The equation for local ion density can be substituted into the Poisson equation under the assumptions that the work being done is only electric work, that our solution is composed of a 1:1 salt (e.g., NaCl), and that the concentration of salt is much higher than the concentration of ions. The electric work to bring a charged cation or charged anion to a surface with potential ψ can be represented by W + = e ψ ψ and W − − = − − e ψ ψ respectively. These work equations can be substituted into the Boltzmann equation, producing two expressions c − − = c 0 ⋅ ⋅ e e ψ ψ ( x , y , z ) k B T and c + = c 0 ⋅ ⋅ e − − e ψ ψ ( x , y , z ) k B T , where e is the charge of an electron, 1.602 × 10 coulombs.