430 Goocli & Scott — Determination of Vanadic Acid. 



by means of a bored rubber stopper and electrical con- 

 nection was made between the iron spindle and the tubular 

 anode by a strip of platinum foil held in contact with both 

 by the pressure of the rubber stopper. 



The current was delivered from a 16-volt storage bat- 

 tery and the rotating motor, on a 110-volt circuit, was run 

 at a rate of 700-900 revolutions per minute. In the action 

 of this cell the vanadic acid was quickly reduced to the 

 condition of the tetroxide and the reduction may pro- 

 ceed further, but the action of the silver sulphate formed 

 by the solution of the anode will automatically restore 

 the condition of oxidation to that of the tetroxide with 

 the precipitation of metallic silver, as shown by Gooch 

 and Gilbert. 3 The end of reduction to the stage of the 

 tetroxide is therefore indicated by the appearance of pre- 

 cipitated silver in suspension. After the electrolytic 

 reduction the liquid was therefore boiled for five minutes 

 to coagulate the suspended silver, filtered, diluted to a 

 volume of 250 cm. 3 , again heated to boiling and titrated 

 with standard potassium permanganate. The results 

 of preliminary experiments showing the rate of reduc- 

 tion under the conditions are given in A of Table II. 

 The results of a final series of experiments in which 

 the reduction was stopped when a distinct cloudiness 

 appeared in the solution are given in B of Table II. The 

 experimentally determined correction for the amount of 

 permanganate needed to produce color in the blank deter- 

 mination (0-12 cm. 3 ) was precisely the same as in the 

 experiments of Table I. It is plain that this procedure 

 yields exact and regular results with great rapidity. 



Reduction with the Aid of the Copper Anode. — The 

 successful use of the silver anode in the reduction of 

 vanadic acid suggested the similar use of the cheaper 

 copper. In the experiments immediately following the 

 procedure was essentially similar to that adopted in the 

 preceding experiments with the silver anode. 



In preliminary 4;ests to determine the rate of reduction 

 the anode was a hollow cylinder of commercial copper 

 exposing a surface of about 25 cm. 2 to the action of 60 cm. 3 

 of solution containing 5 cm. 3 of concentrated sulphuric 

 acid. The platinum cathode measured 2 cm. by 5 cm. 

 The results of these tests are given in Table III. 



3 This Journal, 15, 389, 1903. 



