300 Edgar —Estimation of Vanadic and Arsenic Acids. 



is reduced to arsenious acid and the vanadic acid to tetroxide, 

 so that after boiling off the excess of reagent the reoxidation 

 by iodine in alkaline solution should proceed according to the 

 equation 



As 2 3 + V 2 4 + 3l 2 + 3H 2 0=As 2 B + V 2 5 + 6Hi 



and the iodine thus used should correspond to the sum of 

 the two oxides. If then aliquot parts of the same solution 

 are treated in the manner described above, it is evident that 

 the titration of the solution reduced by tartaric acid should 

 determine the vanadium present and that the titration of the 

 solution reduced by sulphur dioxide should determine the sum 

 of the arsenic and vanadium, the difference in the number of 

 cubic centimeters of iodine used in the two cases being the 

 amount required to oxidize the arsenic. 



In Table (I) are given the results of experiments performed 

 upon solutions containing arsenic and vanadic acids in varying 

 proportions. The method of treatment in detail w T as as fol- 

 lows : The solutions were divided into two portions aud one of 

 these was boiled with one to two grams of tartaric or oxalic acid 

 until the blue color of the vanadium tetroxide indicated complete 

 reduction. The solution was then cooled, nearly neutralized 

 with potassium bicarbonate and an excess of standard iodine 

 solution was added. Neutralization was then completed, an 

 excess of bicarbonate added, and the solution allowed to stand 

 for from fifteen minutes to one half hour. The excess of 

 iodine was then removed with standard arsenious acid and the 

 solution titrated to color after the addition of starch. The 

 results of this titration are given under (I), Table (I). 



The second portion of the solution was placed in a small 

 pressure flask and slightly acidified with sulphuric acid. 

 Twenty-five cubic centimeters of a strong solution of sulphur- 

 ous acid was then added and the flask was closed and heated for 

 one hour on the steam bath.* After cooling, the flask was 

 opened and the solution transferred to an Erlenmeyer flask 

 and boiled to remove the excess of sulphur dioxide, a current 

 of carbon dioxide being passed into the liquid to facilitate the 

 removal of the last traces. The solution was then cooled, 

 nearly neutralized with potassium bicarbonate and an excess 

 of iodine added as before. After completing the neutralization, 

 adding an excess of bicarbonate and allowing to stand for one 

 half hour, the excess of iodine was determined by arsenious 

 acid as before. The results are given in (II), Table (I). 



As before stated, the results in (I) determine the vanadium, 

 and these figures, subtracted from (II), determine the arsenic. 



*McCay: Amer. Chem. Jour., vii, 273. 



