A model is developed to treat reverse osmotic separations of electrolyte solutions. Transport in the model is based on the Extended Nernst-Planck equation, which includes diffusion, convection, and electromigration. Boundary conditions include a distribution coefficient that is due to a specific interaction potential representing repulsion of ions from the membrane material. Boundary conditions also include potential jumps known as Donnan Potentials. The model incorporates a mechanism for varying-membrane-fixed-charge as a function of ion concentrations and pH inside the membrane. In addition to salt ions, hydrogen and hydroxide ions are also considered, as pH changes indicate they take part in the transport. A high Peclet approximation is developed which simplifies calculations.