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Encyclopedia Britannica - Main :: TUM-VAN |
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URANIUM [symbol U, atomic weight 238.5 (0=16)] , a metallic chemical element
oxide
oxide
Berzelius
Berzelius
double
formula
The only practically available raw material for the extraction of uranium is pitchblende (q.v.). Pure pitchblende is U30s, which, in relatively good specimens, forms some 8o% 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
Waited in a platinum crucible leaves a green oxide of the composition 30s, 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 (Peligot). The latter (UO2C204) yields a purer oxide, UO2, or, in the presence of air, U808, on ignition. Metallic uranium, as shown by Peligot, 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 (Comp). rend., 1896, 122, p. 1o88), 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 specificgravity has the high value 18.7; its specific heat is o02765, which, according to Dulong and Petit's law, corresponds to U= 24o. 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 15o or 17o C. catches fire and burns brilliantly into U308; it decomposes water slowly at ordinary temperatures, but rapidly when boiling. It burns in oxygen at 17o, in chlorine at 18o, in bromine at no, in iodine at 26o, in sulphur at 500, and combines with nitrogen at about I000. Dilute sulphuric acid attacks it hut slowly; hydrochloric acid, especially if strong, dissolves it readily, with the formation, more immediately, of a hyacinth-coloured solution of U2C18i which, however, readily absorbs oxygen from the air, with the formation of a green solution of UC14, which in its turn gradually passes into one of yellowuranyl salt, UO2C12. 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 U308 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 octahedra (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)2.8H20, on evaporation. Uranous chloride, UC14, was first prepared by Peligot 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 octahedra. The chloride is very hygroscopic. By heating in hydrogen it yields the trichloride, UC13, and by direct combination with chlorine the pentachloride, UCI5. With hydroflouric acid it yields uranous fluoride, UF4, which forms double
Uranyl or Uranic Compounds.Uranic oxide, UO3 or UO2.0, 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, Na2U2O2, is used as a pigment for painting on glass and porcelain
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 and 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. End of Article: URANIUM [symbol U, atomic weight 238.5 (0=16)] If you wish, you can link directly to this article.
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