434 H. C. BIDDLE 



as noted by Hunt, on heatingcupric oxide with a solution of ferrous 

 chloride.' From an emulsion of ferrous and cupric hydroxides, after 

 long standing, may be separated crystals of cuprous oxide. That 

 further reduction is largely determined by the concentration of 

 the cupric and ferrous ions, appears probable from the action of 

 ammonium carbonate on a solution of ferrous and cupric chlo- 

 rides,^ The precipitate first formed on adding the carbonate to 

 the mixed chlorides, dissolves in an excess of the precipitant to a 

 yellow liquid, from which, on standing twenty-four hours, there 

 is deposited, on the walls of the vessel, a slight but brilliant mir- 

 ror of metallic copper. 



The influence of the concentration of the ions on the reduc- 

 tion of copper is clearly shown in the behavior of the mixed 

 carbonates under varying conditions. 



When one adds a solution of cupric and ferrous chlorides ( i 

 mol. CuClg : 2 mol. FeClg) to a considerable excess of sodium 

 carbonate, there is obtained a greenish precipitate of the carbon- 

 ates which undergoes but slight change on standing. Such a 

 solution, indeed, would naturally little favor the separation of 

 metallic copper since the highly ionized alkaline carbonate would 

 greatly decrease the active masses of the ferrous and cupric 

 ions. 



If the alkaline carbonate employed be only slightly in excess 

 of that required to precipitate the copper and iron salts, the con- 

 centration of the carbonic acid ions will be greatly diminished. 

 Under these more favorable conditions reduction slowly takes 

 place with loss of carbon dioxide. The carbonates gradually 

 change in color to a brick-red precipitate containing metallic 

 copper and basic ferric carbonate. 



As is well known the acid ferrous and cupric carbonates are 

 more soluble than the corresponding normal or basic salts of 

 these metals. The influence of this greater solubility is shown 

 in the precipitation of small amounts of copper even in the pres- 

 ence of large excess of acid alkaline carbonates. If one adds 



' Sterry Hunt : Comp. r., loc. cit. 



" E. Braun : Zeit. fiir Chem., loc. cit. 



