pH dependence in a glass membrane electrode:

When glass electrode is placed in a test solution its glass membrane will have an inner and outer hydrated layers and potential difference is developed due to the difference in hydrogen ion activities between test solution and outer hydrated surface of glass electrode as well as inner solution and inner hydrated surface. This potential is called boundary potential and it varies with the activity or pH of the solution. Overall boundary potential is the potential difference between both the boundary potentials. Concentration of inner solution is kept constant, thus the boundary potential is a measurement of the hydrogen ion activity or the pH of the external solution.