[10] Uranium trioxide, or uranic oxide, shows its feeble basic and acid properties in a great number of its reactions. (1) Solutions of uranic salts give yellow precipitates with alkalis, but these precipitates do not contain the hydrate of the oxide, but compounds of it with bases; for example, 2UO2(NO3)2 + 6KHO = 4KNO3 + 3H2O + K2U2O7. There are other urano-alkali compounds of the same constitution; for example, (NH4)2U2O7 (known commercially as uranic oxide), MgU2O7, BaU2O7. They are the analogues of the dichromates. Sodium uranate is the most generally used under the name of uranium yellow, Na2U2O7. It is used for imparting the characteristic yellow-green tint to glass and porcelain. Neither heat nor water nor acids are able to extract the alkali from sodium uranate, Na2U2O7, and therefore it is a true insoluble salt, of a yellow colour, and clearly indicates the acid character (although feeble) of uranic oxide. (2) The carbonates of the alkaline earths (for instance, barium carbonate) precipitate uranic oxide from its salts, as they do all the salts of feeble bases; for example, R2O3. (3) The alkaline carbonates, when added to solutions of uranic salts, give a precipitate, which is soluble in an excess of the reagent, and particularly so if the acid carbonates be taken. This is due to the fact that (4) the uranyl salts easily form double salts with the salts of the alkali metals, including the salts of ammonium. Uranium, in the form of these double salts, often gives salts of well-defined crystalline form, although the simple salts are little prone to appear in crystals. Such, for example, are the salts obtained by dissolving potassium uranate, K2U2O7, in acids, with the addition of potassium salts of the same acids. Thus, with hydrochloric acid and potassium chloride a well-formed crystalline salt, K2(UO2)Cl4,2H2O, belonging to the monoclinic system, is produced. This salt decomposes in dissolving in pure water. Among these double salts we may mention the double carbonate with the alkalis, R4(UO2)(CO3)3 (equal to 2R2CO3 + UO2CO3); the acetates, R(UO2)(C2H3O2)3—for instance, the sodium salt, Na(UO2)(C2H3O2)3, and the potassium salt, K(UO2)(C2H3O2)3,H2O; the sulphates, R2(UO2)(SO4)3,2H2O, &c. In the preceding formula R = K, Na, NH4, or R2 = Mg, Ba, &c. This property of giving comparatively stable double salts indicates feebly developed basic properties, because double salts are mainly formed by salts of distinctly basic metals (these form, as it were, the basic element of a double salt) and salts of feebly energetic bases (these form the acid element of a double salt), just as the former also give acid salts; the acid of the acid salts is replaced in the double salts by the salt of the feebly energetic base, which, like water, belongs to the class of intermediate bases. For this reason barium does not give double salts with alkalis as magnesium does, and this is why double salts are more easily formed by potassium than by lithium in the series of the alkali metals. (5) The most remarkable property, proving the feeble energy of uranic oxide as a base, is seen in the fact that when their composition is compared with that of other salts those of uranic oxide always appear as basic salts. It is well known that a normal salt, R2X6, corresponds with the oxide R2O3, where X = Cl, NO3, &c., or X2 = SO4, CO3, &c.; but there also exist basic salts of the same type where X = HO or X2 = O. We saw salts of all kinds among the salts of aluminium, chromium, and others. With uranic oxide no salts are known of the types UX6 (UCl6, U(SO4)3, alums, &c., are not known), nor even salts, U(HO)2X4 or UOX4, but it always forms salts of the type U(HO)4X2, or UO2X2. Judging from the fact that nearly all the salts of uranic oxide retain water in crystallising from their solutions, and that this water is difficult to separate from them, it may be thought to be water of hydration. This is seen in part from the fact that the composition of many of the salts of uranic oxide may then be expressed without the presence of water of crystallisation; for instance, U(HO)4K2Cl4 (and the salt of NH4, U(HO)4K2(SO4)2, U(HO)4(C2H3O2)2. Sodium uranyl acetate however does not contain water.
