A Transition or Adsorption-Layer Theory of the E.M.F. of the Voltaic Cell

Transition-Layer Theory of the Difference of Potential between an Electrolyte and a Metal Plate Immersed in It.---Since the layer of solution adjacent to the plate is under the influence of the molecular forces of the metal plate on one side and under those of the liquid on the other, it will, in general, differ from the rest of the solution in density, in concentration of solute (adsorption), and in degree of dissociation; and because of the difference in the diffusion velocities of the positive and negative ions, a segregation of the ions will result which will produce an electric field through the layer and will also charge the plate and the solution oppositely. These two fields, one due to the asymmetrical distribution of ions (volume charge) and the other to the charge on the plate, together cause the potential difference observed between metal and solution. The theory explains the well-known phenomena of the voltaic cell and also leads to the prediction of a number of new effects. When equilibrium exists the rates of gain and of loss of charge, by diffusion of ions to the plate and by chemical action, must be equal. This equilibrium should take an appreciable time to establish initially, it should be affected by a magnetic field and should be different if there is relative motion between the plate and the liquid. Also, if this theory is correct, the e.m.f. of a voltaic cell should depend on the distance apart of the plates and should change when a direct or alternating current is sent through the cell; and two rods of the same metal but of different diameters should show a difference of potential when dipped in the same electrolyte. These effects will be looked for experimentally. The general differential equations for the difference of potential according to this theory are given, but contain various functions whose form is at present unknown.Transition layer at the free surface of a liquid should show not only an adsorption effect but also a segregation of the charges which may be associated with the electrical effects of bubbling through liquid.