Chemical Properties of Cerium

The following results have been recorded: -

[Ce] + (O₂) = [CeO₂] + 224.6 Cals.
[Ce] + 4[Al] = [CeAl₄] + 124.4 Cals.
[Ce(OH)₄] + H₂SO₄aq. = Ce(SO₄)₂aq + 0.90 Cals.
2[Ce(OH)₃.O.OH] + 3H₂SO₄aq. = Ce₂(SO₄)₃aq. + H₂O₂aq. + (O₂) + 29.95 Cals.
2Ce(SO₄)₂aq. + H₂O₂aq. = Ce₂(SO₄)₃aq. + H₂SO₄aq. + (O₂) + 33.58 Cals.
[Ce(OH)₃.O.OH] = [Ce(OH)₄)] + (O) + 20.39 Cals.

The cerous salts are derived from the basic oxide Ce₂O₃ and, if derived from colourless acids, are themselves colourless. Their aqueous solutions are devoid of absorption spectra.

In chemical properties the cerous salts resemble the salts of lanthanum very closely, except for the fact that they may be oxidised to ceric salts. The equivalent conductivities, A, of cerous salts are in harmony with the view that they are derived from a fairly strong triacid base. The following data hold for a temperature of 25° (v - dilution in litres per gram-equivalent): -

CeCl3

v	31.43	62.86	125.72	251.44	502.88	1005.76
λ	107.5	114.2	121.2	126.7	131.0	136.0

Ce2(SO4)3

v	33	66	132	264	528	1056
λ	45.66	53.71	63.63	74.09	87.17	100.90

Numerous cerous salts have been described by Jolin and others. The methods for the conversion of cerous into ceric compounds and vice versa.

Ceric salts are derived from the feebly basic oxide CeO₂, and are yellow, orange, or red in colour. Being salts derived from a weak base, they are considerably hydrolysed in aqueous solution, and normal ceric salts of weak acids are not known. Further, the normal chloride and nitrate are only known in combination as double, or possibly complex, salts. Aqueous solutions of eerie salts, owing to hydrolysis, react strongly acid. The solutions are very unstable and are easily reduced to the cerous state. On the ionic hypothesis, the instability is attributed to the eerie ion, Ce^••••, the oxidation potential of which is greater than that of oxygen; acid solutions of eerie salts accordingly behave as if they were supersaturated with oxygen. In alkaline media, however, eerie compounds are not readily reduced to the cerous state, while the converse change is very easy to effect.

The hydrolysis of a ceric salt in cold, aqueous solution proceeds for a long time after the solution has been prepared, and the colour fades away very perceptibly; the hydrolysis may be hastened and increased by raising the temperature. A freshly prepared solution of a ceric salt immediately darkens in colour when a mineral acid is added, since the degree of hydrolysis is thereby diminished; with an old solution, however, the deepening of the colour takes place very slowly. Again, a. fresh solution is immediately decolorised by hydrogen peroxide, with the formation of a cerous salt and oxygen, but an old solution first turns dark red in colour when similarly treated, and is only slowly reduced. By the hydrolysis of ceric salts basic ceric salts are produced, which, under suitable circumstances, may be utilised for the separation of ceria from the other rare earths.

A cold solution of. ceric nitrate in which hydrolysis has proceeded to a considerable extent, either by long standing or by heating, contains a colloidal hydrosol, which coagulates when nitric acid is added and is almost quantitatively precipitated if 12 cubic centimetres of concentrated nitric acid are added for every 100 cubic centimetres of dilute ceric solution present. The hydrogel thus obtained, when dried over potassium hydroxide, forms an amber-coloured, horny, translucent solid of the composition 4CeO₂.N₂O₅.5H₂O. It easily changes back into the hydrosol in contact with water, giving a greenish, limpid solution unless it is very concentrated, when a faint opalescence is observable. Submitted to dialysis, the solution loses nitric acid; the whole of the acid present in the hydrosol, however, cannot be thus removed, the decomposition ceasing when the ratio 28CeO₂:1N₂O₅ is reached.

When the solution of the basic nitrate hydrosol is treated with one-fifth its volume of concentrated hydrochloric acid, the cerium is almost quantitatively precipitated as a hydrogel which has the composition 4CeO₂.2HCl. 34H₂O and is very similar in properties to the basic nitrate; in solutions of both of these compounds one-half of the acid present may be neutralised with sodium hydroxide before ceric hydroxide begins to be precipitated. When, however, a dilute solution of a dibasic acid (or, better, its ammonium salt) is added to the basic nitrate hydrosol, a hydrogel is precipitated which does not dissolve in water. The basic sulphate, 4CeO₂.SO₃.5H₂O, for instance, which resembles the chloride and nitrate in appearance, loses half its sulphuric acid when washed with warm water, but does not dissolve to any appreciable extent.

The addition of ammonia to the basic nitrate hydrogel converts the latter into a hydroxide which has the composition 8CeO₂.11H₂O when dried over potassium hydroxide, and the same horny appearance as the basic nitrate.

Colloidal compounds similar in properties to the preceding may be prepared containing lanthanum, praseodymium, etc., in addition to cerium.

