 Na+ does cross the membrane via leak channels (the permeability is approximately 1/10 that of K+, the most permeant ion) but, as per the pump-leak model, it is extruded by the Na+/K+-ATPase.Because there is a difference in concentration of ions on either side of the membrane, the pH may also differ when protons are involved.In many instances, from ultrafiltration of proteins to ion exchange chromatography, the pH of the buffer adjacent to the charged groups of the membrane is different from the pH of the rest of the buffer solution.