[11] Uranyl nitrate, or uranium nitrate, UO2(NO3)2,6H2O, crystallises from its solutions in transparent yellowish-green prisms (from an acid solution), or in tabular crystals (from a neutral solution), which effloresce in the air and are easily soluble in water, alcohol, and ether, have a sp. gr. of 2·8, and fuse when heated, losing nitric acid and water in the process. If the salt itself (Berzelius) or its alcoholic solution (Malaguti) be heated up to the temperature at which oxides of nitrogen are evolved, there then remains a mass which, after being evaporated with water, leaves uranyl hydroxide, UO2(HO)2 (sp. gr. 5·93), whilst if the salt be ignited there remains the dioxide, UO2, as a brick-red powder, which on further heating loses oxygen and forms the dark olive uranoso-uranic oxide, U3O8. The solution of the nitrate obtained from the ore is purified in the following manner: sulphurous anhydride is first passed through it in order to reduce the arsenic acid present into arsenious acid; the solution is then heated to 60°, and sulphuretted hydrogen passed through it; this precipitates the lead, arsenic, and tin, and certain other metals, as sulphides, insoluble in water and dilute nitric acid. This liquid is then filtered and evaporated with nitric acid to crystallisation, and the crystals are dissolved in ether. Or else the solution is first treated with chlorine in order to convert the ferrous chloride (produced by the action of the hydrogen sulphide) into ferric chloride, the oxides are then precipitated by ammonia, and the resultant precipitate, containing the oxides Fe2O3, UO3, and compounds of the latter with potash, lime, ammonia, and other bases present in the solution (the latter being due to the property of uranic oxide of combining with bases), is washed and dissolved in a strong, slightly-heated solution of ammonium carbonate, which dissolves the uranic oxide but not the ferric oxide. The solution is filtered, and on cooling deposits a well-crystallising uranyl ammonium carbonate, UO2(NH4)4(CO3)3, in brilliant monoclinic crystals which on exposure to air slowly give off water, carbonic anhydride, and ammonia; the same decomposition is readily effected at 300°, the residue then consisting of uranic oxide. This salt is not very soluble in water, but is readily so in ammonium carbonate; it is obvious that it may readily be converted into all the other salts of oxides of uranium. Uranium salts are also purified in the form of acetate, which is very sparingly soluble, and is therefore directly precipitated from a strong solution of the nitrate by mixing it with acetic acid.
We may also mention the uranyl phosphate, HUPO6, which must be regarded as an orthophosphate in which two hydrogens are replaced by the radicle uranyl, UO2, i.e. as H(UO2)PO4. This salt is formed as a hydrated gelatinous yellow precipitate, on mixing a solution of uranyl nitrate with disodium phosphate. The precipitation occurs in the presence of acetic acid, but not in the presence of hydrochloric acid. If moreover an excess of an ammonium salt be present, the ammonia enters into the composition of the bright yellow gelatinous precipitate formed, in the proportion UO2NH4PO4. This precipitate is not soluble in water and acetic acid, and its solution in inorganic acids when boiled entirely expels all the phosphoric acid. This fact is taken advantage of for removing phosphoric acids from solutions—for instance, from those containing salts of calcium and magnesium.
[12] Uranium dioxide, or uranyl, UO2, which is contained in the salts UO2X2, has the appearance and many of the properties of a metal. Uranic oxide may be regarded as uranyl oxide, (UO2)O, its salts as salts of this uranyl; its hydroxide, (UO2)H2O2, is constituted like CaH2O2. The green oxide of uranium, uranoso-uranic oxide (easily formed from uranic salts by the loss of oxygen), U3O8 = UO2,2UO3, when ignited with charcoal or hydrogen (dry) gives a brilliant crystalline substance of sp. gr. about 11·0 (Urlaub), whose appearance resembles that of metals, and decomposes steam at a red heat with the evolution of hydrogen; it does not, however, decompose hydrochloric or sulphuric acid, but is oxidised by nitric acid. The same substance (i.e. uranium dioxide UO2) is also obtained by igniting the compound (UO2)K2Cl4 in a stream of hydrogen, according to the equation UO2K4Cl4 + H2 = UO2 + 2HCl + 2KCl. It was at first regarded as the metal. In 1841 Peligot found that it contained oxygen, because carbonic oxide and anhydride were evolved when it was ignited with charcoal in a stream of chlorine, and from 272 parts of the substance which was considered to be metal he obtained 382 parts of a volatile product containing 142 parts of chlorine. From this it was concluded that the substance taken contained an equivalent amount of oxygen. As 142 parts of chlorine correspond with 32 parts of oxygen, it followed that 272 - 32 = 240 parts of metal were combined in the substance with 32 parts of oxygen, and also in the chlorine compound obtained with 142 parts of chlorine. These calculations have been made for the now accepted atomic weight of uranium (U = 240, see Note [14]). Peligot took another atomic weight, but this does not alter the principle of the argument.
[13] Uranium tetrachloride, uranous chloride, UCl4, corresponds with uranous oxide as a base. It was obtained by Peligot by igniting uranic oxide mixed with charcoal in a stream of dry chlorine: UO3 + 3C + 2Cl2 = UCl4 + 3CO. This green volatile compound (Note [12]) crystallises in regular octahedra, is very hygroscopic, easily soluble in water, with the development of a considerable amount of heat, and no longer separates out from its solution in an anhydrous state, but disengages hydrochloric acid when evaporated. The solution of uranous chloride in water is green. It is also formed by the action of zinc and copper (forming cuprous chloride) on a solution of uranyl chloride, UO2Cl2, especially in the presence of hydrochloric acid and sal-ammoniac. Solutions of uranyl salts are converted into uranous salts by the action of various reducing agents, and among others by organic substances or by the action of light, whilst the salts UX4 are converted into uranyl salts, UO2X2, by exposure to air or by oxidising agents. Solutions of the green uranyl salts act as powerful reducing agents, and give a brown precipitate of the uranous hydroxide, UH4O4, with potash and other alkalis. This hydroxide is easily soluble in acids but not in alkalis. On ignition it does not form the oxide UO2, because it decomposes water, but when the higher oxides of uranium are ignited in a stream of hydrogen or with charcoal they yield uranous oxide. Both it and the chloride UCl4, dissolve in strong sulphuric acid, forming a green salt, U(SO4)2,2H2O. The same salt, together with uranyl sulphate, UO2(SO4), is formed when the green oxide, U3O8, is dissolved in hot sulphuric acid. The salts obtained in the latter instance may be separated by adding alcohol to the solution, which is left exposed to the light; the alcohol reduces the uranyl salt to uranous salt, an excess of acid being required. An excess of water decomposes this salt, forming a basic salt, which is also easily produced under other circumstances, and contains UO(SO4),2H2O (which corresponds to the uranic salt).
