ELECTRODE POTENTIALS 161 



into a chain, its E.M.F. is measured and taken to represent the 

 difference between the single potentials. Chains may be made up 

 of two different metals, or of electrodes of the same metal in contact 

 with solutions of different concentrations, these latter being the 

 concentration-chains. In accord with the main purpose of this book, 

 we shall concern ourselves chiefly with the theory of such concentra- 

 tion chains. 



Let us take two silver electrodes immersed in solutions of AgNOa 

 whose concentration on the left side is Ci and on the right side q,%. 

 We shall assume that at the place of contact of the two solutions 

 there exists no difference of potentials, a condition frequently re- 

 ahzed and with which we shall later deal separately. We have then 

 in this case 



RT , C RT , C 



Ci ■ F C2 



and the EMF 



RT, C C2 



Jii = iri — TTz = -=r In — • — 

 F Ci C 



E = — — In — 

 F ci 



Hence the constant C disappears from the equation. Such chains 

 are easily reaHzed, and, therefore, we must first discuss the numerical 

 evaluation of the factor R. Since it is desired to obtain E in volts, 

 it is necessary to calculate the gas constant R in terms of electrostatic 

 units. 



Note: When the valence of the current-yielding ions is n, then 

 the equation correspondingly becomes 



E = — — In — 

 nF ci 



47. The numerical evaluation of Nemst's formula in volts 

 The value R is defined according to the gas laws as 



p V po Vo 



R = 



T 273.09 



