488 Gooch and Phelps — Separation of Arsenic from Copper. 



Art. XLII. — The Separation of Arsenic from Copper as 

 Ammonium- Magnesium Arseniate ; by F. A. Gooch and M. 

 A. Phelps. 



[Contribution from the Kent Chemical Laboratory of Yale University---clii.] 



In sl previous paper* from this laboratory it has been shown 

 that arsenic in the higher condition of oxidation is completely 

 removed from solution as the ammonium magnesium arseniate 

 by adding the arsenic in solution with stirring to a sufficient 

 excess of magnesia mixture kept ammoniacal; and, further, 

 that traces of arsenic, even in presence of ammonium salts — 

 which tend to dissolve the ammonium magnesium arseniate — 

 may be precipitated by the use of a sufficient amount of mag- 

 nesia mixture. The fact that ammonium magnesium arseniate 

 is insoluble in presence of an abundance of magnesia mixture 

 while many salts of copper are soluble in ammonia suggests 

 that arsenic existing as an arseniate may be separated from 

 copper in ammoniacal solution by magnesia mixture. The 

 ammonium" magnesium arseniate, as has been shown, may be 

 filtered off upon a mat of fine asbestos under pressure, in a 

 perforated platinum crucible, and ignited to the condition of 

 magnesium pyroarseniate. Obviously, the action of the 

 ammoniacal copper solution on cellulose renders it impossible 

 to make such an estimation by the use of paper filters. 



For the work given here, solutions of arsenic were made by 

 dissolving potassium dihydrogen arseniate in water in definite 

 amounts, and standardized by determining the pyroarseniate 

 obtained in the manner described in the former paper, referred 

 to above. Into an excess of magnesia mixture kept ammo- 

 niacal and contained in a platinum dish, a definite amount of 

 the standardized potassium dihydrogen arseniate solution was 

 added from a burette with stirring. Finally, a few drops of 

 ammonia were added to make sure that the solution at the end 

 was distinctly ammoniacal, while at the point of precipitation 

 the ammonia was never in marked excess. As soon as the 

 precipitate subsided it was filtered off on. fine asbestos under 

 pressure in a perforated platinum crucible ; but the fine pow- 

 dery, crystalline precipitate is so fine that the preparation of a 

 firm mat of finest asbestos is a necessary precaution against 

 loss in filtration. The precipitate remaining on the platinum 

 dish was removed with a policeman and rinsed with portions 

 of the filtrate into the crucible. The precipitate gathered in 

 this way, after carefully draining by pressure, was rinsed with 

 just enough distilled water made faintly ammoniacal to remove 

 * This Journal, ix, p. 55. 



