Inverse Model of the Glass pH Sensor

Introduction The concentration of ion H (pH) in solution plays very important role in chemical, biological and physical processes. Since Cremer (1906) first reported the pH response of oxide glasses, many theoretical researches about the glass electrode and its applications have been carried out. The glass electrode is stable mechanically and chemically and can be used in the wide range of solvents. In spite of new developments of semiconductor sensors and optical sensors for pH measurements (Bakker, 1998), the glass electrode is still of great practical importance. The glass electrode is relatively fast and can be applied effectively in on-line measurements. However, for some applications such as identification tanks (Sung et al., 1995; Obut and Ozgen, 2008), end-point estimations and auto-titrators, faster responses will be preferable. The detailed mechanism of glass electrode is still questionable. Roughly, the dynamic response of glass electrode consists of quick response to a quasi-equilibrium and slow drift due to ion permeation through the glass membrane (Kohler et al., 2005). The former, response to the quasi-equilibrium, can be explained through the equivalent circuit having a very high internal resistance and a capacitance (Buck, 1968). Time constant of the glass electrode is the time that the capacitance is charged through the very large internal resistance. Here a measuring method that reduces this time by an order of magnitude is proposed. External voltage is introduced to charge the capacitance. It is compared and controlled to trace the glass pH electrode voltage by the integral controller.