THE DEPOSITION OF COPPER 433 



is of constant value, hence in the second instance, retaining the 



same letters, we have the expression, —^=^K'. The precipita- 

 tion of copper would then be favored by increasing the concen- 

 tration of ferrous, cuprous, and cupric ions and by decreasing 

 that of the ferric ions. The deposition of the metal, conse- 

 quently, should depend on the relative active masses of the ions 

 present. This assumption is fully sustained by the experimental 

 evidence which follows : 



The precipitation of metallic copper by solutions of ferrous salts is 

 a reversible action, whose direction in any case is determined by the 

 relative concentration of the ferrous, ferric, and copper {^cuprous and 

 cupric^ ions. 



EXPERIMENTAL ' 



((«) In a solution containing an appreciable quantity of ferric 

 ions, or in which these would be formed in the course of the 

 reaction, metallic copper will not be deposited. 



This is shown in the inability of ferrous chloride, or sulphate, 

 to reduce corresponding copper salts, even though the mixed 

 solutions stand for an indefinite period. This inaction is, indeed, 

 to be expected when we consider that solutions of soluble ferric 

 salts, as the sulphate and chloride, easily dissolve metallic cop- 

 per with formation of a cupric salt. 



From this it is readily understood why Wibel '^ obtained no 

 reduction of the copper salt on heating together to 210° C. a 

 solution of the mixed sulphates. 



{b^ In a solution containing few ferric ions and in which the 

 reaction does not result in their appreciable increase, a sufficient 

 concentration of ferrous and copper ions will result in the depo- 

 sition of metallic copper. 



The tendency of ferrous to reduce copper salts is shown in 

 the precipitation of cuprous sulphocyanate by the action of the 

 ammonium salt on a solution of ferrous and cupric chlorides. 

 The same tendency appears in the formation of cuprous chloride, 



^ All the reactions given in the following paragraph have been experimentally 

 determined by its author except as indicated by references. — Ed. JouR. Geol. 

 ^ " Das Gediegen-Kupfer und das Rothkupfrerz," S. 20. 



