CONCENTRATION CELLS. In the extreme ease in which there is no chemical energy to convert into work, the cell becomes a device for the conver sion of heat alone into the energy of the current. It is then known as a concentration cell. Thus, when two electrodes of the same metal are im mersed in a solution of a salt of this metal, the concentration of the solution at one electrode being greater than at the other, the cell has a small electromotive force which will produce a current. When the solutions are so dilute that there is no further heat of dilution, this electro motive force is the residual obtained from the Gibbs-Helmholtz equation, or T X dE/dT. It is proportional to the absolute temperature of the cell. Fig. 16 represents a Zn-ZnSO, concentration cell. The direction of the electromotive force within the cell is shown by the long arrow in the solutions. The short arrows show the tliree• lion of the thermo-electro motive forces at the two electrodes. This thenno-elee tromotive force increases in value with an inerea se in the concentration of the elev. trolyte, and the relation be tween the two is a perfectly definite one. Hence the ori gin of the electromotive force in a concen tration cell may be regarded as thermal en tirely and equal to the difference of the two thermo-electromotive forces on the two sides of the cell. There is also probably a small electro motive force at the contact of the two solutions. Such a cell converts the heat of its surroundings into the energy of a current, hut it does not con tradict the second law of thermodynamics, be cause the process involved does not constitute a cycle of operations. Such a system as this can not by repeated cooling convert into work the heat of neighboring bodies, because the concen trations are equalized by the flow of current, and then there is no further electromotive force.
On the Nernst theory the formula for the elec tromotive force of a concentration cell becomes E = 0.0002 . T
n --I- v c 2 In this formula i expresses the degree of disso ciation of the dissolved salt, Ic and v are the migration velocities of the positive and negative ions respectively, and c, and are the two con centrations.
Nernst has applied his theory to a concentra tion cell consisting of silver electrodes in solu tions of silver nitrate, the concentrations being as 1 to 10. The measured electromotive force was 0.055 volt at C., and the calculated value, 0.0574. The two are in very satisfactory agree ment.
Similar concentration cells may he made by employing a single solution and amalgams in contact with it, the two amalgams containing the metal ion in different degrees of concentra tion. For example, zinc-mercury amalgams ill contact with a solution of zinc sulphate give an electromotive force from the concentrated to the dilute amalgam through the cell. When a current flows the zinc goes into solution from the more concentrated amalgam. and zinc ions are set free at the surface of the weaker amal gam. In this case the larger thenno-electromo tire force is at the junction of the less concen trated amalgam and the solution: and more heat is absorbed at the cathode than is generated at the anode. As in the other case, therefore, this cell converts heat into electric energy.
BIBLIOGRAPHY. Carbart. Primary Batteries Bibliography. Carbart. Primary Batteries (Boston, 1891) ; Benjamin, The Voltaic Cell (New York. 1893) Lehfoldt, Physical Chem istry (London, 1899) ; Walker, Introduction to Physical Chemistry (ib., 1899) ; Cooper, Pri mary Batteries (ib., 1901) ; Arrhenius, Electro (lb., 190.2) ; D le Normal clemcnte (Ifalle, 1902) ; Nernst, Theorelische Chemie (Stuttgart, 1900).