, and quotes numerous analyses carried out to test its accuracy. Far more satisfactory than either of the above is the peroxide method used by de Boisbaudran and Cleve, and later by Wyrouboff and Verneuil.[506] Thorium is completely precipitated as a ‘peroxide salt’ (Th₂O₇,SO₃ or Th₂O₇,N₂O₅ respectively) from warm neutral solutions of the sulphate or nitrate on addition of dilute hydrogen peroxide, a second precipitation being necessary to free it from cerium compounds. Wyrouboff and Verneuil state that the process is rendered difficult by the fact that the peroxide cannot be converted into the dioxide by heating, either alone or with acids, as decrepitation takes place and may cause loss; they accordingly reduce the compound in presence of hydrochloric acid by ammonium iodide, and precipitate thorium hydroxide by ammonia. Benz (_loc. cit._) does not find this difficulty; he states that small quantities of the peroxide dissolve easily in acids without loss, and further finds that if an ammonium salt be added to the neutral solution of the thorium compound before addition of hydrogen peroxide, the precipitate forms much more readily and is very easily handled. Borelli[507] states that the precipitated peroxide can be ignited without loss to the dioxide, and weighed as this. [506] _Compt. rend._ 1898, ~126~, 340. [507] Abstract in _J. Soc. Chem. Ind._ 1909, ~28~, 625. * * * * * The azoimide method of Dennis[508] is of interest rather than of use. He finds that addition of potassium azoimide, N₃K, precipitates thoria quantitatively from a neutral solution, the reaction being expressed by the equation: Th(NO₃)₄ + 4N₃K + 2H₂O = 4KNO₃ + ThO₂ + 4N₃H Cerium, however, if present, is always precipitated with the thorium, and cannot be removed by re-precipitation; this fact, together with the cost of the reagent and the difficulty of obtaining it pure, renders the method quite useless for mineral analysis. [508] _Zeitsch. anorg. Chem._ 1897, ~13~, 412.