For an ideal dilute solution, Δ Φ d is the integral of ∇Φ (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 Di is the diffusion coefficient of species i, z i is the charge number of species i, ci is the concentration of species i, R is the gas constant, T is the thermodynamic temperature, and F is the Faraday constant.