where ψ1 is the potential at xH, n
0
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
εr
ε0
/xH [F/m2] and is usually assumed to be constant with a value of 20 µF/cm2 (with
εr≈11 in the Helmholtz layer, and xH≈5 Å). The potential ψ1
can therefore be
described by