Modelling of non-steady-state concentration profiles at ISFET-based coulometric sensor-actuator systems

Acid or base concentrations can be determmed very rapidly by performmg an acid-base titration with coulometncally generated OHor H+ ions at a noble metal actuator electrode m close proximity to the pH-sensitive gate of an ion-sensitive field effect transistor (ISFET) The ISFET is used as the mdicator electrode to detect the eqmvalence point in the titration curve Typical values for the time needed to reach the equivalence point are 0.5-10 s for acid or base concentrations ranging from 0 5 X 10m3 to 20 x 10e3 mol 1-l A model is presented, gnmg an analytical descnption of the diffusion phenomena that occur ulth the sensor-actuator system. The results of this analytical model descnption, usmg hneanzed concentration gradients, are presented together with the results of numerical simulations Both results are in good agreement with measurements.