26o KANSAS UNIVERSITY QUARTERLY. 



Since arsenic acid and stannous salts react upon each other, of 

 course these two can not exist in the same solution. 



The chemistry of this is simple. Arsenic, in arsenous condition. 

 has but little tendency to form positive ions, and the chlorid, even 

 in as strongly acid solution as the one used, is completely hydro- 

 lized to arsenous acid. Since this is a weak acid in the presence 

 of such strong hydrochloric acid, it will exist, practically all, as 

 the undissociated acid HgAsOg. Hydrogen sulfid acts upon this, 

 the sulfur and oxygen exchanging places. Hydrogen ions accel- 

 erate this, and the stronger the acid the faster it goes, indicating 

 that it is the undissociated hydrogen sulfid acting upon the undis- 

 sociated arsenous acid and not an action between the ions. 



The reaction is as follows: 



H3As03 + 3H.S + H + -fCr^H,AsS3 + 3H20+H+ + Cr 

 2H3AsS3^As.,S3 + 3H3S. 



The concentration of hydrochloric acid being the same on both 

 sides of the equation, the arsenous sulfid should be, and is, just as 

 insoluble in concentrated as in dilute hydrochloric acid. 



Antimony and tin readily form positive ions, and the reaction 

 for the formation of these sulfids is between their ions and the 

 sulfur ions form the hydrogen sulfid. Hydrogen suliid is a very 

 weak acid and breaks up into this 



and hence if the concentration of H"*" be increased by adding 

 h3drochloric acid or any strong acid the concentration of sulfur as 

 ion must decrease; and the greater the concentration of the hydro- 

 gen as ion, the less that of sulfur, and since the product of the 

 concentration of antimony as ion and sulfur as ion must reach a 

 certain value before antimony sulfid can be precipitated, it follows 

 at once that it is only necessary to make the solution highly acid in 

 order to prevent the antimony coming down with the arsenic. 



The chlorine as ion from the hydrochloric acid decreases the dis- 

 sociation of the antimony chlorid and this also tends to increase the 

 solubility of the antimony suliid. 