[14] The atomic weight of uranium was formerly taken as half the present one, U = 120, and the oxides U2O3, suboxide UO, and green oxide U3O4, were of the same types as the oxides of iron. With a certain resemblance to the elements of the iron group, uranium presents many points of distinction which do not permit its being grouped with them. Thus uranium forms a very stable oxide, U2O3(U = 120), but does not give the corresponding chloride U2Cl6 (Roscoe, however, in 1874 obtained UCl5, like MoCl5 and WCl5), and under those circumstances (the ignition of oxide of uranium mixed with charcoal, in a stream of chlorine), when the formation of this compound might be expected, it gives (U = 120) the chloride UCl2, which is characterised by its volatility; this is not a property, to such an extent, of any of the bichlorides, RCl2, of the iron group.
The alteration or doubling of the atomic weight of uranium—i.e. the recognition of U = 240—was made for the first time in the first (Russian) edition of this work (1871), and in my memoir of the same year in Liebig's Annalen, on the ground that with an atomic weight 120, uranium could not be placed in the periodic system. I think it will not be superfluous to add the following remarks on this subject: (1) In the other groups (K—Rb—Cs, Ca—Sr—Ba, Cl—Br—I) the acid character of the oxides decreases and their basic character increases with the rise of atomic weight, and therefore we should expect to find the same in the group Cr—Mo—W—U, and if CrO3, MoO3, WO3 be the anhydrides of acids then we indeed find a decrease in their acid character, and therefore uranium trioxide, UO3, should be a very feeble anhydride, but its basic properties should also be very feeble. Uranic oxide does indeed show these properties, as was pointed out above (Note [10]). (2) Chromium and its analogues, besides the oxides RO3, also form lower grades of oxidation RO2, R2O3, and the same is seen in uranium; it forms UO3, UO2, U2O3 and their compounds. (3) Molybdenum and tungsten, in being reduced from RO3, easily and frequently give an intermediate oxide of a blue colour, and uranium shows the same property; giving the so-called green oxide which, according to present views, must be regarded as U3O8 = UO22UO3, analogous to Mo3O8. (4) The higher chlorides, RCl6, possible for the elements of this group, are either unstable (WCl6) or do not exist at all (Cr); but there is one single lower volatile compound, which is decomposed by water, and liable to further reduction into a non-volatile chlorine product and the metal. The same is observed in uranium, which forms an easily volatile chloride, UCl4, decomposed by water. (5) The high sp. gr. of uranium (18·6) is explained by its analogy to tungsten (sp. gr. 19·1). (6) For uranium, as for chromium and tungsten, yellow tints predominate in the form RO3, whilst the lower forms are green and blue. (7) Zimmermann (1881) determined the vapour densities of uranous bromide, UBr4, and chloride, UCl4 (19·4 and 13·2), and they were found to correspond to the formulæ given above—that is, they confirmed the higher atomic weight U = 240. Roscoe, a great authority on the metals of this group, was the first to accept the proposed atomic weight of uranium, U = 240, which since Zimmermann's work has been generally recognised.
[15] Uranium glass, obtained by the addition of the yellow salt K2U2O7 to glass, has a green yellow fluorescence, and is sometimes employed for ornaments; it absorbs the violet rays, like the other salts of uranic oxide—that is, it possesses an absorption spectrum in which the violet rays are absent. The index of refraction of the absorbed rays is altered, and they are given out again as greenish-yellow rays; hence, compounds of uranic acid, when placed in the violet portion of the spectrum, emit a greenish-yellow light, and this forms one of the best examples (another is found in a solution of quinine sulphate) of the phenomenon of fluorescence. The rays of light which pass through uranic compounds do not contain the rays which excite the phenomena of fluorescence and of chemical transformation, as the researches of Stokes prove.
[16] The comparison of potassium permanganate with potassium perchlorate, or of potassium manganate with potassium sulphate, shows directly that many of the physical and chemical properties of substances do not depend on the nature of the elements, but on the atomic types in which they appear, on the kind of movements, or on the positions in which the atoms forming the molecule occur.
[17] If, however, we compare the spectra (Vol. I. p. [565]) of chlorine, bromine, and iodine with that of manganese, a certain resemblance or analogy is to be found connecting manganese both to iron and to chlorine, bromine, and iodine.