492 Gooch and Phelps — Separation of Arsenic from Copper. 



stand over night before filtering. In such cases it was found 

 that when the solution of arsenic and magnesia mixture with 

 ammonia in distinct excess was frozen, by putting the platinum 

 dish containing the mixture into a good freezing mixture of 

 line ice and salt for five minutes, the ammonium magnesium 

 arseniate was precipitated out during the freezing and remained 

 as a precipitate after the frozen mass was melted. In exjDeri- 

 ments (1) to (6) of Table IY the precipitates were obtained in 

 this way. In every case the precipitate was filtered off on a 

 firm mat of finest asbestos, ignited after careful drying, and 

 weighed. In experiment (7), made in blank, no precipitation 

 occurred. Experiments (5), (6), and (10) show that it is possi- 

 ble to recover satisfactorily the traces of arsenic even in pres- 

 ence of ammonium chloride in larger amount than would be 

 necessary in the ordinary processes of analysis ; and experi- 

 ments (8), (9), and (10), show that double precipitation does 

 not affect unfavorably the delicacy of the process. 



The work as given above shows that arsenic in the higher 

 condition of oxidation may be separated as ammonium mag- 

 nesium arseniate from copper in ammoniacal solution. When 

 the amounts of arsenic present are more than a few milli- 

 grams two precipitations are necessary. Up to two-tenths of 

 a gram of arsenic two precipitations are sufficient. When 

 the amount of arsenic exceeds two-tenths of a gram, three 

 precipitations are requisite. If the arsenic is present in 

 amounts as small as five milligrams, or less, when much stirring 















Table IV. 



Mg 2 As 2 7 





Error 



in terms 



of arsenic. 



No. CuS0 4 



H 2 KAsO 



'4 M 



gMxNH 4 Cl Theory 



Found 



Error 



\ 



*rm. 



cm 3 



cm 3 



grm. 



grm. 



grm. 



grm. 



gram. 



0) 



_ 



0-2 





25 





0-0015 



0-0014 



o-oooi — 



0-0000 + 



(2) 



_ 



0-2 





25 





0-0015 



0-0017 



0-0002 + 



0-0001 + 



(3) 



_ 



1 





25 







0-0077 



0*0078 



0-0001 + 



0-0000 + 



(4) 



_ 



1 





50 





0-0077 



0-0076 



0*0001- 



0-0000 + 



(5) 





0-2 





25 



10 



0-0015 



0-0013 



0-0002 — 



0-0001 — 



(6) 





1 





25 



10 



0-0077 



0-0070 



0-0007 — 



0-0004 — 



(?) 



2 



._ 





25 













_ 





(8) 



2 



0-2 



25 



-25 





0-0015 



0-0015 



0-0000 + 



0*0000 + 



( 9 ) 



2 



0-2 



25- 



-25 





0-0015 



0-0015 



0-0000 + 



0-0000 + 



(10) 



2 



0-2 



25 



-25 



10 



0-0015 



0-0012 



0-0003 — 



o-oooi — 



is required to bring about crystallization, it is decidedly advan- 

 tageous and even necessary in dilute solutions, especially if 

 containing ammonium chloride, to freeze the mixture. The 

 precipitate obtained by pouring into magnesia mixture the 

 cooled solution of the arseniate in hydrochloric acid, in the 

 manner described, is the ammonium magnesium arseniate of 

 ideal constitution ; and this by ignition after carefully drying 

 until all ammonia is driven off yields magnesium pyroarseniate 

 corresponding to the amount of arsenic in solution. 



