526 
CHEMISTRY: D. A. MACINNES 
THE POTENTIALS AT THE JUNCTIONS OF SALT SOLUTIONS 
By Duncan A. Maclnnes 
LABORATORY OF PHYSICAL CHEMISTRY. UNIVERSITY OF ILLINOIS 
Presented to the Academy, August 12, 1915 
A concentration-cell consisting of two identical reversible electrodes 
in contact with two differently concentrated solutions of the same salt 
gives an electromotive force which is the algebraic sum of three poten- 
tials — those at the two electrodes and that at the junction of the solu- 
tions. Since for most theoretical purposes the potential at the electrodes 
alone is desired, many attempts have been made to evaluate the poten- 
tial at the liquid junction. The effect of interposing strong salt solutions 
on the electromotive force of the cell has also been studied with the 
view of eliminating the effect of the liquid junction. 
It has been usual to test the correctness of the value assigned to the 
potential at the junction, or the efficacy of the artifice employed in eHmi- 
nating it, by seeing whether the resulting electromotive force E at the 
electrodes can be calculated by the equation E = {RT/F)\n{C-i/C2) 
where Ci and C2 are the concentrations of the ions in the two solutions 
as calculated from conductance measurements, and R, T, and F are the 
gas-constant, the absolute temperature, and the faraday respectively. 
Recent work^ has indicated, however, that the voltages calculated by 
this formula are too high. It therefore seems desirable to investi- 
gate the subject of the liquid junction connecting two solutions of the 
same salt with a view of finding an expression, involving the fewest 
possible assumptions, for its electromotice force. 
If the cell Ag, AgCl (solid) + KCl (O.ln), AgCl (solid) + KCl 
(O.Oln), Ag operates reversibly until one faraday passes through it, 
one equivalent of chloride-ion enters the dilute solution and the same 
amount of chloride-ion is electrolyzed out of the more concentrated solu- 
tion. The current is carried across the Hquid junction by the movement 
of fic equivalents of potassium-ion in the direction of the current and by 
the migration of (1 — fic) equivalents of chloride-ion in the reverse direc- 
tion, fic being the transference-number of the cation. The total effect 
of the passage of the faraday of electricity is the transference of fic 
equivalents of salt, or 2nc equivalents of ion, from the 0.1 normal KCl 
solution to the 0.01 normal KCl solution. The accompanying electrical 
work is the product E F where E is the total E. M. F. of the cell and F 
is the value of the faraday. Now the electrical work and consequently 
the electromotive force at the liquid junction will be determined by the 
