1911 Encyclopædia Britannica/Uranium

URANIUM [symbol U, atomic weight 238.5 (O=16)], a metallic chemical element. In 1789 Klaproth isolated from pitchblende a yellow oxide which he viewed as the oxide of a new metal, which he named uranium, after the newly discovered planet of Herschel. By reducing the oxide with charcoal at a high temperature, he obtained a product which he took to be metallic uranium. Berzelius about 1823 found that the yellow oxide, when treated with excess of sulphuric acid, gave a sulphate not unlike the ferric salt. He concluded that the uranium salt was Ur₂O₃3SO₃, where Ur₂O₃, according to his analysis, represents 864 parts of yellow oxide (O=16). Like Fe₂O₃, the yellow oxide lost 48 parts of oxygen per Ur₂O₃ (=864 parts) as water, while Ur₂=816 parts of metal remained. These results were adopted until Péligot in 1840 discovered that Berzelius’s (and Klaproth’s) metal contains oxygen, and that his (Ur₂)O₃ really is (U₆O₆)·O₃=3U₂O₃, where U=120 is one equivalent weight of real uranium. Péligot’s results, though called in question by Berzelius, have been amply confirmed by all subsequent investigators; only now, on theoretical grounds, first set forth by Mendeléeff, we double Péligot’s atomic weight, so that U now signifies 240 parts of uranium, while UO₃ stands as the formula of the yellow oxide, and UO₂ as that of Berzelius’s metal.

The only practically available raw material for the extraction of uranium is pitchblende (q.v.). Pure pitchblende is U₃O₈, which, in relatively good specimens, forms some 80% or more of the whole. It is remarkable as always containing helium (q.v.) and radioactive elements (see Radioactivity). To extract the metal, the pitchblende is first roasted in order to remove the arsenic and sulphur. In one process the purified ore is disintegrated with hot nitric acid to produce nitrates, which are then converted into sulphates by evaporation with sulphuric acid. The sulphates are treated with water, which dissolves the uranium and other soluble salts, while silica, lead sulphate, &c., remain; these are removed by filtration. From the solution the arsenic, copper, &c., are precipitated by sulphuretted hydrogen as sulphides, which are filtered off. The filtrate contains the uranium as uranous and the iron as ferrous salt. These are oxidized and precipitated conjointly by excess of ammonia. The precipitate, after having been collected and washed, is digested with a warm concentrated solution of ammonium carbonate, which dissolves the uranium as a yellow solution of ammonium uranate, while the hydrated oxide of iron, the alumina, &c., remain. These are filtered off hot, and the filtrate is allowed to cool, when crystals of the uranate separate out. The mother liquor includes generally more or less of nickel, cobalt, zinc and other heavy metals, which, as Wohler showed, can be removed as insoluble sulphides by the addition of ammonium sulphide; uranium, under the circumstances, is not precipitated by this reagent. The filtrate, on being boiled down, yields a second crop of uranate. This uranate when nited in a platinum crucible leaves a green oxide of the composition U₃O₈, i.e. artificial pitchblende, which serves as a starting-point for the preparation of uranium compounds. The green oxide, as a rule, requires to be further purified. One method for this purpose is to convert it into a solution of the nitrate UO₂(NO₃)₂, and from it to precipitate the metal as oxalate by oxalic acid (Péligot). The latter (UO₂·C₂O₄) yields a purer oxide, UO₂, or, in the presence of air, U₃O₈, on ignition.

Metallic uranium, as shown by Péligot, can be obtained by the reduction of a mixture of dry chloride of potassium and dry uranous chloride, UCl₄, with sodium at a red heat. A better process is that of H. Moissan (Compt. rend., 1896, 122, p. 1088), in which the oxide is heated with sugar charcoal in the electric furnace. Uranium is a white malleable metal, which is pretty hard, though softer than steel. Its specific gravity has the high value 18.7; its specific heat is 0.02765, which, according to Dulong and Petit’s law, corresponds to U=240. It melts at bright redness. The compact metal when exposed to the air tarnishes only very slowly. The powdery metal when heated in air to 150° or 170° C. catches fire and burns brilliantly into U₃O₈; it decomposes water slowly at ordinary temperatures, but rapidly when boiling. It burns in oxygen at 170°, in chlorine at 180°, in bromine at 210°, in iodine at 260°, in sulphur at 500°, and combines with nitrogen at about 1000°. Dilute sulphuric acid attacks it but slowly; hydrochloric acid, especially if strong, dissolves it readily, with the formation, more immediately, of a hyacinth coloured solution of U₂CI₆, which, however, readily absorbs oxygen from the air, with the formation of a green solution of UCl₄, which in its turn gradually passes into one of yellow uranyl salt, UO₂·Cl₂.

