Solution (source code)

= Solution

Multiplying each ion number density by its signed charge gives the local <charge density>
$$
\boxed{\rho(s)=q[n_+(s)-n_-(s)]=-2qn_0\sinh\!\frac{q\psi(s)}{k_BT}.}
$$
The fixed surface charge is supplied separately as a boundary condition; it is not part of this mobile charge density in the liquid.