216 Gooch and Phelps — Reduction of Arsenic Acid. 



Art. XXX. — The Reduction of Arsenic Acid by the Action 

 of Hydrochloric Acid and Potassium Bromide • by F. A. 

 Gooch and I. K. Phelps. 



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



It was shown in a a former paper from this laboratory* that 

 arsenic acid may be easily reduced by the simultaneous action 

 of hydrochloric acid and potassium iodide, and more recently f 

 this reaction has been successfully applied to the rapid detec- 

 tion of arsenic in presence of antimony and tin, and of anti- 

 mony and tin associated with arsenic. This paper is the ac- 

 count of work in the course of which it became evident that 

 the action of hydrobromic acid upon arsenic acid is so similar 

 to that of hydriodic acid that potassium bromide may with 

 propriety, and even with advantage, be substituted for the 

 iodide in the process of reduction. 



The apparatus which we have used in this work is similar to 

 that employed previously, and is essentially a Mohr's distilla 

 tion apparatus — consisting of a small flask, of from 25cm 3 to 

 50cm 3 capacity, fitted by means of a pure rubber stopper to a 

 pipette which was bent, drawn out at the lower end, and 

 dipped into a test-tube supported and cooled in an Erlenmeyer 

 flask nearly filled with water. The arsenic was introduced into- 

 the flask in the form of the pure crystallized dihydrogen 

 potassium arseniate which was dissolved with 3 grm of potassium 

 bromide in 5cm 3 of water, and 5cm 3 of hydrochloric acid of full 

 strength (sp. gr. 1*20) were added. The end of the pipette 

 tube was dipped into ocin 3 of hydrochloric acid of half strength 

 contained in this test tube used as a receiver, and the distilla- 

 tion was carried on until the liquid in the flask had almost 

 entirely passed to the receiver. The residue was treated with 

 10cm 3 of the strongest hydrochloric acid, and the distillation was 

 repeated with the modification that this time the condensation 

 was effected by passing the volatile material into 10cm 3 of water, 

 so that the liquid in the receiver at the end of this operation 

 should have the acidity of hydrochloric acid of half strength. 

 This process of treating the residue with the strongest hydro- 

 chloric acid and distilling was continued until arsenic ceased 

 to be discoverable in this distillate. At the beginning of the 

 distillation bromine is liberated and collects in this distillate ; 

 but later, as the arsenious chloride volatilizes and condenses 

 again, the color of the bromine in the distillate vanishes with 



* Gooch and Danner, this Journal, xliv, 308. 

 f Gooch and Hodge, this Journal, xlvii, 382. 



