Gooch and Gilbert — Estimation of Vanadic Acid. 389 



Art. XXX VII. — The Use of the Zinc Rediictor in the Estima- 

 tion of Vanadic Acid; by F. A. Gooch and E. D. Gilbert. 



[Contributions from the Kent Chemical Laboratory of Yale University — CXVL] 



The reduction of vanadic acid to the condition of vanadium 

 tetroxide preparatory to estimating it by titration with potas- 

 sium permanganate is generally accomplished by the use of 

 sulphurous acid or hydrogen sulphide. The more convenient 

 method of treatment by zinc and free acid is not directly appli- 

 cable on account of the irregular reduction of vanadic acid 

 under the conditions to a stage approximating that of the 

 dioxide, as was first shown by Czudnowicz.* The work herein 

 described is the result of an attempt to find a reliable method 

 for bringing the product of reduction of vanadic acid by zinc 

 and acid definitely to the condition of the tetroxide, in order 

 that the redoctor so useful in the preparation of salts of iron 

 for titration by potassium permanganate may be made simi- 

 larly applicable in the estimation of vanadic acid and its salts. 



A very convenient form of reductorf was made as follows: 

 The contracted end of a piece of glass tubing, 2 cm in inside 

 diameter and 50 cm long, was sealed to a stop-cock prolonged in 

 a smaller tube, 0'5 cm in inside diameter, to a length of 24 cm . 

 At the point of contraction in the larger tube was placed a 

 piece of platinum gauze, next to this a mat of fine glass wool 

 2 cm in thickness, and upon the last a column 40 cm long of amal- 

 gamated zinc of a size to pass a sieve of eight meshes to the 

 centimeter. The smaller tube passed through a rubber stopper 

 fitted to a vacuum flask and the last was connected through a 

 pressure regulator with the vacuum pump. In using this 

 apparatus the pump was started, the regulator set to give a 

 pressure in the flask less than the outside pressure by an 

 amount equal to 20 cm of water, the reducing column of zinc 

 was warmed by passing through it hot distilled water followed 

 by 100 cm3 of hot 1 per cent sulphuric acid, and then the solu- 

 tion of the salt of vanadic acid to be reduced was gradually 

 drawn through the zinc in small portions, alternating with 

 portions of the 1 per cent sulphuric acid amounting to 100 cmS . 

 Finally, the column was washed down with 100 cm3 more of the 

 dilute sulphuric acid and about 250 cm3 of hot distilled water. 

 Throughout the entire series of operations care was taken to 

 keep the zinc covered with liquid and so out of contact with 

 the air. 



The lavendar solution collected in the flask contains vana- 

 dium dioxide, which when exposed to the action of a current 

 of air takes, as Roscoe has shown,;}; the blue color of the 



* Ann. Phys., cxx, 39 (1863). 



f Described in Blair's Chemical Analysis of Iron, p. 93 (edition of 1902). 



% Ann. Pharm. Snppl., vi, 98 (1868). 



