OP ARTS AND SCIENCES. 249 



VANADIO-TUNGSTATES. 



Vanadio-tungstates are very readily formed under conditions pre- 

 cisely the same as tliose which determine the formation of vanadio- 

 molybdates, and which it is unnecessary to recapitulate. As a class, 

 they have a yellow, orange, or orange-red color. They are, so far as 

 examined, readily soluble in water, are crystalline and extremely stable 

 in composition, in the sense at least that it is often difficult to separate 

 the combined vanadio and tungstic oxides, though in many cases com- 

 plex salts easily break up into compounds of simpler type. 



Analytical Methods. — When alkaline vanadates and tungstates ai-e 

 simply mixed together under conditions which do not result in the for- 

 mation of vanadio-tungstates, it is possible to effect the separation of 

 the vanadic and tungstic oxides by means of a solution of amnionic 

 chloride, which, as is well known, precipitates ammonic meta-vauadate 

 in colorless crystals nearly insoluble in a saturated solution of ammonic 

 chloride. This method cannot be applied directly to the separation of 

 the two oxides when combined, as in the vanadio-tungstates, but the 

 difficulty can be overcome by either of the following methods. An 

 excess of ammonia is to be added to the solution, which is then to be 

 digested, or, if necessary, boiled until the liquid becomes colorless. By 

 tliis process, which, however, does not always perfectly succeed, the 

 vanadio-tungstate is resolved into a mixture of ammonic tungstate and 

 meta- vanadate. In place of ammonia, sodic or potassic hydrate in excess 

 may be employed, and frequently with advantage. After the resolu- 

 tion of the vanadio-tungstate, as indicated by the color of the liquid, 

 ammonic meta-vanadate may be precipitated by boiling with an excess 

 of ammonic chloride. From the cold solution completely saturated 

 with ammonic chloride the meta-vanadate my be separated by filtra- 

 tion, and the vanadic pentoxide estimated in the manner already 

 pointed out. A variation of this process, which may often be em- 

 ployed with advantage, consists in fusing the vanadio-tungstate with 

 an excess of potassic or sodic carbonate in a platinum crucible. The 

 fused mass must be boiled with a strong solution of ammonic chloride, 

 a drop or two of ammonia being added from time to time to keep the 

 solution distinctly alkaline. The ammonic meta-vanadate is to be sep- 

 arated and treated as above. In applying these methods it is impor- 

 tant in each case to be sure that the vanadium is exclusively in the 

 form of pentoxide. It is therefore always advisable to add a little 

 bromine water to the solution of the vanadio-tuno-state before adding 

 the excess of ammonia. In fusing with alkaline carbonates a little 



