66 Gooch and Browning — J/< -f hod for the 



Art. YII. — A Method for the Reduction of Arsenic Acid 

 in Analysis; by F. A. Gooch and P. E. Browning. 



[Contributions from the Kent Chemical Laboratory of Tale College — III.] 



Holthoff's development of Molir's suggestion relative to 

 the reduction of arsenic acid to the lower condition of oxidation 

 by the action of sulphurous acid,* with the demonstration that 

 arsenic acid can be evaporated even to dryness in presence of 

 hydrochloric acid without danger of significant volatilization, 

 has placed the analysis of ordinary compounds of arsenic, both 

 natural and artificial, within the scope of Mohr's classical and 

 exact method of determination by titration with iodine. As 

 Holthoff left the method, it is satisfactory so far as regards 

 accuracy, and as modified by McCay.f who substitutes for the 

 four hours' digestion heating for one hour in a pressure-bottle, 

 is eminently successful. In the account of the experiments 

 about to be described we detail our experience in an attempt 

 to shorten still further the process of reduction of arsenic acid 

 by making use of hydriodic acid as the active agent instead of 

 sulphurous acid. 



In a recent paper £ we have described a method for the 

 determination of iodine in haloid salts based upon the action of 

 arsenic acid, in the presence of sulphuric acid, according to the 

 equation, 



H s As0 4 + 2H-I = H 3 As0 3 + H 2 + I-I, 

 the iodine being completely volatilized, but leaving behind in 

 the arsenious acid produced by the action the record of the 

 amount of hydriodic acid originally present. This reaction we 

 propose to utilize conversely, and to employ potassium iodide 

 in excess, in presence of sulphuric acid, to bring about the 

 reduction of the arsenic acid to arsenious acid which may be 

 determined, after neutralization, by the iodine method. The 

 conditions of the methods are different in that, in the former 

 the hydriodic acid is entirely broken up by the action of the 

 arsenic acid, and the iodine volatilizes easily ; while in the 

 latter some hydriodic acid must remain in solution until a very 

 low degree of concentration is reached, and remaining must 

 exhibit its characteristic proneness to retain free iodine. 



"We find in practice that when a solution made up to con- 

 tain sulphuric acid, an arseniate and potassium iodide to an 

 amount somewhat in excess of that theoretically demanded to 

 effect the conversion of the arsenic acid to arsenious acid, 



*Zeit. f. A.nal. Chem., vol. xxiii, p. 3*78. 

 fAm. Chem. Jour., vol. vii, p. 373. 

 JThis Journal, vol. xxxix, p. 188. 



