Note 10 — Chapter XXI · CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE

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.