OTHER PROPERTIES OF ELECTROLYTIC SOLUTIONS 299 



hydration of the ions is not known at the concentrations in question, 

 such corrections cannot be made. The same considerations hold true of 

 the reactions in which the electrolyte is concerned, such as the formation 

 of intermediate or complex ions, complex molecules, etc. 



The electromotive force of a concentration cell may likewise be 

 expressed in terms of the concentrations of the un-ionized fraction, which 

 leads to the equation: 



(93) -rEF = RTlo S ^+J^-J Ui . 



If the conditions of dilute systems are fulfilled, then: 



(94) J* = J- = J u = Q, 



in which case the electromotive force of the cell may be calculated, if the 

 concentration of the ions or of the un-ionized molecules is known. 

 Equation 93, in this case, reduces to: 



(95) 



This is the equation first developed by Nernst. 24 



When the conditions for a dilute system are no longer fulfilled, the 

 function J is involved in the expression for the electromotive force. This 

 function thus measures the change in the potential of the electrolyte due 

 to interaction between the various molecular species present in the 

 mixture. The form of this function is not known, except in so far as it 

 has been determined experimentally. The electromotive force of concen- 

 tration cells has in many cases been employed for this purpose, since 

 it affords a convenient and direct measure of the change in the potential 

 of the electrolyte. In order to determine the true form of the function, 

 however, it is necessary to know the concentrations C + and C" or C . 



Except as the concentration of the ions may be determined from con- 

 ductance measurements, no method appears to be available whereby the 

 concentrations of the ions and of the un-ionized molecules in an electro- 

 lytic solution may be determined. 



For practical purposes, the equation is often written: 



C Q 2 

 (96) rEF = RT log ^ + ZJ^ 27 . 



Nernst, Ztachr. f. phya. (Them. 2 t 613 (1888). 



