(Original post by Chlorophile)
So each actual charge carrier has a higher potential with a high concentration solution than in a low concentration solution?
You are nearly there. Let's put all the previous posts together!
Go back a bit. When a metal electrode is placed within an electrolyte, some of the atoms in the electrolyte go into solution as ions while the electrons create a negative charge on the electrode.
The charge on the electrode therefore increases, but it cannot do this indefinitely because the charge build up at the electrode creates an electric field which makes it increasingly difficult for positive metal ions to go into solution.
The separated charge build up therefore creates an e.m.f. at the electrode and (the equilibrium at which the charge arrests a further increase), is a function of the electrolyte concentration. i.e. a greater density of positive ions in solution transferred from the electrode is made possible by reducing the original concentration of Fe3+
(aq|) electrolyte to allow more electrode ions to enter solution and increase the charge build up on the electrode before the migration is halted by the build in the electric field at the electrode.
So the voltage pressure created at the electrode is a function of the atomic structure of the electrode. i.e. reactivity and atom density whilst the build up of voltage pressure for any given electrode composition is arrested when charge separation equilibrium is reached with the ionic concentration of the electrolyte.
When the conduction path between the electrode pairs in a battery is completed, electrons will flow between the electrodes via the completed circuit. This immediately reduces the charge at the cathode which allows more ions to go into solution.
Hence the completed circuit allows the reaction to continue at a rate governed by the charge separation equilibrium point at the electrodes and the resistance of the conduction path between the electrodes.
It's rather analogous to the energy stored in the electric field between the plates of a capacitor, but this time it's the electric field created between the electrode and the electrolyte and migration of electrons allows the redox reactions to continue until the electrolyte is depleted.
Hence battery capacity is a function of the electrolyte volume.
E.M.F. is a function of the electrode atomic shell configuration and original concentration of ions in the electrolyte solution.