G. Torossian — Reduction Test for Tungstic Acid. 537 



Art. XLIX. — Modifications of the Reduction Test for Tung- 

 stic Acid ; by G. Torossian. 



When acted upon by suitable reducing agents in acid solu- 

 tion tungstic acid is reduced, the solution becoming blue and 

 then brown. Examples of such reductions are the well-known 

 tests with zinc, tin, or aluminium in presence of hydrochloric 

 acid. The test may, however, be made more delicate if the 

 substance to be tested be used in solid (and thus highly concen- 

 trated) form and the reduction brought about either by simple 

 friction (in the case of tungstic acid), or, better, with the aid 

 of a very little hydrochloric acid. 



The Dry Test for Free Tungstic Acid. 



When yellow tungstic acid, or tungsten oxide, is rubbed upon 

 a bright surface of iron, aluminium, zinc, nickel, or copper, in 

 the form of sheet or foil, the yellow color is changed to 

 greenish-blue, while the surface of the metal becomes covered 

 with a deep velvet-blue coating. This action serves as the 

 basis of a convenient test for tungstic acid, and, less favorably, 

 for tungstic oxide. 



In making the test the powdered substance is placed upon 

 a sheet of paper and simply rubbed with a bright iron spatula 

 or with a strip of bright aluminium. In absence of agents capa- 

 ble of exciting marked oxidizing action upon the lower oxides 

 of tungsten, tungstic acid is reduced and a blue coating will 

 appear upon the surface of the metal. Moistening the rubbing 

 surface by breathing upon it tends to facilitate the coloration. 

 Representative mixtures of tungstic acid with sulphur, cal- 

 cium fluoride, potassium silicofluoride, alkali chlorides, alkali 

 carbonates, alkali sulphates, tin oxide, or zinc oxide, gave the 

 color in experimental trials ; but mixtures of tungstic acid with 

 potassium chromate or dichromate, chlorate or nitrate, or 

 with sodium nitrate, failed to respond to the test. 



The Acid Test for Tungstic Acid and Tungstates. 



The presence of hydrochloric acid facilitates the reducing 

 action of the metal upon the substance to be tested, either 

 tungstic acid, a tungstate or any tungsten compound, and the 

 consequent development of the color, even in presence of an 

 oxidizer. Modifications of this mode of testing and the results 

 obtained are shown in the following concise statement : 



