390 Gooch and Gilbert — Use of the Zinc 



tetroxide. The first attempts, therefore, to bring about defi- 

 niteness of condition were made along the lines of Roscoe's 

 observation. The solutions obtained in the receiving flask 

 were treated by air at various temperatures and for various 

 lengths of time, and then, after heating to 80°, were titrated 



1ST 

 with nearly — potassium permanganate standardized by refer- 

 ence to — arsenious oxide. 

 20 



This most exact method of standardizing the permanganate 

 was carried out by adding a convenient volume (43 cm3 ) of the 

 permanganate to a solution of potassium iodide (3 grin.) acidu- 

 lated with sulphuric acid (3 cm3 of the 1 : 1 acid) contained in a 

 glass-stoppered flask fitted with a funnel tube and trapped, 

 introducing through the funnel tube an excess of a standard 



N 

 solution of — arsenious oxide, neutralizing with acid potas- 

 sium carbonate, and titrating in presence of starch with iodine 

 also standardized against the arsenious oxide. 



In blank experiments made from time to time with the 

 reductor it was found that the reading color developed in the 

 acid solution only after the addition of 0-2 cm3 of the standard 

 permanganate, and this correction, due to traces of iron in the 

 zinc and the considerable volume of the solution, was applied 

 in the experiments in which the vanadium salt was treated. 

 In our experiments ammonium vanadate was the salt of vana- 

 dic acid employed and the sample with which we worked con- 

 tained 76*66 per cent of V 2 5 , as determined according to the 

 iodometric method of Holverscheit.* 



The results of some attempts under most favorable condi- 

 tions to effect the oxidation of the reduced trioxide to the 

 condition of the tetroxide by the action on air are given in 

 Table I. These show plainly that, although the oxidation by air 

 appears to proceed rapidly at the outset, the complete conver- 

 sion of the lower oxides to the tetroxide by such action takes 

 place with too great slowness and uncertainty to form the basis 

 of a reliable and rapid quantitative method. 



Table I. 



Time of 

 reatment 







V 2 5 found by 

 titration with 





by air. 



Temperature. 



V 2 5 taken. 



KMn0 4 . • 



Error. 



min. 





grm. 



grm. 



grm. 



75 



56°— 28° 



0-0767 



0-0778 



+ 0-0011 



75 



60°— 29° 



0-0767 



0-0805 



+ 00038 



75 



48°— 29° 



0-0767 



0-0829 



+ 0-0062 



75 



fioo°— 



0-0767 



0-0772 



+ 0-0005 



75 



jioo°— 



0-0767 



0-0769 



+ 0-0002 



*Inaug. Diss. Berlin, 1890, p. 49. 



f Brought twice to the boiling point during this interval. 



