Urano-uranic Oxide, U₃O₈

Urano-uranic Oxide, the Green Oxide of Uranium, U₃O₈ or UO₂.2UO₃, occurs in nature in a more or less pure condition as pitchblende. It may be obtained by heating to redness any of the other oxides of uranium; by strongly igniting, at a temperature above 900° C., ammonium diuranate; or by heating to redness the uranium salt of any volatile acid. The uranyl salts of commerce are invariably impure with alkali, so that it is impossible to obtain the pure oxide from them, unless they are first purified by a series of recrystallisations. To obtain pure urano-uranic oxide, free from alkali, the following method may be used: Sodium uranyl acetate (100 g.) is dissolved in a mixture of water (4 l.) and hydrochloric acid (50 c.c.), and precipitated by adding concentrated ammonia solution (300 c.c.). The precipitate is washed ten times with a 2 per cent, ammonium chloride solution, and then redissolved in hydrochloric acid. The precipitation with ammonia and washing are repeated twice, and the final precipitate, on calcination, yields pure urano-uranic oxide.

Urano-uranic oxide is an amorphous powder, varying in colour, according to its mode of preparation, from green to black, but in all cases it leaves a green streak on unglazed porcelain. The density, according to Karsten, is 7.193; according to Ebelmen, 7.31. The specific heat at ordinary temperatures is 0.07979, but the value falls considerably at lower temperatures, as shown by the following determinations:

Temperature Range, ° C.	Specific Heat.
0 to -77	0.0616
-77 to -190	0.0428

The oxide is paramagnetic, the susceptibility being +0.95×10^-6, a value very near to that for the metal itself. The single electrode potential of the oxide, measured by pasting it in a finely powdered condition with gelatin on a platinum electrode and immersing it in a solution of uranyl nitrate containing 14.3 grams per litre, is:

U₃O₈/UO₂(NO₃)₂ = 0.776 volt.

This value is identical with that obtained for uranous oxide.

Urano-uranic oxide is insoluble in water and in dilute acids; in concentrated mineral acids it dissolves slowly, forming a mixture of uranous and uranyl salts. It is very stable at high temperatures, and for this reason uranium is usually converted into this oxide for gravimetric estimation. It may be heated in a vacuum for three hours at 1000° C. without any appreciable loss of oxygen, and in order to obtain the dioxide from it by simple dissociation a temperature of 2000° C. is necessary. It is reduced when heated in hydrogen, the sole product being the dioxide. The action commences at about 625° C. and can be completed at 650° C. It also yields the dioxide when heated with sulphur. It is reduced to the metal by heating with carbon in the electric furnace; also by heating with magnesium. When heated in a stream of carbon dioxide at about 1120° C. it yields the pentoxide.

The heat of formation of the oxide has been determined by Mixter as follows:

3U + 4O₂ = U₃O₈ + 845,200 calories.

The chemical behaviour of urano-uranic oxide indicates the presence of tetra- and hexa-valent uranium and supports the view that it is an association of uranous and uranic oxides, thus: UO₂.2UO₃. The properties of the substance do not support the view of Groth that it is uranous uranate, U(UO₄)₂.

The oxide is sometimes used for producing a black glaze on porcelain. In uniform films it may be used as a standard of α-ray activity.

When an aqueous solution of uranyl oxalate, or an alcoholic solution of uranyl acetate, is exposed to sunlight, a violet precipitate of hydrated urano-uranic oxide is formed. Moreover, any uranyl salt in solution in presence of a readily oxidisable organic substance, such as alcohol, ether, or glucose, when exposed to light, or better to ultra-violet rays, yields a violet precipitate which invariably contains the acid present in the original salt. The acid, however, can be removed by boiling with water, and a green hydrated oxide, U₃O₈.2H₂O, results. Similar violet compounds are also obtained, without the action of light, whenever uranous and uranyl salts are present together in neutral or feebly acid solution at the requisite temperature.