434 Gooch & Scott — Determination of Vanadic Acid. 



thousand revolutions per minute and exposing a reaction 

 surface of about -9 cm. 2 in 80 cm. 3 of liquid ; on the other 

 hand, under similar conditions, the reduction passed the 

 stage of the tetroxide within two minutes when the 

 diameter of the cylinder was 2 cm., and the exposed sur- 

 face about 25 cm. 2 In the experiments to be described 

 the zinc cylinder was cast from the best commercial spel- 

 ter in the form of a hollow cylinder, closed at one end, 

 having a diameter of 2 cm. and a length of 9-5 cm. In use, 

 this cylinder was immersed to a depth of about 4 cm. and 

 exposed an active surface of about 25 cm. 2 It was 

 attached to the rotating axis by means of a rubber stop- 

 per. At the end of the process of reduction the solution 

 was diluted to a volume of 250 cm. 3 , heated to boiling, and 

 titrated with standard permanganate. The correction in 

 blank for the materials used, which proved to be regular 

 and amounted to 0-18 cm. 3 of the permanganate solution, 

 was applied to the titration figures. The results of 

 determinations of the rapidity of reduction made under 

 the conditions indicated are given in Table V. The 

 details of these preliminary experiments show that the 

 rapidity of the reduction depends upon the initial tem- 

 peratures, as well as upon the rate of rotation and the 

 volume of the solution, but that it makes very little dif- 

 ference whether the zinc cylinder is simply rotated in the 

 solution or is made an anode in the electrolytic cell. 



These preliminary experiments show a considerable 

 degree of variation in the rapidity of the reduction, the 

 influential factors being chiefly the volume of the solution, 

 relative size and rate of rotation of the reducing cylinder, 

 and the temperature. The slight advantage in the use of 

 the electric current would not seem to warrant its use in 

 the reduction of vanadic acid by means of the rotating 

 zinc cylinder. The following determinations, in which 

 over-reduction of the vanadic acid was corrected by the 

 use of silver sulphate, were made therefore without the 

 aid of electrolytic action. In these experiments the con- 

 ditions were the following : viz. — volume of solution 

 heated to boiling at the outset, 80 cm. 3 ; sulphuric acid 

 (cone.) 5 cm. 3 ; diameter of cylinder, 2 cm.; surface of 

 cylinder exposed to action, 25 cm. 2 ; revolutions of cylin- 

 der per minute, 850. At the end of the process of reduc- 

 tion the solution was diluted to a volume of 250 cm. 3 , 

 treated with silver sulphate (10 cm. 3 of the saturated 



