170 HYDROGEN ION CONCENTRATION 



But if into the same solution we immerse such a chemically indiffer- 

 ent electrode as one of platinum or of gold and make it a part of a closed 

 circuit, the Fe++ ions may receive an additional charge (be oxidized) 

 from the current, while the Fe+++ may give up one of its charges 

 (be reduced) to the current. According as to whether the reaction 



Fe++ + ^ Fe 



+++ 



depending chiefly upon the concentration ratio, tends to occur pre- 

 dominatingly in the one or in the other direction, an increase of the 

 oxidation or of the reduction product will result. 



The chemical force with which the above oxidation-reduction 

 reaction tends to occur in the one and in the other direction depends 

 upon the specific chemical nature of the process involved and, 

 secondly, upon the concentrations. There is a definite concentration 

 ratio at which the forces tending towards oxidation and towards 

 reduction are mutually compensating. If we designate this par- 

 ticular concentration ratio by Co, then it can be stated that oxidation 



fFe+++l 

 will predominate when \^ ^^, > Co while the reduction process 



[i^ e++J 



[Fe+++] 

 will prevail when - ^ _^^ < Co 



^ [Fe++] 



Let us now take a galvanic chain: 

 Pt 



[Ye+++] = ci 

 [Fe++] = c'l 



[Fe+++] = C2 

 [Fe++] = c'2 



Upon closing the chain with a metallic conductor between the two 

 electrodes, an electric current will generally be produced in this cir- 

 cuit, let us say from left to right within the chain, hence from right 

 to left in the external circuit. We shall also assume that the volumes 

 of the two solutions are sufficiently great, so that no appreciable 

 change in their concentrations will occur on the passage of a small 

 current. When 1 faraday (96500 coulombs) had flown through any 

 cross section of this circuit, the electric work A = Fx will result, 

 where t is the E.M.F. The same effect which had been produced 

 by the current, could result from an osmotic transport in which one 

 mol of Fe+++ in concentration Ci is isothermally and reversibly com- 

 pressed to the concentration C2, which would entail the work, A = 



RTln - , and at the same time 1 mol of Fe"^ is correspondingly diluted 



Ci 



