For a long time past,We all think that the potential-determining ions are H⁺ and OH^- for hydrated oxide surface(variable charge surface).So the surface potential(Φ₀) is only the function of pH,and irrelative to the electrolyte concentration.Therefore,the surface of hydratied oxide is termed as constant potential surface.From the discussion in this paper,it is clear that the surface potential of variable charge surface is strongly dependent on the electrolyte concentration.So variable charge surface is not a constant potential surface.