Uranyl Phosphate hy the Zinc Reductor. 231 



ting the receiving flask and titrating the solution with perman- 

 ganate. For this purpose there was joined to the stopcock of 

 the burette by a rubber connector a straight glass tube running 

 down through a three-hole stopper into a receiving flask of 

 about a liter capacity. Through the other two holes of the 

 stopper passed pieces of glass tubing bent at right angles, of 

 which one served for introducing the carbon dioxide and the 

 other for applying the suction.* A layer of glass wool about 

 5mm ^jj thickness was placed between the broken glass in the 

 bottom of the burette and the column of zinc so that any 

 particles of zinc might not be drawn into the receiving flask 

 by the suction. The results were constantly very high when 

 operating in this manner. 



Zimmermannf found that uranous salts are fairly stable in 

 the air, an exposure of the dilute solution in a porcelain dish 

 for two hours making a difference of only two milligrams and 

 a half on 0'1568 grams UO3, and also that the same results were 

 obtained whether he poured his reduced solution into a por- 

 celain dish, diluted it with water, and titrated it with perman- 

 ganate, or poured the reduced solution into a porcelain dish 

 containing an excess of permanganate diluted and acidified 

 with sulphuric acid, then destroyed the excess of permanganate 

 by a slight excess of ferrous sulphate, and finally took the end- 

 point with permanganate. 



Since Zimmermann did not find it necessary to keep his 

 reduced solution of uranium from exposure to the air, the idea 

 occurred that possibly the presence of carbon dioxide, as sug- 

 gested by Kern, might be unnecessary. Some experiments 

 were therefore tried in which the solution was passed through 

 the reductor, caught in a flask in which no carbon dioxide was 

 present, and after dilution was titrated directly in the receiving 

 flask with the permanganate. Even this procedure gave posi- 

 tive errors, although not so great as when the carbon dioxide 

 was present, thus seeming, together with the experiments with 

 the carbon dioxide, to point to the fact that the uranyl salts 

 were actually reduced below the uranous state by zinc and sul- 

 phuric acid, and that they tended to oxidize back again when 

 exposed to the air. 



It was thought that if the contents of the flask after passing 

 the reductor were poured through the air into a porcelain dish, 

 any over-reduction might be corrected, and the uranium thus 

 brought exactly to the uranous stage. In each of a set of 

 experiments performed in this manner it was found that the 

 results were sharp and concordant, and the presumption that 

 the uranium salt is actually reduced below the uranous stage by 



* Compare figure p. 334. 



f Ann. Chem. (Liebig), ccxiii, 303. 



