3Ce₂O₃ + 2KMnO₄ + H₂O = 6CeO₂ + 2KOH + 2MnO₂
The precipitated solid is separated, and dissolved in acid; the cerium is then precipitated as the oxalate, which is transformed into nitrate in the usual way. The solution contains the other elements of the cerium group, which are precipitated by means of sodium hydroxide. The yield obtained by this method is very good, practically the whole of the cerium being separated without loss; whilst it has the further advantage that the remaining elements of the group can be precipitated at once after the separation.
Analysis of a Monazite or Monazite Sand for Thorium.
—Since the commercial value of a monazite sand or concentrate, or of the pure mineral, depends, at present, entirely on the percentage of thoria, it is important to have a rapid and reliable method of estimating this constituent. The only reliable methods of quantitatively decomposing the mineral, however, all involve acid treatment, and excess of acid must always be present to prevent precipitation of phosphates. Until recently, no way was known for estimating thorium in an acid solution, and all the earlier methods therefore involved tedious processes for complete removal of phosphoric acid, so that the salts could be obtained in neutral solution. This was usually effected by precipitation of the whole rare earth content with oxalic acid, and thorough washing of the oxalates; these can then be dissolved directly in fuming nitric acid on the water-bath, or ignited to the oxides, which may then be dissolved in the same reagent. The solution of nitrates is evaporated to dryness, to effect removal of the excess of acid, the nitrates dissolved in water, and the thorium estimated in the neutral solution.
Among the earliest methods employed for the estimation in neutral solution was the thiosulphate precipitation.[505] Thorium thiosulphate is not known; when sodium thiosulphate is added to a neutral solution of a thorium salt, a precipitate of thoria mixed with sulphur is obtained, by hydrolysis of the potential thiosulphate, and decomposition of the unstable thiosulphuric acid. The method, however, leaves much to be desired; other earths are partly precipitated, and the separation of thoria is not complete. For analytical purposes the precipitate obtained is redissolved in hydrochloric acid, and a second precipitation with thiosulphate effected. The filtrates from the two precipitations are collected, and the whole earth-content precipitated from these with ammonia; the hydroxides are dissolved in hydrochloric acid, and again treated with thiosulphate to throw down any thoria which has escaped the previous precipitations. The three precipitates of thoria are then collected, dried, and ignited for weighing as pure thorium dioxide, ThO₂.
[505] Full accounts of this and the two following methods will be found in an important paper by Benz, Zeitsch. angew. Chem. 1902, 15, 297
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.