Uranium is chemically related to chromium, molybdenum and tungsten. If forms two series of salts, one, the uranous compounds, are derived from the oxide UO₂, the other, the uranyl compounds, contain the divalent group UO₂.

Uranous Compounds.—Uranium dioxide, UO₂ (Berzelius’s metal), is a brown to copper-coloured powder, obtained by heating U₃O₈ or uranyl oxalate in hydrogen. It fires when heated in air, and dissolves in acids to form uranous salts. It may be obtained as jet black octahedral (isomorphous with thoria) by fusion with borax. Uranous hydrate is obtained as reddish-brown flakes by precipitating a uranous solution with alkali. The solution in sulphuric acid deposits green crystals of the sulphate, U(SO₄)₂·8H₂O, on evaporation. Uranous chloride, UCl₄, was first prepared by Péligot by heating an intimate mixture of the green oxide and, charcoal to redness in a current of dry chlorine; it is obtained as sublimate of black-green metallic-looking octahedral. The chloride is very hygroscopic. By heating in hydrogen it yields the trichloride, UCl₃, and by direct combination with chlorine the pentachloride, UCl₅. With hydrofluoric acid it yields uranous fluoride, UF₄, which forms double salts of the type MF·UF₄. Uranous bromide, UBr₄, and uranous iodide, UI₄, also exist.

Uranyl or Uranic Compounds.—Uranic oxide, UO₃ or UO₂·O, is obtained by heating uranyl nitrate to 250° as a yellow solid, insoluble in water, but soluble in acids with the formation of uranyl salts. Various hydrates have been described, but they cannot be formed by precipitating a uranyl salt with an alkali, this reagent giving rise to salts termed uranates. These salts generally resemble the bichromates; they are yellow in colour, insoluble in Water, soluble in acids, and decomposed by heat. Sodium uranate, Na₂U₂O₇, is used as a pigment for painting on glass and porcelain under the name of uranium yellow. It is manufactured by heating pitchblende with lime, treating the resulting calcium uranate with dilute sulphuric acid, and adding sodium carbonate in excess. Dilute sulphuric acid precipitates uranium yellow, Na₂U₂O₇·6H₂O, from the solution so obtained. Ammonium uranate heated to redness yields pure U₃O₈, which serves as a raw material for uranium compounds. Uranyl nitrate, UO₂(NO₃)₂·6H₂O, is the most important uranium salt. It is obtained as fine lemon yellow deliquescent prisms by evaporating a solution of any of the oxides in nitric acid. By electrolysis it yields uranium dioxide as a pyrophoric powder, and peruranic hydroxide, UO₄·2H₂O, when treated with hydrogen peroxide. The latter gives rise to salts, the peruranates, e.g. (Na₂O₂)₂UO₄·8H₂O. Uranyl nitrate is used in photography, and also in analytical chemistry as a precipitant for phosphoric acid (as uranyl ammonium phosphate, UO₂·NH₄·PO₄). Uranyl chloride, UO₂Cl₂, is a yellow crystalline mass formed when chlorine is passed over uranium dioxide at a red heat. It is also obtained by dissolving the oxide in hydrochloric acid and evaporating. It forms double salts with metallic chlorides and with the hydro chlorides of organic bases. Uranyl sulphide, UO₂S, is a black precipitate obtained by adding ammonium sulphide to a uranyl solution. Exposed to air this mixture is oxidized to the pigment uranium red, U₆(NH₄)₂SO₉, which is a fine blood-coloured amorphous powder.

Analysis.—A borax bead dissolves uranium oxides in the reducing flame with a green, in the oxidizing flame with a yellow, colour. Solutions of uranyl salts (nitrate, &c.) behave to reagents as follows: sulphuretted hydrogen produces green uranous salt with precipitation of sulphur; sulphide of ammonium in neutral solutions gives a black precipitate of UO₂S, which settles slowly and, while being washed in the filter, breaks up partially into hydrated UO₂ an sulphur; ammonia gives a yellow precipitate of uranate of ammonia, characteristically soluble in hot carbonate of ammonia solution; prussiate of potash gives a brown precipitate which in appearance is not unlike the precipitate produced by the same reagent in cupric salts.