58 IOWA ACADEMY OF SCIENCE 



change, for any given electrode, is most rapid at first, the rate 

 of change then gradually decreasing to zero at equilibrium. In 

 order to eliminate any errors from this source, the whole battery 

 of half-cells with their respective electrodes and solutions was 

 set up and allowed to stand for at least one and one-half hours 

 at constant temperature. That this time sufficed for the attain- 

 ment of equilibrium betwen electrode and solution may be seen 

 from the following table : 



There are four sources of electromotive force in any cell : the 

 thermo-electric potential at the junction of the wire leads with 

 the electrodes, the diffusion potential at the junction of the two 

 solutions, and the electrode potentials at the surfaces of contact 

 between the electrodes and their respective solutions. The first 

 is entirely eliminated by compensation, and it is assumed that the 

 diffusion potential has been made negligible by the interposi- 

 tion of the 0.1 N solution of ammonium nitrate. There is left 

 for consideration, therefore, only the two electrode potentials. 



According to the equation of Nernst, based on the osmotic 

 theory of the cell, the electrode potentials of a metal in contact 

 with two solutions of its ions are given by the expressions : 



/Ft 1 RT , P j rri RT , P ,. x 



II 3 = • In — and II t = . In — • (1) 



nf p 2 nf p! 



where R represents the gas-constant, (1.985 calores), T the ab- 

 solute temperature, n the valence of the cation, and f the faraday 

 (96540 coulombs), P represents the solution pressure of the 

 metal, and p x and p 2 the osmotic pressures of the cation in the 

 two solutions, the pressure being measured in atmospheres. 



Assuming the absence of a diffusion potential, the electromo- 

 tive force of a concentration cell is therefore given by the ex- 

 pression : 



Tf Ti t? RT i P RT i P / ^ \ 



II =n 2 — ii ]= __ . in — — — - -. In — > — (pi>p 8 ). 

 nf p 2 nf p t 



This by rearrangement becomes, 



TT = ^. In 2jl (2) 



nf p 2 



