SOLUTION OF GOLD IN POTASSIUM CYANIDE 217 



If it were possible to supply oxygen to the platinized platinum at a high 

 enough speed and at the same time exclude all air from the cyanide 

 (other things being equal, such as physical condition of the gold, etc.), 

 the solubility would undoubtedly be directly proportional to the current, 

 according to Faraday's law. As a matter of fact, the physical condition 

 of the gold varying and causing a varying occlusion of oxygen on its 

 part, the oxygen held in solution by the cyanide, and the varying amount 

 of oxygen occluded by the platinized platinum undoubtedly cause 

 the gold to dissolve at a rate not proportional to the current flowing 

 through the cell. Hence the solubilities indicated in the following 

 table are not to be considered more than fair approximations. 



We also measured the potentials of gold in varying concentrations 

 of cyanide against the oxygen electrode, with results which agree with 

 the measurements of Christy and others made against the calomel 

 electrode. 



Strength of KCN Gold potential. 



.10 per cent. .12 volt 



.15 per cent. .14 volt 



.65 per cent. .21 volt 



.80 per cent. .22 volt 



1 .00 per cent. .29 volt 



The equation for the solubility of gold in potassium cyanide, often 

 called Eisner's equation, is 



4 Au+8KCN-f 2H 2 + 2 = 4 KAu(CN) 2 + 4 KOH . 

 The gold dissolves on account of the formation of innumerable short- 

 circuited gold-oxygen cells. The gold sends 4 Au ions into the solution 

 by virtue of its solution pressure, and at the same time the dissolved 



rr 



oxygen sends 2O ions into the solution by virtue of its solution pressure. 



The 4 Au ions react with 8CN ions of the dissociated KCN forming 



ft 

 4 Au(CN) 2 ions, and the 2O ions react with 2H ions (present from the 



