Modelling of the migration effect occurring ISFET-based coulometric sensor-actuator system

The migration effect, in addition to diffusion, occurring at an ion-selective field-effect transistor (ISFET)-based coulometric sensor-actuator system has been studied. A diffusion-migration model is presented, based on the numerical solution of the Nemst-Planck equations of which a digital simulation is realized. Corresponding experiments were carried out and compared with the simulation. The results are in good agreement with the simulation. Typical titration times of this system were found to be 0.5-10 s, corresponding to fully dissociated acid concentrations of 0.5 x lo-‘-6.5 x 10W3 mol 1-l with excess of supporting electrolyte. Both the simulation and experimental results show that if the concentration of the supporting electrolyte is 2 20 times higher than that of the species to be titrated, the deviation caused by migration is less than 5% and within the experimental error when pure diffusion is considered. At relatively low concentrations of supporting electrolyte, the migration effect should be taken into account to determine the concentrations of titrated species.