where ψ1 is the potential at xH, n0 the concentration of each ion in the bulk solution in number/litre, and z the valence of the ions. The parameters εr, ε0, k, q and T have their usual meaning. The difference between the potential ψ1 at xH and the surface potential ψ0 is the potential difference across the Stern capacitance. The Stern capacitance has a value of εrε0/xH [F/m2] and is usually assumed to be constant with a value of 20 µF/cm2 (with ε r≈11 in the Helmholtz layer, and xH≈5 Å). The potential ψ1 can therefore be described by