Ceric oxide, CeO₂

Ceric oxide, cerium dioxide, or ceria, CeO₂, may be prepared by the ignition of cerous or ceric hydroxide, nitrate, sulphate, etc., or by the ignition of the cerous salt of any volatile oxyacid; it is perhaps most commonly prepared by the ignition of cerous oxalate.

Ceria is an amorphous powder of specific gravity 6.405 at 17° when prepared from the oxalate, and 6.99 when obtained from the nitrate (Sterba); its specific heat is 0.0877 (0°-100°). There has been a great deal of discussion over the question of the colour of ceria. It might be anticipated that ceria, like zirconia and thoria, would be white. Most experimenters agree that pure ceria has a pale yellow colour. The depth of colour depends upon the temperature at which the oxide has been calcined and the salt from which it has been prepared. When prepared by the prolonged ignition of cerous sulphate at a white heat, its tint is so slight that it may almost be said that the ceria is white, but when obtained at a lower temperature by the ignition of cerous oxalate or ceric ammonium nitrate it has a more pronounced tint, usually described as that of pale chamois. On the other hand, Spencer claims that when ceric sulphate is heated for a prolonged period at temperatures below a red heat, cerium dioxide is formed, which is pure white; further, that when the ceria is heated above a red heat it shrinks in volume and becomes pale yellow in colour. Ceria darkens in colour very markedly when heated, but returns (practically) to its original colour when cooled. When ceria is contaminated with a little of the other earths of the cerium group, it is salmon-coloured, reddish-brown, or brown, according to the extent of contamination and the temperatnre of ignition. The coloration is attributed mainly to the presence of praseodymia, or rather, its peroxide.

Ceria is readily obtained in the crystalline form by adding anhydrous cerous sulphate to molten magnesium chloride, allowing to cool slowly, and extracting the mass with hydrochloric acid. The crystals belong to the regular system and exhibit faces of the cube and octahedron. They are practically colourless, very hard and very brilliant, the refractive index being high (about 1.9); they have a density of 7.3. If small quantities of another rare earth sulphate are added in the preparation, e.g. neodymium, praseodymium or erbium sulphate, beautiful coloured crystals may be obtained.

Ceria does not melt at c. 1900°, but it volatilises in vacuo quite rapidly at that temperature. It readily melts in the electric furnace and attacks the containing vessel.

Ceria is a very stable oxide, but it can be reduced to the metallic state by heating it with aluminium or magnesium. By neither of these methods, however, has a regulus of the metal been obtained. The action of other reducing agents is discussed in connection with cerous oxide and ceroceric oxide.

Crystalline ceria is very resistant towards acids and alkalis. The amorphous substance after ignition is insoluble in hydrochloric or nitric acid, except in the presence of a suitable reducing agent, e.g. hydrogen peroxide, hydriodic acid, or stannous chloride, when it passes into solution as a cerous salt. Concentrated sulphuric acid converts it into ceric sulphate, while moderately concentrated acid causes partial reduction to cerous sulphate and dilute acid has no perceptible action.

Ceria acts as an oxygen-carrier towards other substances, in a manner that is not at present understood. It may therefore be employed as the catalyst in Dennstedt's method for the combustion of organic compounds.

Ceria is a very weakly basic oxide; it is possible that it can also act as a feebly acidic oxide. It has been pointed out in describing the preparation of ceria that rare earth mixtures obtained by the ignition of the mixed oxalates are completely soluble in nitric or hydrochloric acid, -provided that the ceria does not exceed 45 to 50 per cent, of the mixture; pure ceria, however, is insoluble in these acids. The usual explanation of these results is that the ceria acts as a feeble acid and combines with the other strong bases present to form salts; these salts are decomposed by a strong acid with the liberation of ceric acid, i.e. ceric hydroxide, which is soluble, as a base, in the excess of strong acid present. If such is the case, the salts must apparently be of the type M₂O₃.2CeO₂ or 2M₂O₃.3CeO₂ in order to account for the 45-50 per cent, limit to the solubility of the ceria.

Ceria combines with uranium dioxide. When a dry mixture of cerous and uranyl sulphates is heated with molten magnesium chloride in a covered crucible for fifteen hours, deep blue cubic crystals are produced which can be separated from the accompanying substances by reason of the relative stability towards dilute acids. The composition of the crystals approximates to that required by the compound UO₂.2CeO₂. A similar deep blue compound may be obtained by precipitating an aqueous solution of uranyl and cerous nitrates with excess of ammonium hydroxide or dilute potassium hydroxide solution; the precipitate is at first yellow, but soon changes to a denser blue solid.