H. L. Ward — Oxalate- Permanganate Process. 427 



acid present to 20 cm3 does not entirely eliminate the error when 

 the volume is kept at 100 cm3 . Another method, used in the 

 last experiment of the table, is to precipitate in water solution 

 and add the acetic acid afterward to throw out the last traces 

 of copper. This secures a somewhat sharper separation in the 

 case of the larger amounts of the metals. 



The Separation of Copper from Arsenic as Oxalate. 

 In the first attempt to separate copper from arsenic the evap- 

 oration process used for cadmium was tried. No nitric acid 

 was added on precipitation, as the arsenic comes down only on 

 evaporation and may be dissolved again. Less nitric acid was 

 also used in the extraction. In the experiments of Table IY 

 the arsenic was present in the form of potassium arsenate, as 

 it was found much easier to make a sharp separation when the 

 arsenic was in the higher condition of oxidation. Even then 

 it was difficult with the larger amounts to bring the arsenic 

 entirely into solution. 



Table IV. 

 Separation of Copper from Arsenic by Desiccation and Extraction. 



Copper 

 present 



Arsenic 



present as 



arsenate 



Volume 

 at pre- 

 cipita- 

 tion 



Oxalic 

 acid 



Solution 

 used in 

 extrac- 

 tion 



Nitric 

 acid used 



in ex- Copper 

 traction found 



Error 



grm. 

 0-0051 



grm. 



o-io 



cm 3 

 50 



grm. 

 1 



cm 3 

 50 



cm 3 



2 



grm. 

 0-0047 



grm. 

 — 0-0004 



0*0504 



0-05 



50 



1 



50 



2 



0-0499 



— 0-0005 



0-0504 



0*05 



50 



1 



50 



2 



0-0501 



— 0-0003 



0-0504 



o-io 



50 



1 



50 



2 



0-0503 



—o-oooi 



0-0504 



o-io 



50 



1 



50 



2 



0-049Y 



— 0-0007 



0-0504 



0-20 



50 



1 



50 



2 



0-0498 



-0-0006 



0-1533 



0-20 



50 



1 



50 



2 



0-1528 



— 0-0005 



On precipitation from a solution containing a large amount 

 of acetic acid according to the procedure before described for 

 cadmium, a very sharp and satisfactory separation was secured. 

 The results of some experiments appear in Table Y. In the 

 case of the larger amounts of copper it was found necessary to 

 add nitric acid before precipitation to render the precipitate 

 crystalline and suitable for filtration. 



Table V. 



Separation of Copper from Arsenic by Precipitation in a Solution Containing 



a Large Amount of Acetic Acid. 





Arsenic 



Volume 















present 



at pre- 





Acetic 









Copper 



as arsen- 



cipita- 



Oxalic 



acid 



Nitric 



Copper 





present 



ate 



tion 



acid 



present 



acid 



found 



Error 



grm. 



grm. 



cm 3 



grm. 



cm 5 



cm 3 



grm. 



grm. 



0-0051 



0-20 



100 



2 



50 



0-2 



0-0052 



+ 0-0001 



0-0051 



0-30 



100 



4 



50 



0-4 



0-0054 



+ 0-0003 



0-0511 



0*30 



100 



4 



50 



0-4 



0-0505 



— 0-0006 



0*1530 



0-20 



100 



4 



50 



10-0 



0-1530 



±0-0000 



0-1533 



0-30 



100 



5 



50 



10-0 



0-1530 



— 0-0003 



