Gooch and Morris — Estimation of Arsenic Acid. 151 



Akt. XYI. — The Iodometric Estimation of Arsenic Acid / 

 by F. A. Gooch and Julia C. Morris. 



[Contributions from the Kent Laboratory of Tale University — XCVL] 



The interaction of a soluble arseniate and a soluble iodide 

 in a suitably acidulated solution results, as is well known, in 

 the reduction of the arsenic acid (more or less completely 

 according to conditions of temperature and proportions of 

 reagents and solvents) with the corresponding liberation of two 

 atoms of iodine for every molecule of arsenic acid (H 3 3 AsO) 

 reduced. Inasmuch, however, as the reaction of this process is 

 reversible, it is necessary, in order that the reduction may be 

 complete, to nullify the oxidizing action of the iodine liberated. 

 Theoretically this end may be accomplished in either of two 

 ways, by volatilizing the free iodine bodily or by destroying 

 the oxidizing power of the iodine by converting it to hydri- 

 odic acid. The former method was followed in a process 

 devised for the estimation of arsenic acid and elaborated in 

 this laboratory.* This method, as originally put forward, con- 

 sisted in adding to the solution of the arseniate potassium 

 iodide in excess of the amount theoretically indicated, with 

 10 cm3 of sulphuric acid of half strength, and so arranging the 

 dilution that the total volume of the liquid should be about 

 100 cm3 , boiling until the volume decreased to 40 cm3 , bleaching 

 by the cautious addition of sulphurous acid the trace of free 

 iodine still held by the hydriodic acids, diluting, cooling, neu- 

 tralizing with acid potassium carbonate, and titrating with 

 iodine, after adding the starch indicator. This process, depend- 

 ing upon the removal by volatilization of all but the last traces 

 of liberated iodine and the conversion of this minute residue 

 by sulphurous acid, involves no secondary reactions of a sort 

 likely to influence the main effect. It is exact and fairly rapid. 



The method of Williamson, f brought forward more recently, 

 depends upon the conversion of the liberated iodine to hydri- 

 odic acid. The interaction at ordinary temperatures of a suitably 

 strong acid, hydrochloric or sulphuric acid, upon the mixture 

 of the arseniate and iodide sets free iodine, and the liberated 

 iodine is converted to hydriodic acid by the action of sodium 

 thiosulphate, the end point being the disappearance of the 

 iodine color. 



According to Williamson's directions, 25 cm3 portions of the 

 solution of the arseniate are treated with potassium iodide and 

 mixed with an equal volume of hydrochloric acid of sp. gr. 



* Gooch and Browning, this Journal, xl (1890), 66. 

 f Jour. Soc. Dyers and Colorists, 1896, 86-89. 



