298 PRINCIPLES OF 'CHEMISTRY 



highest salt-forming oxide, UO 3 , shows very feeble acid properties, 

 Although it gives sparingly-soluble yellow compounds with alkalis, 

 which fully correspond with the dichromates for example, Na. 2 U 2 O v 

 ==Na 2 0,2U0 3 , 10 yet it more frequently and easily reacts with acids,-HX, 



Solution of peroxide of hydrogen for several minutes. The solution rapidly turns yellow, 

 and no longer gives a precipitate of tungstic anhydride when treated with nitric acid. 

 When evaporated in vacuo the solution leaves a thick syrupy liquid from which ray-like 

 Crystals separate out ; these crystals are more soluble in, water than the salt originally 

 taken. When heated they also lose water and oxygen. Their composition answers to 

 the formula M 2 W 2 O 8 2H,,O, where M = Na, NH 4) &c. The permolybdates and per- 

 tungstates have similar properties. When treated with oxygen acids they give peroxide 

 of hydrogen, and disengage chlorine, and iodiae from hydrochloric acid and potassium 

 iodide. 



Piccini (1891) showed that peroxide of hydrogen not only combines with the oxygen 

 compounds of Mo and W, but also with their fluo-compounds, among which ammonium 

 fluo-molybdate MoO 2 F 2 2NH 4 and others have long been known. (A few new salts of 

 similar composition have been obtained by F. Moureu in 1893.) The action of peroxide 

 of hydrogen upon these compounds gi-ves salts containing a larger amount of oxygen ; for 

 instance, a solution of MoO 2 F,/2KFH 2 with peroxide Of hydrogen gives a yellow solu- 

 tion which after cooling separates out yellow crystalline flakes of Mo0 3 F 2 2KFHoO, resem- 

 bling the salt originally taken in their external appearance. By employing a similar method 

 Piccini also obtained : MoO 3 Fo2RbFH 2 yellow monoclinic crystals ; MoO 3 F 8 2CsFHjO, 

 yellow flakes, and the corresponding tungstic compounds. All these salts re-act like 

 peroxide of hydrogen. 



In speaking of these compounds I for my part think it may be well to call attention 

 to the fact that, in the first place, the composition of Piccini's oxy-fluo compounds does 

 not correspond to that of permolybdic and pertungstic acid. If the latter be expressed 

 by formulae with one equivalent of an element, they will be HMoO 4 and HWO^ and the 

 oxy-fluo form corresponding to them should have the composition Mo0 3 F and WO 5 F 

 while it contains M0 3 F 2 and WO 3 F 2 , i.e. answers as it were to a higher degree of oxida- 

 tion, MoH 2 O 5 and W HO 5 . But if permolybdic acid be regarded as 2MoO 3 + H 2 O 2 , i.e. 

 as containing the elements of peroxide of hydrogen, then Piccini's compound will also be 

 found to contain the original salts + H 2 O ; for example, from MoOjF 2 2KFH 3 O there is 

 obtained a compound MoO 2 F 2 2KFH.jO 2 , i.e. instead of H 2 O they contain H 2 O. ; . In the 

 second place the capacity of the salts of molybdenum and tungsten to retain a further 

 amount of oxygen or H 4 O 2 probably bears some relation to their property of giving com- 

 plex acids and of polymerising which has been considered in Note 8 bis. There is, 

 however, a great chemical interest in the accumulation of data respecting these high 

 peroxide compounds corresponding to molybdic and tungstic acids. With regard to the 

 peroxide form of uranium, see Chapter XX., Note 66. 



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 tho oxide, but compounds of 

 it with bases ; for example, 2UO 2 (NO 3 ) 3 + 6KHO = 4KNO 3 + 3H 2 O + K 2 U.,O 7 . There are 

 other urano-alkali compounds of the same constitution ; for example, (NH 4 ). i U. 4 O 7 

 (known commercially as uranic oxide), MgU 2 7 , BaU 2 O 7 . They are the analogues of the 

 dichromates. Sodium uranate is the most generally used under the name of uranium 

 yellow, Na.jU 2 O 7 . 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, Na 2 U^O 7 , 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, R 2 O 3 . (3), The alkaline car- 

 bonates, when added to solutions of uranic salts, give a precipitate, which is soluble in 



