208 GUGGENHEIM art. e 



separating these phases "breaking the circuit." When the 

 conditions (104) are satisfied simultaneously we may say that 

 the cell is "run down." 



More complicated "purely chemical cells" might be described, 

 containing a larger number of phases, membranes and com- 

 ponents, but the general nature of any such cell and the condi- 

 tions of equilibrium will be similar to that of the above simple 

 example. 



The "purely chemical cell" is not of practical importance and, 

 possibly for this reason, is not usually described or discussed in 

 text-books. It has been described here since a clear understand- 

 ing of a "purely chemical cell" should facilitate a complete 

 comprehension of the nature of an "electrochemical cell," which 

 will be discussed next. It is especially to be emphasized that 

 from a theoretical thermodynamic point of view the electric 

 charges of the ions are rather incidental, the fundamental factors 

 at the base of any cell, whether "purely chemical" or "electro- 

 chemical," being the membrane or contact equilibria between 

 successive phases. 



24. Electrochemical Cells. The only essential difference 

 between an "electrochemical cell" and a "purely chemical cell" 

 is that in the former the membrane equilibria involve charged 

 ions. Let us consider the following system, somewhat similar 

 to the purely chemical cell discussed above, in which however 

 the various species concerned are ions. 



Phase a. Containing ions E and A. 

 Membrane 1. Permeable to ions A only. 

 Phase /3. Containing ions A and B. 

 Membrane 2. Permeable to ions B only. 

 Phase 7. Containing ions B and E. 

 Membrane 3. Permeable to ions E only. 

 Phase a'. Chemically identical with phase a. 



The three membrane equihbria are defined completely by the 

 conditions : 



. WV = M", (106.1) 



[fJiBp = M^ (106.2) 



M"' = My, (106.3) 