Concerning the constitution of these colloidal substances, little can be said beyond the statement that they do not appear to be basic salts of the ordinary type. Wyrouboff and Verneuil regard them as derivatives of polymerised or " condensed " ceric hydroxides, such as Ce₂₄O₁₅(OH)₆₀(OH)₆, in which only part of the hydroxyl (the (OH)₆ of the preceding formula, for example) is capable of reacting with acids; they speak of the compounds as being derived from certain metaoxides or mixed metaoxides, the term "meta-" being used as in naming "condensed" acids, e.g. metastannic and metatungstic acids. There is very little evidence, however, for the molecular formulae ascribed to these compounds by the French chemists.

Colloidal basic ceric compounds of another type have also been prepared by Wyrouboff and Verneuil, who speak of them as derivatives of a paraoxide. A description of the basic nitrate may be given. When cerous oxalate is calcined in air at the lowest possible temperature, a canary-yellow residue of ceria is obtained containing 2.9 per cent, of water. It is quite indifferent towards concentrated nitric acid, but is transformed into a white, gelatinous substance when heated to 100° with 3 per cent, nitric acid for several hours. This substance, when separated from the dilute acid by decantation, may be dissolved in water. The solution has a decidedly milky appearance; the hydrosol it contains may be completely precipitated by the addition of nitric acid (2 per cent.) or ammonium nitrate, and dries at 100° to a very pale-coloured, horny, translucent mass, soluble in water. Wyrouboff and Verneuil propose the molecular formula 20CeO₂.N₂O₅.5H₂O for the substance; basic chlorides, sulphates, etc., and another modification of ceric hydroxide may be prepared from it as in the case of the meta-nitrate. Moreover, the French chemists state that when cerous hydroxide, precipitated from a cerous salt by means of ammonia, is oxidised to ceric hydroxide by a current of air, the product is almost entirely insoluble in boiling concentrated nitric acid, and that the insoluble portion is colloidal, dissolving in water to form a solution of the para-hydroxide.

Owing to the great instability of ceric chloride, this transformation cannot be effected in solutions acidified with hydrochloric acid.

1. Oxidation in nitric acid solution. - A solution of cerous nitrate in concentrated nitric acid may be oxidised to ceric nitrate to the extent of 6-8 per cent, by evaporation at 100°. In the presence of the requisite amount of alkali nitrate, some 30 per cent, or more of the cerous salt may be oxidised, a result that is attributed to the transformation of the ceric ions Ce^••••, as they are produced, into the complex ion Ce(NO₃)₆'', the probable existence of which has been shown by Meyer and Jacoby. Provided that (i.) a cerous salt is readily soluble, and the corresponding ceric salt only sparingly soluble in concentrated nitric acid, and (ii.) the acid from which the cerous salt is derived is not attacked by nitric acid, e.g. iodic, phosphoric, and arsenic acids, the oxidation of the cerous to the ceric salt may be readily accomplished by boiling it with concentrated nitric acid.
   
   Oxidation in nitric acid solution may be effected by the use of various oxidising agents, e.g. lead peroxide and bismuth tetroxide; the processes are useful in connection with the analytical chemistry of cerium.
2. Oxidation in sulphuric acid solution. - This may be accomplished by means of ammonium persulphate or sodium bismuthate. The processes are useful for analytical purposes, and the former may also be utilised for the preparation of pure ceria.
3. Electrolytic oxidation. - The electrolysis of cerous nitrate or sulphate between platinum electrodes in neutral or slightly acid solution leads to the separation of ceric hydroxide or basic ceric salt at the anode; but in the presence of sufficient mineral acid, ceric salt is produced in solution. As much as 95 per cent, of the cerium may be oxidised under suitable conditions.
4. Oxidation in alkaline media. - Cerous hydroxide is readily oxidised to ceric hydroxide by alkali hypochlorite or hypobromite. Cerous salts are also readily converted into ceric hydroxide by potassium permanganate in the presence of a base, e.g. sodium hydroxide or magnesia. These reactions are of considerable value both in preparation work and in analytical chemistry. A solution of cerous carbonate in potassium carbonate is easily oxidised by oxygen or hydrogen peroxide.

In alkaline or acetic acid solution, cerous compounds may be oxidised to perceric compounds by means of hydrogen peroxide.

In acid solution ceric salts may be reduced to the cerous state with great ease by numerous reducing agents, e.g. hydrogen peroxide, sulphurous acid, hydrochloric, hydrobromic, and hydriodic acids, oxalic acid, stannous chloride, ferrous sulphate, etc. The neatest method is that involving the use of hydrogen peroxide, but on a large scale it is rather expensive. The transformation from ceric nitrate or sulphate into cerous oxalate is readily effected in warm, acid solution by the addition of oxalic acid.

The conversion of cerium dioxide into cerous salts is worthy of special notice, inasmuch as the dioxide is insoluble in hot, concentrated hydrochloric or nitric acid. The conversion into cerous nitrate may be accomplished very neatly by warming the oxide with moderately concentrated nitric acid and adding hydrogen peroxide from time to time. The conversion into cerous sulphate or chloride may be brought about by heating the oxide with hydro-quinone and an excess of the requisite acid in aqueous solution, the hydro- quinone being converted into benzoquinone and quinhydrone. Ceria may also be converted into cerous sulphate by heating it with concentrated sulphuric acid until it has been converted into ceric sulphate and reducing its aqueous solution with sulphurous acid; while it may be converted into anhydrous cerous chloride by heating in the vapour of disulphur dichloride, and into a solution of cerous chloride (plus alkali chloride) by heating with concentrated hydrochloric acid and an alkali iodide.