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atomic weight 238.5 URANIUM [symbol U...

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Originally appearing in Volume V27, Page 788 of the 1911 Encyclopedia Britannica.
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atomic

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weight 238.5 URANIUM [symbol U (0=16)]  , a metallic chemical element . In 1789 Klaproth isolated from pitchblende a yellow
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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
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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 Ur2033S03, where Ur203, according to his analysis, represents 864 parts of yellow oxide (0=16) . Like Fe203, the yellow oxide lost 48 parts of oxygen per Ur203 (= 864 parts) as
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water, while Ur2 = 816 parts of metal remained . These results were adopted until Peligot in 184o discovered that Berzelius's (and Klaproth's) metal contains oxygen, and that his (Ur2)03 really is (UsOs)•03=3U203, where U=12o is one
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equivalent
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weight of real uranium . Peligot'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 Mendeleeff, we double Peligot'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 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
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RADIOACTIVITY) . To extract the metal, the pitchblende is first roasted in order to remove the arsenic and
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sulphur .

In one

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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
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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
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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
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separate out . The
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mother liquor includes generally more or less of nickel, cobalt,
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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 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
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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 (UO2•C204) 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
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sugar charcoal in the electric
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furnace . Uranium is a white malleable metal, which is
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pretty hard, though softer than steel . Its specificgravity has the high value 18.7; its specific heat is o•02765, 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 yellow uranyl salt, UO2•C12 .

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
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group UO2 . Uranous Compounds.—Uranium dioxide, UO2 (Berzelius's metal), is a brown to copper-coloured powder, obtained by
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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
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fusion with borax . Uranous
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hydrate is obtained as reddish-brown flakes by precipitating a uranous solution with
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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
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direct combination with chlorine the pentachloride, UCI5 . With hydroflouric 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 UO2.0, 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
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painting on glass and
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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 U308, which serves as a raw material for uranium compounds . Uranyl nitrate, UO2(NO3)2.6H20, is the most important uranium salt . It is obtained as
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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, U04.2H20, when treated with hydrogen peroxide . The latter gives rise to salts, the peruranates, e.g . (Na202)2U04.8H20 .

Uranyl nitrate is used in

photography, and also in
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analytical chemistry as a precipitant for phosphoric acid (as uranyl ammonium phosphate, Up02.NH4•PO4) . Uranyl chloride, UO2C12, 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 hydrochlorides 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, U2(NH4)2SO4, which is a fine
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blood-coloured amorphous powder.' Analysis.—A borax
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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: atomic weight 238.5 URANIUM [symbol U (0=16)]
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