For an ideal dilute solution, \(\Delta \mathit{\Phi }_{\rm{d}}\) is the integral of \(\nabla \mathit{\Phi }\) (given by the following equation) across the boundary between two regions of different concentrations. \[\nabla \mathit{\Phi }=\frac{R\ T\ \sum D_{i}\ z_{i}\ \nabla c_{i}}{F\ \sum s_{i}^{2}\ D_{i}\ c_{i}}\] where \(D_{i}\) is the diffusion coefficient of species \(i\), \(z_{i}\) is the charge number of species \(i\), \(c_{i}\) is the concentration of species \(i\), \(R\) is the gas constant, \(T\) is the thermodynamic temperature, and \(F\) is the Faraday constant.