Of all the rare earth elements, cerium, by virtue of its property of forming ceric salts corresponding to the dioxide CeO₂, is the one most easily separated and obtained in the pure state. In those compounds in which it is tetravalent, cerium functions as a much less strongly electropositive element than in the cerous compounds, and all the methods of separation are based on this fact. Mosander, who first demonstrated that the old ‘ceria’ was a mixture, separated the element by treating a suspension of the hydroxides in potassium hydroxide with chlorine; yellow ceric hydroxide remains undissolved, whilst the other elements go into solution as the chlorides and hypochlorites. This method was extensively used until quite recently; it has the advantage of separating the cerium completely, but the product is very impure, and several repetitions are required to give good results. The basic nitrate method, which is now used on the commercial scale in extracting cerium from monazite (see p. 284), is also due to Mosander, though it has been employed subsequently by many workers. Several methods take advantage of the ease with which the ceric salts, as compared with salts of the trivalent elements, may be hydrolysed. Brauner[196] dissolves the oxides in nitric acid, and after removal of excess of acid, boils with a large volume of water--basic ceric nitrate is thrown down, the other elements remaining in solution as nitrates. The precipitate is redissolved, and the process repeated until the cerium is found spectroscopically to be free from didymium. The hydrolysis of the ceric salt may be effected more quickly and completely by the addition of ammonium sulphate or magnesium acetate.[197] James[198] boils the solution of the nitrates with potassium bromate, keeping the whole neutral by addition of powdered marble; the cerium is completely and very quickly precipitated as basic nitrate. [196] _Trans. Chem. Soc._ 1885, ~47~, 879. [197] Meyer and Koss, _Ber._ 1902, ~35~, 672. [198] _J. Amer. Chem. Soc._ 1912, ~34~, 757; this paper gives a complete scheme for a full separation of all the elements. An interesting method is due to Koppel[199]; the oxides are dissolved in a solution of hydrogen chloride in methyl alcohol, and treated with pyridine, when the sparingly soluble double chloride, (C₅H₅NH)₂CeCl₆, separates, and may be obtained pure by recrystallisation from alcohol and ether. The permanganate method of Drossbach, which is used on the commercial scale, is described on p. 285. [199] _Zeitsch. anorg. Chem._ 1898, ~18~, 305.