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Cerium (symbol Ce, atomic weight 140.25), a metallic chemical element which occurs with the rare earths in the minerals cerite, samarskite, euxenite, monazite, parisite and many yttrium minerals. The particular earth containing cerium was discovered by M.H. Klaproth in 1803, whilst J. Berzelius at about the same time also examined it and came to the conclusion that it was the oxide of a new metal, which he termed cerium. The crude oxide of the metal is obtained from cerite, by evaporating the mineral with strong sulphuric acid, removing excess of acid and dissolving the residue in ice-cold water; sulphuretted hydrogen is passed through the solution, which is then filtered, acidified with hydrochloric acid, and precipitated as oxalate by oxalic acid; the oxalate is then converted into oxide by ignition. From the crude oxide so obtained (which contains lanthanum and didymium oxides) the cerium may be separated by conversion into its double sulphate on the addition of potassium sulphate, the sulphates of the cerium group being insoluble in a saturated solution of potassium sulphate. The sulphate is subsequently boiled with water, when a basic sulphate is precipitated. For the preparation of pure cerium compounds see Auer v. Welsbach, Monatshefte, 1884, v. 508.

The metal was first obtained, in an impure state, by C.G. Mosander, by fusing its chloride with sodium. W.F. Hillebrand and T. Norton have prepared it by the electrolysis of the melted chloride (Pogg. Ann., 1875, 156, p. 466); and C. Winkler (Berichte, 1891, xxiv. 884) obtained it by heating the dioxide with magnesium powder. The metal has somewhat the appearance of iron, and has a specific gravity of 6.628, which, after melting, is increased to 6.728. Its specific heat is 0.04479 (W.F. Hillebrand). It is permanent in dry air, but tarnishes in moist air; it can be hammered and rolled; it melts at 623° C. It burns readily on heating, with a brilliant flame; and it also combines with chlorine, bromine, iodine, sulphur, phosphorus and cyanogen. In the case of the two former elements the combination is accompanied by combustion of the metal. With water it is slowly converted into the dioxide. Cold concentrated nitric and sulphuric acids are without action on the metal, but it reacts rapidly with dilute nitric and hydrochloric acids. The dioxide is used in incandescent gas mantles (see Lighting).

Three oxides of cerium are known. The sesquioxide, Ce2O3, is obtained by heating the carbonate in a current of hydrogen. It is a bluish-green powder, which on exposure rapidly combines with the oxygen of the air. By the addition of caustic soda to cerous salts, a white precipitate of cerous hydroxide is formed. Cerium dioxide, CeO2, is produced when cerium carbonate, nitrate, sulphate or oxalate is heated in air. It is a white or pale yellow compound, which becomes reddish on heating. Its specific gravity is 6.739, and its specific heat 0.0877. It is not reduced to the metallic condition on heating with carbon. Concentrated sulphuric acid dissolves this oxide, forming a yellowish solution and ozone. By suspending the precipitated cerous hydroxide in water and passing chlorine through the solution, a hydrated form of the dioxide, 2CeO2·3H2O, is obtained, which is readily soluble in nitric and sulphuric acids, forming ceric salts, and in hydrochloric acid, where it forms cerous chloride, with liberation of chlorine. A higher hydrated oxide, CeO3·xH2O, is formed by the interaction of cerous sulphate with sodium acetate and hydrogen peroxide (Lecoq de Boisbaudran, Comptes rendus, 1885, 100, p. 605).

Cerous chloride, CeCl3, is obtained when the metal is burned in chlorine; when a mixture of cerous oxide and carbon is heated in chlorine; or by rapid heating of the dioxide in a stream of carbon monoxide and chlorine. It is a colourless substance, which is easily fusible. A hydrated chloride of composition 2CeCl3·15H2O is also known, and is obtained when a solution of cerous oxide in hydrochloric acid is evaporated over sulphuric acid. Double salts of cerous chloride with stannic chloride, mercuric chloride, and platinic chloride are also known. Cerous bromide, 2CeBr3·3H2O, and iodide, CeI3·9H2O, are known. Cerous sulphide, Ce2S3, results on heating cerium with sulphur or cerium oxide in carbon bisulphide vapour. It is a red infusible mass of specific gravity 5.1, and is slowly decomposed by warm water. The sulphate, Ce2(SO4)3, is formed on dissolving the carbonate in sulphuric acid, or on dissolving the basic sulphate in sulphuric acid, in the presence of sulphur dioxide, evaporating the solution, and drying the product obtained, at high temperature (B. Brauner, Monatshefte, 1885, vi. 793). It is a white powder of specific gravity 3.912, easily soluble in cold water. Many hydrated forms of the sulphate are known, as are also double salts of the sulphate with potassium, sodium, ammonium, thallium and cadmium sulphates. Ceric fluoride, CeF4·H2O, is obtained when the hydrated dioxide is dissolved in hydrofluoric acid and the solution evaporated on the water bath (B. Brauner). The sulphate, Ce(SO4)2·4H2O, is formed when the basic sulphate is dissolved in sulphuric acid; or when the dioxide is dissolved in dilute sulphuric acid, and evaporated in vacuo over sulphuric acid. It forms yellow crystals soluble in water; the aqueous solution on standing gradually depositing a basic salt. Double sulphates of composition 2Ce(SO4)2·2K2SO4·2H2O, Ce(SO4)2·3(NH4)2SO4·4H2O are known. Nitrates of cerium have been described, as have also phosphates, carbonates and a carbide.

Cerium compounds may be recognized by the red precipitate of ceric hydroxide, which is formed when sodium hypochlorite is added to a colourless cerous salt. For the quantitative determination of the metal, the salts are precipitated by caustic potash, the precipitate washed, dried and heated, and finally weighed as the dioxide.

The atomic weight of cerium has been determined by B. Brauner (Chem. News, 1895, lxxi. 283) from the analysis of the oxalate; the values obtained varying from 140.07 to 140.35.