Based on the Nernst equation, the relationship between the electromotive force (EMF) of NaCl (denoted as C(Na+)(sw)) and the concentration of sodium ion (denoted as c Na+)(sw)) in seawater desalination solution was derived as follow: E-sw = E-0 + Slg ((gamma(sw)(+/- NaCl))(2)c(Na+)(sw)c(Cl-)(sw)). The value of the standard EMF (E-0) and the parameter S can be obtained with the binary system of NaCl-H2O by correcting electrode. The activity coefficient of NaCl in seawater desalination solution (denoted as gamma(sw)(+/- NaCl)) was calculated by the simplified Pitzer equation, and shown good agreement with the experimental result. Through this model, the functional relationship between c(Na+)(sw) and E-sw can be obtained once the functional relationship between gamma(sw)(+/- NaCl) and c(Na+)(sw) is regressed and E-0, S are available. Compared with the direct potentiomety method, this model can provide wider measuring range including linear and nonlinear parts and higher accuracy whose deviation is 137%, as well as no need for standard buffer solution. (C) 2013 Elsevier B.V. All rights reserved.