202 GUGGENHEIM art. b 



volume of the species Si at infinite dilution at the given tem- 

 perature, and at zero pressure and at the given pressure p respec- 

 tively; A'",- is the mol fraction of the species Si] Zi its valency; and 

 fi its activity coefficient which, at given temperature and pres- 

 sure, tends to unity at infinite dilution. Finally, V has the 

 same value for all ionic species in the given phase. 



Formula (77) will always lead to correct physical results, but 

 it is partly ambiguous because there is no experimental method 

 of distinguishing between the last two terms, 



At\ogfi + ZiFV. (78) 



Thus the activity coefficient of a single ionic species is physically 

 indeterminate, as in each phase an arbitrary value may be 

 assigned to V and the value of /» will vary in such a way that 

 the sum (78) remains invariant. If, however, we consider 

 combinations of ions with zero net electric charge, the cor- 

 responding combinations of electrochemical potentials will be 

 given by 



i i i 



-\-At^\i\ogNi-^At^\i\ogU (79) 



> i 



since by supposition the X/s satisfy the relation (73). It follows 

 that, although the individual ionic activity coefficients /,• are 

 physically indefinite, certain combinations of them of the form 



^ ^i log fi, (80) 



or 



n (/')' 



(81) 



are completely determinate whenever the Xi's satisfy (73). 



19. Mean Activity Coefficient of Electrolyte. Of the various 

 possible products of activity coefficients of the type (81) which 



