Figure 2. Course of electric potential, ϕ, and charge density, ρ, within a pore of a membrane in a Donnan system.

The total cross-sectional area of the pores is very small compared to the area of the membrane, so that almost all the change in ϕ and ρ occurs within the pore itself. This diagram depicts a system containing a negatively charged impermeant polymer. Thus, if the potential at –∞ is set to 0, ϕ at +∞ is negative, while the ρ in the pore is positive. Electroneutrality requires that the total positive charge within the few pores to the left of the interface be equal to the total negative charge in the large volume to the right.